JAVA-Technical

(PART-I)
Interview Questions in Core Java
1.what is a transient variable?
A transient variable is a variable that may not be serialized.
2.which containers use a border Layout as their default layout?
The window, Frame and Dialog classes use a border layout as their default layout.
3.Why do threads block on I/O?
Threads block on i/o (that is enters the waiting state) so that other threads may execute while the i/o
Operation is performed.
4. How are Observer and Observable used?
Objects that subclass the Observable class maintain a list of observers. When an Observable object is
updated it invokes the update() method of each of its observers to notify the observers that it has changed
state. The Observer interface is implemented by objects that observe Observable objects.
5. What is synchronization and why is it important?
With respect to multithreading, synchronization is the capability to control the access of multiple threads to
shared resources. Without synchronization, it is possible for one thread to modify a shared object while
another thread is in the process of using or updating that object's value. This often leads to significant
errors.
6. Can a lock be acquired on a class?
Yes, a lock can be acquired on a class. This lock is acquired on the class's Class object.
7. What's new with the stop(), suspend() and resume() methods in JDK 1.2?
The stop(), suspend() and resume() methods have been deprecated in JDK 1.2.
8. Is null a keyword?
The null value is not a keyword.
9. What is the preferred size of a component?
The preferred size of a component is the minimum component size that will allow the component to display
normally.
10. What method is used to specify a container's layout?
The setLayout() method is used to specify a container's layout.
11. Which containers use a FlowLayout as their default layout?
The Panel and Applet classes use the FlowLayout as their default layout.
12. What state does a thread enter when it terminates its processing?
When a thread terminates its processing, it enters the dead state.
13. What is the Collections API?
The Collections API is a set of classes and interfaces that support operations on collections of objects.
14. Which characters may be used as the second character of an identifier, but not as the first
character of an identifier?
The digits 0 through 9 may not be used as the first character of an identifier but they may be used after the
first character of an identifier.
15. What is the List interface?
The List interface provides support for ordered collections of objects.
16. How does Java handle integer overflows and underflows?
It uses those low order bytes of the result that can fit into the size of the type allowed by the operation.
17. What is the Vector class?
The Vector class provides the capability to implement a growable array of objects
18. What modifiers may be used with an inner class that is a member of an outer class?
A (non-local) inner class may be declared as public, protected, private, static, final, or abstract.
19. What is an Iterator interface?
The Iterator interface is used to step through the elements of a Collection.
20. What is the difference between the >> and >>> operators?
The >> operator carries the sign bit when shifting right. The >>> zero-fills bits that have been shifted out.
21. Which method of the Component class is used to set the position and size of a component?
setBounds()
22. How many bits are used to represent Unicode, ASCII, UTF-16, and UTF-8 characters?
Unicode requires 16 bits and ASCII require 7 bits. Although the ASCII character set uses only 7 bits, it is
usually represented as 8 bits. UTF-8 represents characters using 8, 16, and 18 bit patterns. UTF-16 uses 16-
bit and larger bit patterns.
23What is the difference between yielding and sleeping?
When a task invokes its yield() method, it returns to the ready state. When a task invokes its sleep()
method, it returns to the waiting state.
24. Which java.util classes and interfaces support event handling?
The EventObject class and the EventListener interface support event processing.
25. Is sizeof a keyword?
The sizeof operator is not a keyword.
26. What are wrapped classes?
Wrapped classes are classes that allow primitive types to be accessed as objects.
27. Does garbage collection guarantee that a program will not run out of memory?
Garbage collection does not guarantee that a program will not run out of memory. It is possible for
programs to use up memory resources faster than they are garbage collected. It is also possible for
programs to create objects that are not subject to garbage collection
28. What restrictions are placed on the location of a package statement within a source code file?
A package statement must appear as the first line in a source code file (excluding blank lines and
comments).
29. Can an object's finalize() method be invoked while it is reachable?
An object's finalize() method cannot be invoked by the garbage collector while the object is still reachable.
However, an object's finalize() method may be invoked by other objects.
30. What is the immediate superclass of the Applet class?
Panel
31. What is the difference between preemptive scheduling and time slicing?
Under preemptive scheduling, the highest priority task executes until it enters the waiting or dead states or
a higher priority task comes into existence. Under time slicing, a task executes for a predefined slice of
time and then reenters the pool of ready tasks. The scheduler then determines which task should execute
next, based on priority and other factors.
32. Name three Component subclasses that support painting.
The Canvas, Frame, Panel, and Applet classes support painting.
33. What value does readLine() return when it has reached the end of a file?
The readLine() method returns null when it has reached the end of a file.
34. What is the immediate superclass of the Dialog class?
Window
35. What is clipping?
Clipping is the process of confining paint operations to a limited area or shape.
36. What is a native method?
A native method is a method that is implemented in a language other than Java.
37. Can a for statement loop indefinitely?
Yes, a for statement can loop indefinitely. For example, consider the following:
for(;;) ;
38. What are order of precedence and associativity, and how are they used?
Order of precedence determines the order in which operators are evaluated in expressions. Associatity
determines whether an expression is evaluated left-to-right or right-to-left
39. When a thread blocks on I/O, what state does it enter?
A thread enters the waiting state when it blocks on I/O.
40. To what value is a variable of the String type automatically initialized?
The default value of an String type is null.
41. What is the catch or declare rule for method declarations?
If a checked exception may be thrown within the body of a method, the method must either catch the
exception or declare it in its throws clause.
42. What is the difference between a MenuItem and a CheckboxMenuItem?
The CheckboxMenuItem class extends the MenuItem class to support a menu item that may be checked or
unchecked.
43. What is a task's priority and how is it used in scheduling?
A task's priority is an integer value that identifies the relative order in which it should be executed with
respect to other tasks. The scheduler attempts to schedule higher priority tasks before lower priority tasks.
44. What class is the top of the AWT event hierarchy?
The java.awt.AWTEvent class is the highest-level class in the AWT event-class hierarchy.
45. When a thread is created and started, what is its initial state?
A thread is in the ready state after it has been created and started.
46. Can an anonymous class be declared as implementing an interface and extending a class?
An anonymous class may implement an interface or extend a superclass, but may not be declared to do
both.
47. What is the range of the short type?
The range of the short type is -(2^15) to 2^15 - 1.
48. What is the range of the char type?
The range of the char type is 0 to 2^16 - 1.
49. In which package are most of the AWT events that support the event-delegation model defined?
Most of the AWT-related events of the event-delegation model are defined in the java.awt.event package.
The AWTEvent class is defined in the java.awt package.
50. What is the immediate superclass of Menu?
MenuItem
51. What is the purpose of finalization?
The purpose of finalization is to give an unreachable object the opportunity to perform any cleanup
processing before the object is garbage collected.
52. Which class is the immediate superclass of the MenuComponent class.
Object
53. What invokes a thread's run() method?
After a thread is started, via its start() method or that of the Thread class, the JVM invokes the thread's
run() method when the thread is initially executed.
54. What is the difference between the Boolean & operator and the && operator?
If an expression involving the Boolean & operator is evaluated, both operands are evaluated. Then the &
operator is applied to the operand. When an expression involving the && operator is evaluated, the first
operand is evaluated. If the first operand returns a value of true then the second operand is evaluated. The
&& operator is then applied to the first and second operands. If the first operand evaluates to false, the
evaluation of the second operand is skipped.
55. Name three subclasses of the Component class.
Box.Filler, Button, Canvas, Checkbox, Choice, Container, Label, List, Scrollbar, or TextComponent
56. What is the GregorianCalendar class?
The GregorianCalendar provides support for traditional Western calendars.
57. Which Container method is used to cause a container to be laid out and redisplayed?
validate()
58. What is the purpose of the Runtime class?
The purpose of the Runtime class is to provide access to the Java runtime system.
59. How many times may an object's finalize() method be invoked by the garbage collector?
An object's finalize() method may only be invoked once by the garbage collector.
60. What is the purpose of the finally clause of a try-catch-finally statement?
The finally clause is used to provide the capability to execute code no matter whether or not an exception is
thrown or caught.
61. What is the argument type of a program's main() method?
A program's main() method takes an argument of the String[] type.
62. Which Java operator is right associative?
The = operator is right associative.
63. What is the Locale class?
The Locale class is used to tailor program output to the conventions of a particular geographic, political, or
cultural region.
64. Can a double value be cast to a byte?
Yes, a double value can be cast to a byte.
65. What is the difference between a break statement and a continue statement?
A break statement results in the termination of the statement to which it applies (switch, for, do, or while).
A continue statement is used to end the current loop iteration and return control to the loop statement.
66. What must a class do to implement an interface?
It must provide all of the methods in the interface and identify the interface in its implements clause.
67. What method is invoked to cause an object to begin executing as a separate thread?
The start() method of the Thread class is invoked to cause an object to begin executing as a separate thread.
68. Name two subclasses of the TextComponent class.
TextField and TextArea
69. What is the advantage of the event-delegation model over the earlier event-inheritance model?
The event-delegation model has two advantages over the event-inheritance model. First, it enables event
handling to be handled by objects other than the ones that generate the events (or their containers). This
allows a clean separation between a component's design and its use. The other advantage of the eventdelegation
model is that it performs much better in applications where many events are generated. This
performance improvement is due to the fact that the event-delegation model does not have to repeatedly
process unhandled events, as is the case of the event-inheritance model.
70. Which containers may have a MenuBar?
Frame
71. How are commas used in the initialization and iteration parts of a for statement?
Commas are used to separate multiple statements within the initialization and iteration parts of a for
statement.
72. What is the purpose of the wait(), notify(), and notifyAll() methods?
The wait(),notify(), and notifyAll() methods are used to provide an efficient way for threads to wait for a
shared resource. When a thread executes an object's wait() method, it enters the waiting state. It only enters
the ready state after another thread invokes the object's notify() or notifyAll() methods.
73. What is an abstract method?
An abstract method is a method whose implementation is deferred to a subclass.
74. How are Java source code files named?
A Java source code file takes the name of a public class or interface that is defined within the file. A source
code file may contain at most one public class or interface. If a public class or interface is defined within a
source code file, then the source code file must take the name of the public class or interface. If no public
class or interface is defined within a source code file, then the file must take on a name that is different than
its classes and interfaces. Source code files use the .java extension.
75. What is the relationship between the Canvas class and the Graphics class?
A Canvas object provides access to a Graphics object via its paint() method.
76. What are the high-level thread states?
The high-level thread states are ready, running, waiting, and dead.
77. What value does read() return when it has reached the end of a file?
The read() method returns -1 when it has reached the end of a file.
78. Can a Byte object be cast to a double value?
No, an object cannot be cast to a primitive value.
79. What is the difference between a static and a non-static inner class?
A non-static inner class may have object instances that are associated with instances of the class's outer
class. A static inner class does not have any object instances.
80. What is the difference between the String and StringBuffer classes?
String objects are constants. StringBuffer objects are not.
81. If a variable is declared as private, where may the variable be accessed?
A private variable may only be accessed within the class in which it is declared.
82. What is an object's lock and which object's have locks?
An object's lock is a mechanism that is used by multiple threads to obtain synchronized access to the object.
A thread may execute a synchronized method of an object only after it has acquired the object's lock. All
objects and classes have locks. A class's lock is acquired on the class's Class object.
83. What is the Dictionary class?
The Dictionary class provides the capability to store key-value pairs.
84. How are the elements of a BorderLayout organized?
The elements of a BorderLayout are organized at the borders (North, South, East, and West) and the center
of a container.
85. What is the % operator?
It is referred to as the modulo or remainder operator. It returns the remainder of dividing the first operand
by the second operand.
86. When can an object reference be cast to an interface reference?
An object reference be cast to an interface reference when the object implements the referenced interface.
87. What is the difference between a Window and a Frame?
The Frame class extends Window to define a main application window that can have a menu bar.
88. Which class is extended by all other classes?
The Object class is extended by all other classes.
89. Can an object be garbage collected while it is still reachable?
A reachable object cannot be garbage collected. Only unreachable objects may be garbage collected..
90. Is the ternary operator written x : y ? z or x ? y : z ?
It is written x ? y : z.
91. What is the difference between the Font and FontMetrics classes?
The FontMetrics class is used to define implementation-specific properties, such as ascent and descent, of a
Font object.
92. How is rounding performed under integer division?
The fractional part of the result is truncated. This is known as rounding toward zero.
93. What happens when a thread cannot acquire a lock on an object?
If a thread attempts to execute a synchronized method or synchronized statement and is unable to acquire
an object's lock, it enters the waiting state until the lock becomes available.
94. What is the difference between the Reader/Writer class hierarchy and the InputStream/
OutputStream class hierarchy?
The Reader/Writer class hierarchy is character-oriented, and the InputStream/OutputStream class hierarchy
is byte-oriented.
95. What classes of exceptions may be caught by a catch clause?
A catch clause can catch any exception that may be assigned to the Throwable type. This includes the Error
and Exception types.
96. If a class is declared without any access modifiers, where may the class be accessed?
A class that is declared without any access modifiers is said to have package access. This means that the
class can only be accessed by other classes and interfaces that are defined within the same package.
97. What is the SimpleTimeZone class?
The SimpleTimeZone class provides support for a Gregorian calendar.
98. What is the Map interface?
The Map interface replaces the JDK 1.1 Dictionary class and is used associate keys with values.
99. Does a class inherit the constructors of its superclass?
A class does not inherit constructors from any of its superclasses.
100. For which statements does it make sense to use a label?
The only statements for which it makes sense to use a label are those statements that can enclose a break or
continue statement.
101. What is the purpose of the System class?
The purpose of the System class is to provide access to system resources.
102. Which TextComponent method is used to set a TextComponent to the read-only state?
setEditable()
103. How are the elements of a CardLayout organized?
The elements of a CardLayout are stacked, one on top of the other, like a deck of cards.
104. Is &&= a valid Java operator?
No, it is not.
105. Name the eight primitive Java types.
The eight primitive types are byte, char, short, int, long, float, double, and boolean.
106. Which class should you use to obtain design information about an object?
The Class class is used to obtain information about an object's design.
107. What is the relationship between clipping and repainting?
When a window is repainted by the AWT painting thread, it sets the clipping regions to the area of the
window that requires repainting.
108. Is "abc" a primitive value?
The String literal "abc" is not a primitive value. It is a String object.
109. What is the relationship between an event-listener interface and an event-adapter class?
An event-listener interface defines the methods that must be implemented by an event handler for a
particular kind of event. An event adapter provides a default implementation of an event-listener interface.
110. What restrictions are placed on the values of each case of a switch statement?
During compilation, the values of each case of a switch statement must evaluate to a value that can be
promoted to an int value.
111. What modifiers may be used with an interface declaration?
An interface may be declared as public or abstract.
112. Is a class a subclass of itself?
A class is a subclass of itself.
113. What is the highest-level event class of the event-delegation model?
The java.util.EventObject class is the highest-level class in the event-delegation class hierarchy.
114. What event results from the clicking of a button?
The ActionEvent event is generated as the result of the clicking of a button.
115. How can a GUI component handle its own events?
A component can handle its own events by implementing the required event-listener interface and adding
itself as its own event listener.
116. What is the difference between a while statement and a do statement?
A while statement checks at the beginning of a loop to see whether the next loop iteration should occur. A
do statement checks at the end of a loop to see whether the next iteration of a loop should occur. The do
statement will always execute the body of a loop at least once.
117. How are the elements of a GridBagLayout organized?
The elements of a GridBagLayout are organized according to a grid. However, the elements are of different
sizes and may occupy more than one row or column of the grid. In addition, the rows and columns may
have different sizes.
118. What advantage do Java's layout managers provide over traditional windowing systems?
Java uses layout managers to lay out components in a consistent manner across all windowing platforms.
Since Java's layout managers aren't tied to absolute sizing and positioning, they are able to accomodate
platform-specific differences among windowing systems.
119. What is the Collection interface?
The Collection interface provides support for the implementation of a mathematical bag - an unordered
collection of objects that may contain duplicates.
120. What modifiers can be used with a local inner class?
A local inner class may be final or abstract.
121. What is the difference between static and non-static variables?
A static variable is associated with the class as a whole rather than with specific instances of a class. Nonstatic
variables take on unique values with each object instance.
122. What is the difference between the paint() and repaint() methods?
The paint() method supports painting via a Graphics object. The repaint() method is used to cause paint() to
be invoked by the AWT painting thread.
123. What is the purpose of the File class?
The File class is used to create objects that provide access to the files and directories of a local file system.
124. Can an exception be rethrown?
Yes, an exception can be rethrown.
125. Which Math method is used to calculate the absolute value of a number?
The abs() method is used to calculate absolute values.
126. How does multithreading take place on a computer with a single CPU?
The operating system's task scheduler allocates execution time to multiple tasks. By quickly switching
between executing tasks, it creates the impression that tasks execute sequentially.
127. When does the compiler supply a default constructor for a class?
The compiler supplies a default constructor for a class if no other constructors are provided.
128. When is the finally clause of a try-catch-finally statement executed?
The finally clause of the try-catch-finally statement is always executed unless the thread of execution
terminates or an exception occurs within the execution of the finally clause.
129. Which class is the immediate superclass of the Container class?
Component
130. If a method is declared as protected, where may the method be accessed?
A protected method may only be accessed by classes or interfaces of the same package or by subclasses of
the class in which it is declared.
131. How can the Checkbox class be used to create a radio button?
By associating Checkbox objects with a CheckboxGroup.
132. Which non-Unicode letter characters may be used as the first character of an identifier?
The non-Unicode letter characters $ and _ may appear as the first character of an identifier
133. What restrictions are placed on method overloading?
Two methods may not have the same name and argument list but different return types.
134. What happens when you invoke a thread's interrupt method while it is sleeping or waiting?
When a task's interrupt() method is executed, the task enters the ready state. The next time the task enters
the running state, an InterruptedException is thrown.
135. What is casting?
There are two types of casting, casting between primitive numeric types and casting between object
references. Casting between numeric types is used to convert larger values, such as double values, to
smaller values, such as byte values. Casting between object references is used to refer to an object by a
compatible class, interface, or array type reference.
136. What is the return type of a program's main() method?
A program's main() method has a void return type.
137. Name four Container classes.
Window, Frame, Dialog, FileDialog, Panel, Applet, or ScrollPane
138. What is the difference between a Choice and a List?
A Choice is displayed in a compact form that requires you to pull it down to see the list of available
choices. Only one item may be selected from a Choice. A List may be displayed in such a way that several
List items are visible. A List supports the selection of one or more List items.
139. What class of exceptions are generated by the Java run-time system?
The Java runtime system generates RuntimeException and Error exceptions.
140. What class allows you to read objects directly from a stream?
The ObjectInputStream class supports the reading of objects from input streams.
141. What is the difference between a field variable and a local variable?
A field variable is a variable that is declared as a member of a class. A local variable is a variable that is
declared local to a method.
142. Under what conditions is an object's finalize() method invoked by the garbage collector?
The garbage collector invokes an object's finalize() method when it detects that the object has become
unreachable.
143. How are this() and super() used with constructors?
this() is used to invoke a constructor of the same class. super() is used to invoke a superclass constructor.
144. What is the relationship between a method's throws clause and the exceptions that can be
thrown during the method's execution?
A method's throws clause must declare any checked exceptions that are not caught within the body of the
method.
145. What is the difference between the JDK 1.02 event model and the event-delegation model
introduced with JDK 1.1?
The JDK 1.02 event model uses an event inheritance or bubbling approach. In this model, components are
required to handle their own events. If they do not handle a particular event, the event is inherited by (or
bubbled up to) the component's container. The container then either handles the event or it is bubbled up to
its container and so on, until the highest-level container has been tried.
In the event-delegation model, specific objects are designated as event handlers for GUI components.
These objects implement event-listener interfaces. The event-delegation model is more efficient than the
event-inheritance model because it eliminates the processing required to support the bubbling of unhandled
events.
146. How is it possible for two String objects with identical values not to be equal under the ==
operator?
The == operator compares two objects to determine if they are the same object in memory. It is possible for
two String objects to have the same value, but located indifferent areas of memory.
147. Why are the methods of the Math class static?
So they can be invoked as if they are a mathematical code library.
148. What Checkbox method allows you to tell if a Checkbox is checked?
getState()
149. What state is a thread in when it is executing?
An executing thread is in the running state.
150. What are the legal operands of the instanceof operator?
The left operand is an object reference or null value and the right operand is a class, interface, or array type.
151. How are the elements of a GridLayout organized?
The elements of a GridBad layout are of equal size and are laid out using the squares of a grid.
152. What an I/O filter?
An I/O filter is an object that reads from one stream and writes to another, usually altering the data in some
way as it is passed from one stream to another.
153. If an object is garbage collected, can it become reachable again?
Once an object is garbage collected, it ceases to exist. It can no longer become reachable again.
154. What is the Set interface?
The Set interface provides methods for accessing the elements of a finite mathematical set. Sets do not
allow duplicate elements.
155. What classes of exceptions may be thrown by a throw statement?
A throw statement may throw any expression that may be assigned to the Throwable type.
156. What are E and PI?
E is the base of the natural logarithm and PI is mathematical value pi.
157. Are true and false keywords?
The values true and false are not keywords.
158. What is a void return type?
A void return type indicates that a method does not return a value.
159. What is the purpose of the enableEvents() method?
The enableEvents() method is used to enable an event for a particular object. Normally, an event is enabled
when a listener is added to an object for a particular event. The enableEvents() method is used by objects
that handle events by overriding their event-dispatch methods.
160. What is the difference between the File and RandomAccessFile classes?
The File class encapsulates the files and directories of the local file system. The RandomAccessFile class
provides the methods needed to directly access data contained in any part of a file.
161. What happens when you add a double value to a String?
The result is a String object.
162. What is your platform's default character encoding?
If you are running Java on English Windows platforms, it is probably Cp1252. If you are running Java on
English Solaris platforms, it is most likely 8859_1..
163. Which package is always imported by default?
The java.lang package is always imported by default.
164. What interface must an object implement before it can be written to a stream as an object?
An object must implement the Serializable or Externalizable interface before it can be written to a stream as
an object.
165. How are this and super used?
this is used to refer to the current object instance. super is used to refer to the variables and methods of the
superclass of the current object instance.
166. What is the purpose of garbage collection?
The purpose of garbage collection is to identify and discard objects that are no longer needed by a program
so that their resources may be reclaimed and reused.
167. What is a compilation unit?
A compilation unit is a Java source code file.
168. What interface is extended by AWT event listeners?
All AWT event listeners extend the java.util.EventListener interface.
169. What restrictions are placed on method overriding?
Overridden methods must have the same name, argument list, and return type. The overriding method may
not limit the access of the method it overrides. The overriding method may not throw any exceptions that
may not be thrown by the overridden method.
170. How can a dead thread be restarted?
A dead thread cannot be restarted.
171. What happens if an exception is not caught?
An uncaught exception results in the uncaughtException() method of the thread's ThreadGroup being
invoked, which eventually results in the termination of the program in which it is thrown.
172. What is a layout manager?
A layout manager is an object that is used to organize components in a container.
173. Which arithmetic operations can result in the throwing of an ArithmeticException?
Integer / and % can result in the throwing of an ArithmeticException.
174. What are three ways in which a thread can enter the waiting state?
A thread can enter the waiting state by invoking its sleep() method, by blocking on I/O, by unsuccessfully
attempting to acquire an object's lock, or by invoking an object's wait() method. It can also enter the
waiting state by invoking its (deprecated) suspend() method.
175. Can an abstract class be final?
An abstract class may not be declared as final.
176. What is the ResourceBundle class?
The ResourceBundle class is used to store locale-specific resources that can be loaded by a program to
tailor the program's appearance to the particular locale in which it is being run.
177. What happens if a try-catch-finally statement does not have a catch clause to handle an
exception that is thrown within the body of the try statement?
The exception propagates up to the next higher level try-catch statement (if any) or results in the program's
termination.
178. What is numeric promotion?
Numeric promotion is the conversion of a smaller numeric type to a larger numeric type, so that integer and
floating-point operations may take place. In numerical promotion, byte, char, and short values are
converted to int values. The int values are also converted to long values, if necessary. The long and float
values are converted to double values, as required.
179. What is the difference between a Scrollbar and a ScrollPane?
A Scrollbar is a Component, but not a Container. A ScrollPane is a Container. A ScrollPane handles its
own events and performs its own scrolling.
180. What is the difference between a public and a non-public class?
A public class may be accessed outside of its package. A non-public class may not be accessed outside of
its package.
181. To what value is a variable of the boolean type automatically initialized?
The default value of the boolean type is false.
182. Can try statements be nested?
Try statements may be tested.
183. What is the difference between the prefix and postfix forms of the ++ operator?
The prefix form performs the increment operation and returns the value of the increment operation. The
postfix form returns the current value all of the expression and then performs the increment operation on
that value.
184. What is the purpose of a statement block?
A statement block is used to organize a sequence of statements as a single statement group.
185. What is a Java package and how is it used?
A Java package is a naming context for classes and interfaces. A package is used to create a separate name
space for groups of classes and interfaces. Packages are also used to organize related classes and interfaces
into a single API unit and to control accessibility to these classes and interfaces.
186. What modifiers may be used with a top-level class?
A top-level class may be public, abstract, or final.
187. What are the Object and Class classes used for?
The Object class is the highest-level class in the Java class hierarchy. The Class class is used to represent
the classes and interfaces that are loaded by a Java program.
188. How does a try statement determine which catch clause should be used to handle an exception?
When an exception is thrown within the body of a try statement, the catch clauses of the try statement are
examined in the order in which they appear. The first catch clause that is capable of handling the exception
is executed. The remaining catch clauses are ignored.
189. Can an unreachable object become reachable again?
An unreachable object may become reachable again. This can happen when the object's finalize() method is
invoked and the object performs an operation which causes it to become accessible to reachable objects.
190. When is an object subject to garbage collection?
An object is subject to garbage collection when it becomes unreachable to the program in which it is used.
191. What method must be implemented by all threads?
All tasks must implement the run() method, whether they are a subclass of Thread or implement the
Runnable interface.
192. What methods are used to get and set the text label displayed by a Button object?
getLabel() and setLabel()
193. Which Component subclass is used for drawing and painting?
Canvas
194. What are synchronized methods and synchronized statements?
Synchronized methods are methods that are used to control access to an object. A thread only executes a
synchronized method after it has acquired the lock for the method's object or class. Synchronized
statements are similar to synchronized methods. A synchronized statement can only be executed after a
thread has acquired the lock for the object or class referenced in the synchronized statement.
195. What are the two basic ways in which classes that can be run as threads may be defined?
A thread class may be declared as a subclass of Thread, or it may implement the Runnable interface.
196. What are the problems faced by Java programmers who don't use layout managers?
Without layout managers, Java programmers are faced with determining how their GUI will be displayed
across multiple windowing systems and finding a common sizing and positioning that will work within the
constraints imposed by each windowing system.
197. What is the difference between an if statement and a switch statement?
The if statement is used to select among two alternatives. It uses a boolean expression to decide which
alternative should be executed. The switch statement is used to select among multiple alternatives. It uses
an int expression to determine which alternative should be executed.
198. What is the List interface?
The List interface provides support for ordered collections of objects.
Most frequent questions
0) Q: Java and C++
A: Some of the similarities and differences are in the table:
Features Java C/C++
Pointer No Yes
Operator Overload No Yes
Typedef, Define,
Preprocessors No Yes
Structures, Unions No Yes
Enums No Yes
Functions No (only methods within classes) Yes
Goto statement No Yes
Automatic CoercionsNo(types should be converted explicitly) Yes
Global Variables No. Variable is part of a class Yes
Templates No Yes
Private, Protected, Public
Inheritance No Yes
Default parameters No Yes
Garbage Collection Yes No
Multi-thread support Yes No
Multiple Inheritance
Yes. Supports only interface inheritance and not implementation inheritance!
Yes
Exception Handling
Yes. try/catch must be defined if the function declares that it may throw an exception.
Yes. You may not include the try/catch even if the function
throws an exception.
Function Overload Yes Yes
Internationalization Yes Yes
Include of other Objects #import #include
Comments "//","/* */,/** */ "//","/* */"
1) Q: What is the purpose of the toolkit in the Abstract Window Toolkit (AWT)? How does AWT
work ?
A: The AWT toolkit is an interface between the abstract window layer and a specific windowing
implementation.
2) Q: What is layout manager ? How does it work ?
A: A layout manager is an object that positions and resizes the components in a Container according to
some algorithm; for example, the FlowLayout layout manager lays out components from left to right until it
runs out of room and then continues laying out components below that row.
3) Q: Advantages and disadvantages of layout manager ?
4) Q: Compare SWING components to standard AWT.
A: Swing is an extension of, and not a replacement for the AWT. There is some overlap between AWT and
Swing (for example a Swing JButton component might be viewed as an improved functional replacement
for an AWT Button component.) One of the advantages of Swing components is that because the
components are not rendered on the screen by the operating system, the look and feel of a component does
not change as the application or applet is executed on different platforms running under different operating
systems. Furthermore, it is possible to cause Swing components to mimic the look and feel of a specific
platform no matter what platform the program is running on. This is known as pluggable look and feel.
Swing components support the JDK 1.1
Delegation Event Model. From an event handling viewpoint, Swing components operate the same as
AWT components (except that Swing provides a number of new event types). Many Swing components
don't have an AWT counterpart. A number of new and exciting components are included in the Swing
library that don't exist in the AWT (tooltips, progress bars, trees, etc.)
5) Q: What is Java Beans ?
A: According to JavaSoft, "A Java Bean is a reusable software component that can be manipulated visually
in a builder tool."
6) Q: What you know about Corba implementation in Java ?
A: Java 1.2 promises full CORBA IDL support.
7) Q: What do you know about networking support in Java ?
A: Java supports "low-level" and "high-level" classes. "Low-level" classes provide support for socket
programming: Socket, DatagramSocket, and ServerSocket classes. "High-level" classes provide "Web
programming": URL, URLEncoder, and URLConnection classes. Networking programming classes ease
the programming of network applications, but do not substitute your knowledge of networking. Java
networking like anything else in Java is platform-independent.
8) Q: What is it object serialization ?
A: Serialization is a way to convert objects (including complex data structures such as lists and
trees) into a stream of bytes.
9) Q: How to make application thread-safe ?
A: You should use the word synchronized to mark the critical section of code. You may also use other
methods of thread synchronization (see wait(), notify(), notifyAll() etc.
10) Q: What is it reflection (introspection) ? Why is reflection possible in the Java language?
A: Reflection (introspection) is querying a class about its properties, and operating on methods and
fields by the name for a given object instance. Reflection is possible in the Java language because of late
binding.
11) Q: Why are Java ARchive (JAR) files important?
A: JAR files bundle .class files and optimize applet downloads.
Following answer may not be correct
12) Describe what happens when an object is created in Java
Several things happen in a particular order to ensure the object is constructed properly:
1. Memory is allocated from heap to hold all instance variables and implementation-specific data of the
object and its superclasses. Implemenation-specific data includes pointers to class and method data.
2. The instance variables of the objects are initialized to their default values.
3. The constructor for the most derived class is invoked. The first thing a constructor does is call the
consctructor for its superclasses. This process continues until the constrcutor for java.lang.Object is called,
as java.lang.Object is the base class for all objects in java.
4. Before the body of the constructor is executed, all instance variable initializers and initialization blocks
are executed. Then the body of the constructor is executed. Thus, the constructor for the base class
completes first and constructor for the most derived class completes last.
13) In Java, You can create a String object as below : String str = "abc"; & String str = new
String("abc");
Why cant a button object be created as : Button bt = "abc" Why is it compulsory to create a button object
as: Button bt = new Button("abc"); Why this is not compulsory in String's case.
The main reason you cannot create a button by
Button bt1= "abc";
is because "abc" is a literal string (something slightly different than a String object, by-the-way) and bt1 is
a Button object. That simple. The only object in Java that can be assigned a literal String is java.lang.String.
Important to not that you are NOT calling a java.lang.String constuctor when you type String s = "abc";
For example
String x = "abc";
String y = "abc";
refer to the same object. While
String x1 = new String("abc");
String x2 = new String("abc");
refer to two different objects.
14) What are the main differences between Java and C++?
Everything is an object in Java( Single root hierarchy as everything gets derived from java.lang.Object)
Java does not have all the complicated aspects of C++ ( For ex: Pointers, templates, unions, operator
overloading, structures etc..)
The Java language promoters initially said "No pointers!", but when many programmers questioned how
you can work without pointers, the promoters began saying "Restricted pointers." You can make up your
mind whether it’s really a pointer or not. In any event, there’s no pointer arithmetic.
There are no destructors in Java. (automatic garbage collection)
Java does not support conditional compile (#ifdef/#ifndef type).
Thread support is built into java but not in C++.
Java does not support default arguments. There’s no scope resolution operator :: in Java. Java uses the
dot for everything, but can get away with it since you can define elements only within a class. Even the
method definitions must always occur within a class, so there is no need for scope
resolution there either.
There’s no "goto " statement in Java.
Java doesn’t provide multiple inheritance (MI), at least not in the same sense that C++ does.
Exception handling in Java is different because there are no destructors.
Java has method overloading, but no operator overloading. The String class does use the + and +=
operators to concatenate strings and String expressions use automatic type conversion, but that’s a special
built-in case.
Java is interpreted for the most part and hence platform independent.
15) What are interfaces?
Interfaces provide more sophisticated ways to organize and control the objects in your system.
The interface keyword takes the abstract concept one step further. You could think of it as a “pure” abstract
class. It allows the creator to establish the form for a class: method names, argument lists, and return
types, but no method bodies. An interface can also contain fields, but The interface keyword takes the
abstract concept one step further. You could think of it as a “pure” abstract class. It allows the creator to
establish the form for a class: method names, argument lists, and return types, but no method bodies.
An interface can also contain fields, but
An interface says: “This is what all classes that implement this particular interface will look like.” Thus,
any code that uses a particular interface knows what methods might be called for that interface, and that’s
all. So the interface is used to establish a “protocol” between classes. (Some object-oriented programming
languages have a keyword called protocolto do the same thing.)
15) How can you achieve Multiple Inheritance in Java?
Java's interface mechanism can be used to implement multiple inheritance, with one important difference
from c++ way of doing MI: the inherited interfaces must be abstract. This obviates the need to choose
between different implementations, as with interfaces there are no implementations.
16) What is the difference between StringBuffer and String class?
A string buffer implements a mutable sequence of characters. A string buffer is like a String, but can be
modified. At any point in time it contains some particular sequence of characters, but the length and content
of the sequence can be changed through certain method calls.
The String class represents character strings. All string literals in Java programs, such as "abc" are constant
and implemented as instances of this class; their values cannot be changed after they are created.
17) Describe, in general, how java's garbage collector works?
The Java runtime environment deletes objects when it determines that they are no longer being used. This
process is known as garbage collection.
The Java runtime environment supports a garbage collector that periodically frees the memory used by
objects that are no longer needed. The Java garbage collector is a mark-sweep garbage collector that scans
Java's dynamic memory areas for objects, marking those that are referenced. After all possible paths to
objects are investigated, those objects that are not marked (i.e. are not referenced) are known to be garbage
and are collected.
18) What's the difference between == and equals method?
The equals method can be considered to perform a deep comparison of the value of an object, whereas the
== operator performs a shallow comparison.
The equals() method compares the characters inside a string object. == operator compares two object
references to check whether they refer to the same instances or not.
19) What are abstract classes, abstract methods?
Simply speaking a class or a method qualified with "abstract" keyword is an abstract class or abstract
method.
You create an abstract class when you want to manipulate a set of classes through a common interface. All
derived-class methods that match the signature of the base-class declaration will be called using the
dynamic binding mechanism.
An abstract method is an incomplete method. It has only a declaration and no method body. Here is the
syntax for an abstract method declaration: abstract void f();
20) How can you force all derived classes to implement a method present in the base class?
Creating and implementing an interface would be the best way for this situation. Just create an interface
with empty methods which forces a programmer to implement all the methods present under it.
Another way of achieving this task is to declare a class as abstract with all its methods abstract.
21) What is the difference between an Applet and an Application?
1. Applets can be embedded in HTML pages and downloaded over the Internet whereas Applications have
no special support in HTML for embedding or downloading.
2. Applets can only be executed inside a java compatible container, such as a browser or appletviewer
whereas Applications are executed at command line by java.exe or jview.exe.
3. Applets execute under strict security limitations that disallow certain operations(sandbox model security)
whereas Applications have no inherent security restrictions.
4. Applets don't have the main() method as in applications. Instead they operate on an entirely different
mechanism where they are initialized by init(),started by start(),stopped by stop() or destroyed by destroy().
22) Java says "write once, run anywhere". What are some ways this isn't quite true?
Any time you use system calls specific to one operating system and do not create alternative calls for
another operating system, your program will not function correctly.
Solaris systems and Intel systems order the bits of an integer differently. (You may have heard of little
endian vs. big endian)
If your code uses bit shifting, or other binary operators, they will not work on systems that have opposide
endianism.
23) Describe java's security model.
Java's security model is one of the most interesting and unique aspects of the language. For the most part
it's broken into two pieces: the user adjustable security manager that checks various API operations like file
access, and the byte code verifier that asserts the validity of compiled byte code.
public abstract class SecurityManager java.lang.SecurityManager is an abstract class which different
applications subclass to implement a particular security policy. It allows an application to determine
whether or not a particular operation will generate a security exception.
24) What is the difference between a Vector and an Array. Discuss the advantages and disadvantages
of both?
The vector container class generalizes the concept of an ordinary C array. Like an array, a vector is an
indexed data structure, with index values that range from 0 to one less than the number of elements
contained in the structure. Also like an array, values are most commonly assigned to and extracted from the
vector using the subscript operator. However, the vector differs from an array in the following important
respects:
The size of the vector can change dynamically. New elements can be inserted on to the end of a vector, or
into the middle. It is important to note, however, that while these abilities are provided, insertion into the
middle of a vector is not as efficient as insertion into the middle of a list.
A vector has more "self-knowledge" than an ordinary array. In particular, a vector can be queried about its
size, about the number of elements it can potentially hold (which may be different from its current size),
and so on.
A vector can only hold references to objects and not primitive types.
Vector Implementaions are usually slower then array because of all the functionality that comes with them.
As implemented in Java, vector is a thread-safe class and hence all methods are synchronous methods,
which makes them considerably slow.
25) How many different types of JDBC drivers are present? Discuss them.
Type 1: JDBC-ODBC Bridge plus ODBC Driver:
The first type of JDBC driver is the JDBC-ODBC Bridge. It is a driver that provides JDBC access to
databases through ODBC drivers. The ODBC driver must be configured on the client for the bridge to
work. This driver type is commonly used for prototyping or when there is no JDBC driver available for a
particular DBMS.
Type 2: Native-API partly-Java Driver:
The Native to API driver converts JDBC commands to DBMS-specific native calls. This is much like the
restriction of Type 1 drivers. The client must have some binary code loaded on its machine. These drivers
do have an advantage over Type 1 drivers because they interface directly with the database.
Type 3: JDBC-Net Pure Java Driver:
The JDBC-Net drivers are a three-tier solution. This type of driver translates JDBC calls into a databaseindependent
network protocol that is sent to a middleware server. This server then translates this DBMSindependent
protocol into a DBMS-specific protocol, which is sent to a particular database. The results are
then routed back through the middleware server and sent back to the client. This type of solution makes it
possible to implement a pure Java client. It also makes it possible to swap databases without affecting the
client.
Type 4: Native-Protocol Pur Java Driver
These are pure Java drivers that communicate directly with the vendor's database. They do this by
converting JDBC commands directly into the database engine's native protocol. This driver has no
additional translation or middleware layer, which improves performance tremendously.
26) What does the keyword "synchronize" mean in java. When do you use it? What are the
disadvantages of synchronization?
Synchronize is used when u want to make ur methods thread safe. The disadvantage of synchronise is it
will end up in slowing down the program. Also if not handled properly it will end up in dead lock.
1. Only use (and minimize it's use)synchronization when writing multithreaded code as there is a speed (up
to five to six time slower, depending on the execution time of the synchronized/non-synchronized method )
cost associated with its use.
2. In case of syncronized method modifier, the byte code generated is the exact same as non-syncronized
method. The only difference is that a flag called ACC_SYNCRONIZED property flag in method's
method_info structure is set if the syncronized method modifier is present.
3. Also, syncronized keyword can make the code larger in size if used in the body of the method as
bytecode for monitorenter/monitorexit is generated in addition to any exception handling.
27) What are native methods? How do you use them?
Native methods are methods that are defined as public static methods within a java class, but whose
implementation is provided in another programming language such as C.
28) What is RMI?
RMI stands for Remote Method Invocation. Traditional approaches to executing code on other machines
across a network have been confusing as well as tedious and error-prone to implement. The nicest way to
think about this problem is that some object happens to live on another machine, and that you can send a
message to the remote object and get a result as if the object lived on your local machine. This
simplification is exactly what Java Remote Method Invocation (RMI) allows you to do.
29) What is JDBC? Describe the steps needed to execute a SQL query using JDBC.
The JDBC is a pure Java API used to execute SQL statements. It provides a set of classes and interfaces
that can be used by developers to write database applications.
The steps needed to execute a SQL query using JDBC:
1. Open a connection to the database.
2. Execute a SQL statement.
3. Process th results.
4. Close the connection to the database.
30) Access specifiers: "public", "protected", "private", nothing?
Public – any other class from any package can instantiate and execute the classes and methods
Protected – only subclasses and classes inside of the package can access the classes and methods
Private – the original class is the only class allowed to executed the methods.
31) What does the "final" keyword mean in front of a variable? A method? A class?
FINAL for a variable : value is constant
FINAL for a method : cannot be overridden
FINAL for a class : cannot be derived
32) Does Java have "goto"?
no
33) Why "bytecode"? Can you reverse-engineer the code from bytecode?
34) What synchronization constructs does Java provide? How do they work?
35) Are constructors inherited? Can a subclass call the parent's class constructor? When?
You cannot inherit a constructor. That is, you cannot create a instance of a subclass using a constructor of
one of it's superclasses. One of the main reasons is because you probably don't want to overide the
superclasses constructor, which would be possible if they were inherited. By giving the developer the
ability to override a superclasses constructor you would erode the encapsulation abilities of the language.
36) Does Java have destructors?
No garbage collector does the job working in the background
37) What does the "abstract" keyword mean in front of a method? A class?
Abstract keyword declares either a method or a class. If a method has a abstract keyword in front of it,it is
called abstract method.Abstract method hs no body.It has only arguments and return type.Abstract methods
act as placeholder methods that are implemented in the subclasses.
Abstract classes can't be instantiated.If a class is declared as abstract,no objects of that class can be
created.If a class contains any abstract method it must be declared as abstract
38) Name four methods every Java class will have.
public String toString();
public Object clone();
public boolean equals();
public int hashCode();
39) Given a text file, input.txt, provide the statement required to open
this file with the appropriate I/O stream to be able to read and process this file.
40) Discuss the differences between creating a new class, extending a class and implementing an
interface; and when each would be appropriate.
*Creating a new class is simply creating a class with no extensions and no
implementations. The signature is as follows
public class MyClass()
{
}
*Extending a class is when you want to use the functionality of another class or
classes. The extended class inherits all of the functionality of the previous class. An
example of this when you create your own applet class and extend from
java.applet.Applet. This gives you all of the functionality of the java.applet.Applet class.
The signature would look like this
public class MyClass extends MyBaseClass
{
}
*Implementing an interface simply forces you to use the methods of the interface
implemented. This gives you two advantages. This forces you to follow a standard
(forces you to use certain methods) and in doing so gives you a channel for
polymorphism. This isn’t the only way you can do polymorphism but this is one of the
ways.
public class Fish implements Animal
{
}
40) What's the difference between the == operator and the equals() method? What test does
Object.equals() use, and why?
The == operator would be used, in an object sense, to see if the two objects were
actually the same object. This operator looks at the actually memory address to see if it
actually the same object. The equals() method is used to compare the values of the
object respectively. This is used in a higher level to see if the object values are equal.
Of course the the equals() method would be overloaded in a meaningful way for
whatever object that you were working with.
41) why do you create interfaces, and when MUST you use one.
You would create interfaces when you have two or more functionalities talking to each other. Doing it this
way help you in creating a protocol between the parties involved.
42) What is the difference between instanceof and isInstance?
instanceof is used to check to see if an object can be cast into a specified type without throwing a cast class
exception.
isInstance()
Determines if the specified Object is assignment-compatible with the object represented
by this Class. This method is the dynamic equivalent of the Java language instanceof
operator. The method returns true if the specified Object argument is non-null and can
be cast to the reference type represented by this Class object without raising a
ClassCastException. It returns false otherwise.
43) How many methods do u implement if implement the Serializable Interface?
The Serializable interface is just a "marker" interface, with no methods of its own to implement.
Are there any other 'marker' interfaces?
java.rmi.Remote
java.util.EventListener
44) *What are the advantages of developing an n-tiered system?
45) *Why is it often difficult to separate the business layer from the data access layer?
46) . Diff between ArrayList and Vector
47) Variable shadowing with example
1. What is the diffrence between an Abstract class and Interface ?
2. What is user defined exception ?
3. What do you know about the garbate collector ?
4. What is the difference between C++ & Java ?
5. Explain RMI Architecture?
6. How do you communicate in between Applets & Servlets ?
7. What is the use of Servlets ?
8. What is JDBC? How do you connect to the Database ?
9. In an HTML form I have a Button which makes us to open another page in 15 seconds. How will do you
that ?
10. What is the difference between Process and Threads ?
11. What is the difference between RMI & Corba ?
12. What are the services in RMI ?
13. How will you initialize an Applet ?
14. What is the order of method invocation in an Applet ?
15. When is update method called ?
16. How will you pass values from HTML page to the Servlet ?
17. Have you ever used HashTable and Dictionary ?
18. How will you communicate between two Applets ?
19. What are statements in JAVA ?
20. What is JAR file ?
21. What is JNI ?
22. What is the base class for all swing components ?
23. What is JFC ?
24. What is Difference between AWT and Swing ?
25. Considering notepad/IE or any other thing as process, What > will happen if you start notepad or IE 3
times? Where 3 processes are started or 3 threads are started ?
26. How does thread synchronization occurs inside a monitor ?
27. How will you call an Applet using a Java Script function ?
28. Is there any tag in HTML to upload and download files ?
29. Why do you Canvas ?
30. How can you push data from an Applet to Servlet ?
31. What are 4 drivers available in JDBC ?
32. How you can know about drivers and database information ?
33. If you are truncated using JDBC, How can you know ..that how much > > > data is truncated ?
34. And What situation , each of the 4 drivers used ?
35. How will you perform transaction using JDBC ?
36. In RMI, server object first loaded into the memory and then the stub reference is sent to the client ? or
whether a stub reference is directly sent to the client ?
37. Suppose server object is not loaded into the memory, and theclient request for it , what will happen?
38. What is serialization ?
39. Can you load the server object dynamically? If so, what are the major 3 steps involved in it ?
40. What is difference RMI registry and OSAgent ?
41. To a server method, the client wants to send a value 20, with this value exceeds to 20,. a message
should be sent to the client ? What will you do for achieving for this ?
42. What are the benefits of Swing over AWT ?
43. Where the CardLayout is used ?
44. What is the Layout for ToolBar ?
45. What is the difference between Grid and GridbagLayout ?
46. How will you add panel to a Frame ?
47. What is the corresponding Layout for Card in Swing ?
48. What is light weight component ?
49. Can you run the product development on all operating systems ?
50. What is the webserver used for running the Servlets ?
51. What is Servlet API used for conneting database ?
52. What is bean ? Where it can be used ?
53. What is difference in between Java Class and Bean ?
54. Can we send object using Sockets ?
55. What is the RMI and Socket ?
56. How to communicate 2 threads each other ?
57. What are the files generated after using IDL to Java Compilet ?
58. What is the protocol used by server and client ?
59. Can I modify an object in CORBA ?
60. What is the functionality stubs and skeletons ?
61. What is the mapping mechanism used by Java to identify IDL language ?
62. Diff between Application and Applet ?
63. What is serializable Interface ?
64. What is the difference between CGI and Servlet ?
65. What is the use of Interface ?
66. Why Java is not fully objective oriented ?
67. Why does not support multiple Inheritance ?
68. What it the root class for all Java classes ?
69. What is polymorphism ?
70. Suppose If we have variable ' I ' in run method, If I can create one or more thread each thread will
occupy a separate copy or same variable will be shared ?
71. In servlets, we are having a web page that is invoking servlets username and password ? which is cheks
in the database ? Suppose the second page also If we want to verify the same information whether it will
connect to the database or it will be used previous information?
72. What are virtual functions ?
73. Write down how will you create a binary Tree ?
74. What are the traverses in Binary Tree ?
75. Write a program for recursive Traverse ?
76. What are session variable in Servlets ?
77. What is client server computing ?
78. What is Constructor and Virtual function? Can we call Virtual funciton in a constructor ?
79. Why we use OOPS concepts? What is its advantage ?
80. What is the middleware ? What is the functionality of Webserver ?
81. Why Java is not 100 % pure OOPS ? ( EcomServer )
82. When we will use an Interface and Abstract class ?
83. What is an RMI?
84. How will you pass parameters in RMI ? Why u serialize?
85. What is the exact difference in between Unicast and Multicast object ? Where we will use ?
86. What is the main functionality of the Remote Reference Layer ?
87. How do you download stubs from a Remote place ?
88. What is the difference in between C++ and Java ? can u explain in detail ?
89. I want to store more than 10 objects in a remote server ? Which methodology will follow ?
90. What is the main functionality of the Prepared Statement ?
91. What is meant by static query and dynamic query ?
92. What are the Normalization Rules ? Define the Normalization ?
93. What is meant by Servelet? What are the parameters of the service method ?
94. What is meant by Session ? Tell me something about HTTPSession Class ?
95. How do you invoke a Servelt? What is the difference in between doPost and doGet methods ?
96. What is the difference in between the HTTPServlet and Generic Servlet ? Expalin their methods ? Tell
me their parameter names also ?
97. Have you used threads in Servelet ?
98. Write a program on RMI and JDBC using StoredProcedure ?
99. How do you sing an Applet ?
100. In a Container there are 5 components. I want to display the all the components names, how will you
do that one ?
101. Why there are some null interface in java ? What does it mean ? Give me some null interfaces in
JAVA ?
102. Tell me the latest versions in JAVA related areas ?
103. What is meant by class loader ? How many types are there? When will we use them ?
104. How do you load an Image in a Servlet ?
105. What is meant by flickering ?
106. What is meant by distributed Application ? Why we are using that in our applications ?
107. What is the functionality of the stub ?
108. Have you used any version control ?
109. What is the latest version of JDBC ? What are the new features are added in that ?
110. Explain 2 tier and 3 -tier Architecture ?
111. What is the role of the webserver ?
112. How have you done validation of the fileds in your project ?
113. What is the main difficulties that you are faced in your project ?
114. What is meant by cookies ? Explain ?
Make sure you have a copy of your resume in front of you. OK to have a cheat sheet or two - just don't let
anyone hear papers shuffling.
Know your OOA&D definitions, such as polymorphism, inheritance, etc.
Know the difference between an interface and an abstract class.
Know that Java does not support multiple inheritance the way C++ does.
Know that you implement an interface (can implement more than one).
Know that you extend an abstract class (can only extend more than one).
Know about the access modifiers: public/friendly(default)/protected/private. Be able to explain in one or
two sentences for each case. This is where you can get tangled up in a phone conversation and confuse the
heck out of the interviewer and yourself.
Know AWT Event Model - tough to do over the phone - but you may get hit on a question.
Know the two ways to start a thread - "extending Thread" or "implementing Runnable".
Know that the method is "run" but to run a thread you use "start".
1. How can I improve the performance of a java application, what are the java optimization
techniques.
That question is about as meaningful as "how do you code stuff?" There's no one right answer, and the
interviewer probably just wanted to see if you knew *anything* about optimization. The answer is to go to
the Performance thread and read the postings. If you search there for "books", you should find some good
references which will get you started. You should also search for "tools".
quote:
2. What should you do to ensure that your applet works exactly the same way on both IE and netscape.
Short answers include: make sure they use the same JVM, don't use complex GUIs, and most importantly
test to be certain! Personally, I'd question the benefit of the applet and ask if could be done with JSPs.

(PART-II)


Q1. What is the difference between an Abstract class and Interface?
 1. Abstract classes may have some executable methods and methods left unimplemented. Interfaces contain no implementation code. 

2. An class can implement any number of interfaces, but subclass at most one abstract class.

 3. An abstract class can have nonabstract methods. All methods of an interface are abstract.

 4. An abstract class can have instance variables. An interface cannot.

 5. An abstract class can define constructor. An interface cannot. 

6. An abstract class can have any visibility: public, protected, private or none (package). An interface's visibility must be public or none (package). 

7. An abstract class inherits from Object and includes methods such as clone() and equals().

Q2.What are checked and unchecked exceptions? 

Java defines two kinds of exceptions : •

 Checked exceptions : Exceptions that inherit from the Exception class are checked exceptions. Client code has to handle the checked exceptions thrown by the API, either in a catch clause or by forwarding it outward with the throws clause. Examples - SQLException, IOxception.

 • Unchecked exceptions : RuntimeException also extends from Exception. However, all of the exceptions that inherit from RuntimeException get special treatment. There is no requirement for the client code to deal with them, and hence they are called unchecked exceptions. Example Unchecked exceptions are NullPointerException, OutOfMemoryError, DivideByZeroException typically, programming errors.

Q3.What is a user defined exception? 

User-defined exceptions may be implemented by 

• defining a class to respond to the exception and 

• embedding a throw statement in the try block where the exception can occur or declaring that the method throws the exception (to another method where it is handled). The developer can define a new exception by deriving it from the Exception class as follows: 

public class MyException extends Exception 

{ /* class definition of constructors (but NOT the exception handling code) goes here public MyException() 

{ 

super();

 } 

public MyException( String errorMessage )

 { 

super( errorMessage );

 } } 

The throw statement is used to signal the occurance of the exception within a try block. Often, exceptions are instantiated in the same statement in which they are thrown using the syntax. throw new MyException("I threw my own exception.") To handle the exception within the method where it is thrown, a catch statement that handles MyException, must follow the try block. If the developer does not want to handle

 exception in the method itself, the method must pass the exception using the syntax: public myMethodName() throws MyException

Q4.What is the difference between C++ & Java? 

Well as Bjarne Stroustrup says "..despite the syntactic similarities,

 C++ and Java are very different languages.

 In many ways, Java seems closer to Smalltalk than to C++.."

. Here are few I discovered:

 • Java is multithreaded 

• Java has no pointers

 • Java has automatic memory management (garbage collection)

 • Java is platform independent (Stroustrup may differ by saying "Java is a platform"

 • Java has built-in support for comment documentation

 • Java has no operator overloading

 • Java doesn’t provide multiple inheritance

 • There are no destructors in Java

Q5.What are statements in JAVA ? 

Statements are equivalent to sentences in natural languages. A statement forms a complete unit of execution. The following types of expressions can be made into a statement by terminating the expression with a semicolon 

• Assignment expressions

 • Any use of ++ or --

 • Method calls

 • Object creation expressions These kinds of statements are called expression statements. In addition to these kinds of expression statements, there are two other kinds of statements. A declaration statement declares a variable. A control flow statement regulates the order in which statements get executed. The for loop and the if statement are both examples of control flow statements.

Q6.What is JAR file? 

JavaARchive files are a big glob of Java classes, images, audio, etc., compressed to make one simple, smaller file to ease Applet downloading. Normally when a browser encounters an applet, it goes and downloads all the files, images, audio, used by the Applet separately. This can lead to slower downloads.

Q7.What is JNI? 

JNI is an acronym of Java Native Interface. Using JNI we can call functions which are written in other languages from Java. Following are its advantages and disadvantages.

 Advantages:

 • You want to use your existing library which was previously written in other language.

 • You want to call Windows API function.

 • For the sake of execution speed. 

• You want to call API function of some server product which is in c or c++ from java client. Disadvantages:

 • You can’t say write once run anywhere. 

• Difficult to debug runtime error in native code.

• Potential security risk.

 • You can’t call it from Applet.

Q8.What is serialization? 

Quite simply, object serialization provides a program the ability to read or write a whole object to and from a raw byte stream. It allows Java objects and primitives to be encoded into a byte stream suitable for streaming to some type of network or to a file-system, or more generally, to a transmission medium or storage facility. A seralizable object must implement the Serilizable interface. We use ObjectOutputStream to write this object to a stream and ObjectInputStream to read it from the stream

Q9.Why there are some null interface in java ? What does it mean ? Give me some null interfaces in JAVA?

Null interfaces act as markers..they just tell the compiler that the objects of this class need to be treated differently..some marker interfaces are : Serializable, Remote, Cloneable

Q10. Is synchronised a modifier?indentifier??what is it?? 

It's a modifier. Synchronized methods are methods that are used to control access to an object. A thread only executes a synchronized method after it has acquired the lock for the method's object or class. Synchronized statements are similar to synchronized methods. A synchronized statement can only be executed after a thread has acquired the lock for the object or class referenced in the synchronized statement.

Q11.What is singleton class?where is it used? 

Singleton is a design pattern meant to provide one and only one instance of an object. Other objects can get a reference to this instance through a static method (class constructor is kept private). 

Why do we need one? 

Sometimes it is necessary, and often sufficient, to create a single instance of a given class. This has advantages in memory management, and for Java, in garbage collection. Moreover, restricting the number of instances may be necessary or desirable for technological or business reasons--for example, we may only want a single instance of a pool of database connections.

Q12.What is a compilation unit?

 The smallest unit of source code that can be compiled, i.e. a .java file.

Q13.Is string a wrapper class? 

String is a class, but not a wrapper class. Wrapper classes like (Integer) exist for each primitive type. They can be used to convert a primitive data value into an object, and viceversa.

Q14.Why java does not have multiple inheritance? 

The Java design team strove to make Java: 

• Simple, object oriented, and familiar 

• Robust and secure 

• Architecture neutral and portable 

• High performance

 • Interpreted, threaded, and dynamic The reasons for omitting multiple inheritance from the Java language mostly stem from the "simple, object oriented, and familiar" goal. As a simple language, Java's creators wanted a

language that most developers could grasp without extensive training. To that end, they worked to make the language as similar to C++ as possible (familiar) without carrying over C++'s unnecessary complexity (simple). In the designers' opinion, multiple inheritance causes more problems and confusion than it solves. So they cut multiple inheritance from the language (just as they cut operator overloading). The designers' extensive C++ experience taught them that multiple inheritance just wasn't worth the headache.

Q15.Why java is not a 100% oops? 

Many people say this because Java uses primitive types such as int, char, double. But then all the rest are objects. Confusing question.

Q16.What is a resource bundle? 

In its simplest form, a resource bundle is represented by a text file containing keys and a text value for each key.

Q17.What is transient variable? 

Transient variable can't be serialize. For example if a variable is declared as transient in a Serializable class and the class is written to an ObjectStream, the value of the variable can't be written to the stream instead when the class is retrieved from the ObjectStream the value of the variable becomes null.

Q18.What is Collection API? 

The Collection API is a set of classes and interfaces that support operation on collections of objects. These classes and interfaces are more flexible, more powerful, and more regular than the vectors, arrays, and hashtables if effectively replaces. Example of classes: HashSet, HashMap, ArrayList, LinkedList, TreeSet and TreeMap. Example of interfaces: Collection, Set, List and Map.

Q19.Is Iterator a Class or Interface? What is its use? 

Iterator is an interface which is used to step through the elements of a Collection.

Q20.What is similarities/difference between an Abstract class and Interface?

 Differences are as follows:

 • Interfaces provide a form of multiple inheritance. A class can extend only one other class. 

• Interfaces are limited to public methods and constants with no implementation. Abstract classes can have a partial implementation, protected parts, static methods, etc.

 • A Class may implement several interfaces. But in case of abstract class, a class may extend only one abstract class.

 • Interfaces are slow as it requires extra indirection to to find corresponding method in in the actual class. Abstract classes are fast. Similarities:

 • Neither Abstract classes or Interface can be instantiated.

Q22.Which containers use a border Layout as their default layout?

 The window, Frame and Dialog classes use a border layout as their default layout.

Q23.Why do threads block on I/O?

 Threads block on i/o (that is enters the waiting state) so that other threads may execute while the i/o Operation is performed.

Q24.How are Observer and Observable used? 

Objects that subclass the Observable class maintain a list of observers. When an Observable object is updated it invokes the update() method of each of its observers to notify the observers that it has changed state. The Observer interface is implemented by objects that observe Observable objects.

Q25.What is synchronization and why is it important?

 With respect to multithreading, synchronization is the capability to control the access of multiple threads to shared resources. Without synchronization, it is possible for one thread to modify a shared object while another thread is in the process of using or updating that object's value. This often leads to significant errors.

Q26. Can a lock be acquired on a class? 

Yes, a lock can be acquired on a class. This lock is acquired on the class's Class object.

Q27. What's new with the stop(), suspend() and resume() methods in JDK 1.2? The stop(), suspend() and resume() methods have been deprecated in JDK 1.2.

Q28. Is null a keyword? The null value is not a keyword.

Q29. What is the preferred size of a component? The preferred size of a component is the minimum component size that will allow the component to display normally.

Q30. What method is used to specify a container's layout? The setLayout() method is used to specify a container's layout.

Q31. Which containers use a FlowLayout as their default layout? The Panel and Applet classes use the FlowLayout as their default layout.

Q32. What state does a thread enter when it terminates its processing? When a thread terminates its processing, it enters the dead state.

Q33. What is the Collections API? The Collections API is a set of classes and interfaces that support operations on collections of objects.

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Q34. Which characters may be used as the second character of an identifier, but not as the first character of an identifier? The digits 0 through 9 may not be used as the first character of an identifier but they may be used after the first character of an identifier.

Q35. What is the List interface? The List interface provides support for ordered collections of objects.

Q36. How does Java handle integer overflows and underflows? It uses those low order bytes of the result that can fit into the size of the type allowed by the operation.

Q37. What is the Vector class? The Vector class provides the capability to implement a growable array of objects

Q38. What modifiers may be used with an inner class that is a member of an outer class? A (non-local) inner class may be declared as public, protected, private, static, final, or abstract.

Q39. What is an Iterator interface? The Iterator interface is used to step through the elements of a Collection.

Q40. What is the difference between the >> and >>> operators? The >> operator carries the sign bit when shifting right. The >>> zero-fills bits that have been shifted out.

Q41. Which method of the Component class is used to set the position and size of a component? setBounds()

Q42. How many bits are used to represent Unicode, ASCII, UTF-16, and UTF-8 characters? Unicode requires 16 bits and ASCII require 7 bits. Although the ASCII character set uses only 7 bits, it is usually represented as 8 bits. UTF-8 represents characters using 8, 16, and 18 bit patterns. UTF-16 uses 16-bit and larger bit patterns.

Q43. What is the difference between yielding and sleeping? When a task invokes its yield() method, it returns to the ready state. When a task invokes its sleep() method, it returns to the waiting state.

Q44. Which java.util classes and interfaces support event handling? The EventObject class and the EventListener interface support event processing.

Q45. Is sizeof a keyword? The sizeof operator is not a keyword.

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Q46. What are wrapped classes? Wrapped classes are classes that allow primitive types to be accessed as objects.

Q47. Does garbage collection guarantee that a program will not run out of memory? Garbage collection does not guarantee that a program will not run out of memory. It is possible for programs to use up memory resources faster than they are garbage collected. It is also possible for programs to create objects that are not subject to garbage collection

Q48. What restrictions are placed on the location of a package statement within a source code file? A package statement must appear as the first line in a source code file (excluding blank lines and comments).

Q49. Can an object's finalize() method be invoked while it is reachable? An object's finalize() method cannot be invoked by the garbage collector while the object is still reachable. However, an object's finalize() method may be invoked by other objects.

Q50. What is the immediate superclass of the Applet class? Panel

Q51. What is the difference between preemptive scheduling and time slicing? Under preemptive scheduling, the highest priority task executes until it enters the waiting or dead states or a higher priority task comes into existence. Under time slicing, a task executes for a predefined slice of time and then reenters the pool of ready tasks. The scheduler then determines which task should execute next, based on priority and other factors.

Q52 Name three Component subclasses that support painting. The Canvas, Frame, Panel, and Applet classes support painting.

Q53. What value does readLine() return when it has reached the end of a file? The readLine() method returns null when it has reached the end of a file.

Q54. What is the immediate superclass of the Dialog class? Window

Q55. What is clipping? Clipping is the process of confining paint operations to a limited area or shape.

Q56. What is a native method? A native method is a method that is implemented in a language other than Java.

Q57. Can a for statement loop indefinitely? Yes, a for statement can loop indefinitely. For example, consider the following: for(;;) ;

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Q58. What are order of precedence and associativity, and how are they used? Order of precedence determines the order in which operators are evaluated in expressions. Associatity determines whether an expression is evaluated left-to-right or right-to-left

Q59. When a thread blocks on I/O, what state does it enter? A thread enters the waiting state when it blocks on I/O.

Q60. To what value is a variable of the String type automatically initialized? The default value of an String type is null.

Q61. What is the catch or declare rule for method declarations? If a checked exception may be thrown within the body of a method, the method must either catch the exception or declare it in its throws clause.

Q62. What is the difference between a MenuItem and a CheckboxMenuItem? The CheckboxMenuItem class extends the MenuItem class to support a menu item that may be checked or unchecked.

Q63. What is a task's priority and how is it used in scheduling? A task's priority is an integer value that identifies the relative order in which it should be executed with respect to other tasks. The scheduler attempts to schedule higher priority tasks before lower priority tasks.

Q64. What class is the top of the AWT event hierarchy? The java.awt.AWTEvent class is the highest-level class in the AWT event-class hierarchy.

Q65. When a thread is created and started, what is its initial state? A thread is in the ready state after it has been created and started.

Q66. Can an anonymous class be declared as implementing an interface and extending a class? An anonymous class may implement an interface or extend a superclass, but may not be declared to do both.

Q67. What is the range of the short type? The range of the short type is -(2^15) to 2^15 - 1.

Q68. What is the range of the char type? The range of the char type is 0 to 2^16 - 1.

Q69. In which package are most of the AWT events that support the event-delegation model defined? Most of the AWT-related events of the event-delegation model are defined in the java.awt.event package. The AWTEvent class is defined in the java.awt package.

Q70. What is the immediate superclass of Menu? MenuItem

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Q71. What is the purpose of finalization? The purpose of finalization is to give an unreachable object the opportunity to perform any cleanup processing before the object is garbage collected.

Q72. Which class is the immediate superclass of the MenuComponent class. Object

Q73. What invokes a thread's run() method? After a thread is started, via its start() method or that of the Thread class, the JVM invokes the thread's run() method when the thread is initially executed.

Q74. What is the difference between the Boolean & operator and the && operator? If an expression involving the Boolean & operator is evaluated, both operands are evaluated. Then the & operator is applied to the operand. When an expression involving the && operator is evaluated, the first operand is evaluated. If the first operand returns a value of true then the second operand is evaluated. The && operator is then applied to the first and second operands. If the first operand evaluates to false, the evaluation of the second operand is skipped.

Q75. Name three subclasses of the Component class. Box.Filler, Button, Canvas, Checkbox, Choice, Container, Label, List, Scrollbar, or TextComponent

Q76. What is the GregorianCalendar class? The GregorianCalendar provides support for traditional Western calendars.

Q77. Which Container method is used to cause a container to be laid out and redisplayed? validate()

Q78. What is the purpose of the Runtime class? The purpose of the Runtime class is to provide access to the Java runtime system.

Q79. How many times may an object's finalize() method be invoked by the garbage collector? An object's finalize() method may only be invoked once by the garbage collector.

Q80. What is the purpose of the finally clause of a try-catchfinally statement? The finally clause is used to provide the capability to execute code no matter whether or not an exception is thrown or caught.

Q81. What is the argument type of a program's main() method? A program's main() method takes an argument of the String[] type.

Q82. Which Java operator is right associative? The = operator is right associative.

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Q83. What is the Locale class? The Locale class is used to tailor program output to the conventions of a particular geographic, political, or cultural region.

Q84. Can a double value be cast to a byte? Yes, a double value can be cast to a byte.

Q85. What is the difference between a break statement and a continue statement? A break statement results in the termination of the statement to which it applies (switch, for, do, or while). A continue statement is used to end the current loop iteration and return control to the loop statement.

Q86. What must a class do to implement an interface? It must provide all of the methods in the interface and identify the interface in its implements clause.

Q87. What method is invoked to cause an object to begin executing as a separate thread? The start() method of the Thread class is invoked to cause an object to begin executing as a separate thread.

Q88. Name two subclasses of the TextComponent class. TextField and TextArea

Q89. What is the advantage of the event-delegation model over the earlier eventinheritance model? The event-delegation model has two advantages over the event-inheritance model. First, it enables event handling to be handled by objects other than the ones that generate the events (or their containers). This allows a clean separation between a component's design and its use. The other advantage of the event-delegation model is that it performs much better in applications where many events are generated. This performance improvement is due to the fact that the event-delegation model does not have to repeatedly process unhandled events, as is the case of the event-inheritance model.

Q90. Which containers may have a MenuBar? Frame

Q91. How are commas used in the intialization and iteration parts of a for statement? Commas are used to separate multiple statements within the initialization and iteration parts of a for statement.

Q92. What is the purpose of the wait(), notify(), and notifyAll() methods? The wait(),notify(), and notifyAll() methods are used to provide an efficient way for threads to wait for a shared resource. When a thread executes an object's wait() method, it enters the waiting state. It only enters the ready state after another thread invokes the object's notify() or notifyAll() methods..

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Q93. What is an abstract method? An abstract method is a method whose implementation is deferred to a subclass.

Q94. How are Java source code files named? A Java source code file takes the name of a public class or interface that is defined within the file. A source code file may contain at most one public class or interface. If a public class or interface is defined within a source code file, then the source code file must take the name of the public class or interface. If no public class or interface is defined within a source code file, then the file must take on a name that is different than its classes and interfaces. Source code files use the .java extension.

Q95. What is the relationship between the Canvas class and the Graphics class? A Canvas object provides access to a Graphics object via its paint() method.

Q96. What are the high-level thread states? The high-level thread states are ready, running, waiting, and dead.

Q97. What value does read() return when it has reached the end of a file? The read() method returns -1 when it has reached the end of a file.

Q98. Can a Byte object be cast to a double value? No, an object cannot be cast to a primitive value.

Q99. What is the difference between a static and a nonstatic inner class? A non-static inner class may have object instances that are associated with instances of the class's outer class. A static inner class does not have any object instances.

Q100. What is the difference between the String and StringBuffer classes? String objects are constants. StringBuffer objects are not.

Q101. If a variable is declared as private, where may the variable be accessed? A private variable may only be accessed within the class in which it is declared.

Q102. What is an object's lock and which object's have locks? An object's lock is a mechanism that is used by multiple threads to obtain synchronized access to the object. A thread may execute a synchronized method of an object only after it has acquired the object's lock. All objects and classes have locks. A class's lock is acquired on the class's Class object.

Q103. What is the Dictionary class? The Dictionary class provides the capability to store key-value pairs.

Q104. How are the elements of a BorderLayout organized? The elements of a BorderLayout are organized at the borders (North, South, East, and West) and the center of a container.

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Q105. What is the % operator? It is referred to as the modulo or remainder operator. It returns the remainder of dividing the first operand by the second operand.

Q106. When can an object reference be cast to an interface reference? An object reference be cast to an interface reference when the object implements the referenced interface.

Q107. What is the difference between a Window and a Frame? The Frame class extends Window to define a main application window that can have a menu bar.

Q108. Which class is extended by all other classes? The Object class is extended by all other classes.

Q109. Can an object be garbage collected while it is still reachable? A reachable object cannot be garbage collected. Only unreachable objects may be garbage collected..

Q110. Is the ternary operator written x : y ? z or x ? y : z ? It is written x ? y : z.

Q111. What is the difference between the Font and FontMetrics classes? The FontMetrics class is used to define implementation-specific properties, such as ascent and descent, of a Font object.

Q112. How is rounding performed under integer division? The fractional part of the result is truncated. This is known as rounding toward zero.

Q113. What happens when a thread cannot acquire a lock on an object? If a thread attempts to execute a synchronized method or synchronized statement and is unable to acquire an object's lock, it enters the waiting state until the lock becomes available.

Q114. What is the difference between the Reader/Writer class hierarchy and the InputStream/OutputStream classhierarchy?

The Reader/Writer class hierarchy is character-oriented, and the InputStream/ OutputStream class hierarchy is byte-oriented.

Q115. What classes of exceptions may be caught by a catch clause? A catch clause can catch any exception that may be assigned to the Throwable type. This includes the Error and Exception types.

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Q116. If a class is declared without any access modifiers, where may the class be accessed? A class that is declared without any access modifiers is said to have package access. This means that the class can only be accessed by other classes and interfaces that are defined within the same package.

Q117. What is the SimpleTimeZone class? The SimpleTimeZone class provides support for a Gregorian calendar.

Q118. What is the Map interface? The Map interface replaces the JDK 1.1 Dictionary class and is used associate keys with values.

Q119. Does a class inherit the constructors of its superclass? A class does not inherit constructors from any of its superclasses.

Q120. For which statements does it make sense to use a label? The only statements for which it makes sense to use a label are those statements that can enclose a break or continue statement.

Q121. What is the purpose of the System class? The purpose of the System class is to provide access to system resources.

Q122. Which TextComponent method is used to set a TextComponent to the read-only state? setEditable()

Q123. How are the elements of a CardLayout organized? The elements of a CardLayout are stacked, one on top of the other, like a deck of cards.

Q124. Is &&= a valid Java operator? No, it is not.

Q125. Name the eight primitive Java types? The eight primitive types are byte, char, short, int, long, float, double, and boolean.

Q126. Which class should you use to obtain design information about an object? The Class class is used to obtain information about an object's design.

Q127. What is the relationship between clipping and repainting? When a window is repainted by the AWT painting thread, it sets the clipping regions to the area of the window that requires repainting.

Q128. Is "abc" a primitive value? The String literal "abc" is not a primitive value. It is a String object.

Q129. What is the relationship between an event-listener interface and an event-adapter class?

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An event-listener interface defines the methods that must be implemented by an event handler for a particular kind of event. An event adapter provides a default implementation of an event-listener interface.

Q130. What restrictions are placed on the values of each case of a switch statement? During compilation, the values of each case of a switch statement must evaluate to a value that can be promoted to an int value.

Q131. What modifiers may be used with an interface declaration? An interface may be declared as public or abstract.

Q132. Is a class a subclass of itself? A class is a subclass of itself.

Q133. What is the highest-level event class of the eventdelegation model? The java.util.EventObject class is the highest-level class in the event-delegation class hierarchy.

Q134. What event results from the clicking of a button? The ActionEvent event is generated as the result of the clicking of a button.

Q135. How can a GUI component handle its own events? A component can handle its own events by implementing the required event-listener interface and adding itself as its own event listener.

Q136. What is the difference between a while statement and a do statement? A while statement checks at the beginning of a loop to see whether the next loop iteration should occur. A do statement checks at the end of a loop to see whether the next iteration of a loop should occur. The do statement will always execute the body of a loop at least once.

Q137. How are the elements of a GridBagLayout organized? The elements of a GridBagLayout are organized according to a grid. However, the elements are of different sizes and may occupy more than one row or column of the grid. In addition, the rows and columns may have different sizes.

Q138. What advantage do Java's layout managers provide over traditional windowing systems? Java uses layout managers to lay out components in a consistent manner across all windowing platforms. Since Java's layout managers aren't tied to absolute sizing and positioning, they are able to accomodate platform-specific differences among windowing systems.

Q139. What is the Collection interface? The Collection interface provides support for the implementation of a mathematical bag – an unordered collection of objects that may contain duplicates.

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Q140. What modifiers can be used with a local inner class? A local inner class may be final or abstract.

Q141. What is the difference between static and non-staticvariables? A static variable is associated with the class as a whole rather than with specific instances of a class. Non-static variables take on unique values with each object instance.

Q142. What is the difference between the paint() and repaint() methods? The paint() method supports painting via a Graphics object. The repaint() method is used to cause paint() to be invoked by the AWT painting thread.

Q143. What is the purpose of the File class? The File class is used to create objects that provide access to the files and directories of a local file system.

Q144. Can an exception be rethrown? Yes, an exception can be rethrown.

Q145. Which Math method is used to calculate the absolute value of a number? The abs() method is used to calculate absolute values.

Q146. How does multithreading take place on a computer with a single CPU?

The operating system's task scheduler allocates execution time to multiple tasks. By quickly switching between executing tasks, it creates the impression that tasks execute sequentially.

Q147. When does the compiler supply a default constructor for a class? The compiler supplies a default constructor for a class if no other constructors are provided.

Q148. When is the finally clause of a try-catch-finally statement executed? The finally clause of the try-catch-finally statement is always executed unless the thread of execution terminates or an exception occurs within the execution of the finally clause.

Q149. Which class is the immediate superclass of the Container class? Component

Q150. If a method is declared as protected, where may the method be accessed? A protected method may only be accessed by classes or interfaces of the same package or by subclasses of the class in which it is declared.

Q151. How can the Checkbox class be used to create a radio button? By associating Checkbox objects with a CheckboxGroup.

Q152. Which non-Unicode letter characters may be used as the first character of an identifier? The non-Unicode letter characters $ and _ may appear as the first character of an identifier

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Q153. What restrictions are placed on method overloading? Two methods may not have the same name and argument list but different return types.

Q154. What happens when you invoke a thread's interrupt method while it is sleeping or waiting? When a task's interrupt() method is executed, the task enters the ready state. The next time the task enters the running state, an InterruptedException is thrown.

Q155. What is casting? There are two types of casting, casting between primitive numeric types and casting between object references. Casting between numeric types is used to convert larger values, such as double values, to smaller values, such as byte values. Casting between object references is used to refer to an object by a compatible class, interface, or array type reference.

Q156. What is the return type of a program's main() method? A program's main() method has a void return type.

Q157. Name four Container classes. Window, Frame, Dialog, FileDialog, Panel, Applet, or ScrollPane

Q158. What is the difference between a Choice and a List? A Choice is displayed in a compact form that requires you to pull it down to see the list of available choices. Only one item may be selected from a Choice. A List may be displayed in such a way that several List items are visible. A List supports the selection of one or more List items.

Q159. What class of exceptions are generated by the Java run-time system? The Java runtime system generates RuntimeException and Error exceptions.

Q160. What class allows you to read objects directly from a stream? The ObjectInputStream class supports the reading of objects from input streams.

Q161. What is the difference between a field variable and a local variable? A field variable is a variable that is declared as a member of a class. A local variable is a variable that is declared local to a method.

Q162. Under what conditions is an object's finalize() method invoked by the garbage collector? The garbage collector invokes an object's finalize() method when it detects that the object has become unreachable.

Q163. How are this() and super() used with constructors? this() is used to invoke a constructor of the same class. super() is used to invoke a superclass constructor.

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Q164. What is the relationship between a method's throws clause and the exceptions that can be thrown during themethod's execution? A method's throws clause must declare any checked exceptions that are not caught within the body of the method.

Q165. What is the difference between the JDK 1.02 event model and the event-delegation model introduced withJDK 1.1? The JDK 1.02 event model uses an event inheritance or bubbling approach. In this model, components are required to handle their own events. If they do not handle a particular event, the event is inherited by (or bubbled up to) the component's container. The container then either handles the event or it is bubbled up to its container and so on, until the highest-level container has been tried..In the event-delegation model, specific objects are designated as event handlers for GUI components. These objects implement event-listener interfaces. The event-delegation model is more efficient than the event-inheritance model because it eliminates the processing required to support the bubbling of unhandled events.

Q166. How is it possible for two String objects with identical values not to be equal under the == operator? The == operator compares two objects to determine if they are the same object in memory. It is possible for two String objects to have the same value, but located indifferent areas of memory.

Q167. Why are the methods of the Math class static? So they can be invoked as if they are a mathematical code library.

Q168. What Checkbox method allows you to tell if a Checkbox is checked? getState()

Q169. What state is a thread in when it is executing? An executing thread is in the running state.

Q170. What are the legal operands of the instanceof operator? The left operand is an object reference or null value and the right operand is a class, interface, or array type.

Q171. How are the elements of a GridLayout organized? The elements of a GridBad layout are of equal size and are laid out using the squares of a grid.

Q172. What an I/O filter? An I/O filter is an object that reads from one stream and writes to another, usually altering the data in some way as it is passed from one stream to another.

Q173. If an object is garbage collected, can it become reachable again? Once an object is garbage collected, it ceases to exist. It can no longer become reachable again.

Q174. What is the Set interface?

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The Set interface provides methods for accessing the elements of a finite mathematical set. Sets do not allow duplicate elements.

Q175. What classes of exceptions may be thrown by a throw statement? A throw statement may throw any expression that may be assigned to the Throwable type.

Q176. What are E and PI? E is the base of the natural logarithm and PI is mathematical value pi.

Q177. Are true and false keywords? The values true and false are not keywords.

Q178. What is a void return type? A void return type indicates that a method does not return a value.

Q179. What is the purpose of the enableEvents() method? The enableEvents() method is used to enable an event for a particular object. Normally, an event is enabled when a listener is added to an object for a particular event. The enableEvents() method is used by objects that handle events by overriding their eventdispatch methods.

Q180. What is the difference between the File and RandomAccessFile classes? The File class encapsulates the files and directories of the local file system. The RandomAccessFile class provides the methods needed to directly access data contained in any part of a file.

Q181. What happens when you add a double value to a String? The result is a String object.

Q182. What is your platform's default character encoding? If you are running Java on English Windows platforms, it is probably Cp1252. If you are running Java on English Solaris platforms, it is most likely 8859_1..

Q183. Which package is always imported by default? The java.lang package is always imported by default.

Q184. What interface must an object implement before it can be written to a stream as an object? An object must implement the Serializable or Externalizable interface before it can be written to a stream as an object.

Q185. How are this and super used? this is used to refer to the current object instance. super is used to refer to the variables and methods of the superclass of the current object instance.

Q186. What is the purpose of garbage collection? The purpose of garbage collection is to identify and discard objects that are no longer needed by a program so that their resources may be reclaimed and reused.

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Q187. What is a compilation unit? A compilation unit is a Java source code file.

Q188. What interface is extended by AWT event listeners? All AWT event listeners extend the java.util.EventListener interface.

Q189. What restrictions are placed on method overriding? • Overridden methods must have the same name, argument list, and return type. • The overriding method may not limit the access of the method it overrides. • The overriding method may not throw any exceptions that may not be thrownby the overridden method.

Q190. How can a dead thread be restarted? A dead thread cannot be restarted.

Q191. What happens if an exception is not caught? An uncaught exception results in the uncaughtException() method of the thread's ThreadGroup being invoked, which eventually results in the termination of the program in which it is thrown.

Q192. What is a layout manager? A layout manager is an object that is used to organize components in a container.

Q193. Which arithmetic operations can result in the throwing of an ArithmeticException? Integer / and % can result in the throwing of an ArithmeticException.

Q194. What are three ways in which a thread can enter the waiting state? A thread can enter the waiting state by invoking its sleep() method, by blocking on I/O, by unsuccessfully attempting to acquire an object's lock, or by invoking an object's wait() method. It can also enter the waiting state by invoking its (deprecated) suspend() method.

Q195. Can an abstract class be final? An abstract class may not be declared as final.

Q196. What is the ResourceBundle class?

The ResourceBundle class is used to store locale-specific resources that can be loaded by a program to tailor the program's appearance to the particular locale in which it is being run.

Q197. What happens if a try-catch-finally statement doesnot have a catch clause to handle an exception that is thrown within the body of the try statement? The exception propagates up to the next higher level try-catch statement (if any) or results in the program's termination.

Q198. What is numeric promotion? Numeric promotion is the conversion of a smaller numeric type to a larger numeric type, so

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that integer and floating-point operations may take place. In numerical promotion, byte, char, and short values are converted to int values. The int values are also converted to long values, if necessary. The long and float values are converted to double values, as required.

Q199. What is the difference between a Scrollbar and a ScrollPane? A Scrollbar is a Component, but not a Container. A ScrollPane is a Container. A ScrollPane handles its own events and performs its own scrolling.

Q200. What is the difference between a public and a nonpublic class? A public class may be accessed outside of its package. A non-public class may not be accessed outside of its package.

Q201. To what value is a variable of the boolean type automatically initialized? The default value of the boolean type is false.

Q202. Can try statements be nested? Try statements may be tested.

Q203. What is the difference between the prefix and postfix forms of the ++ operator?

The prefix form performs the increment operation and returns the value of the increment operation. The postfix form returns the current value all of the expression and then performs the increment operation on that value.

Q204. What is the purpose of a statement block? A statement block is used to organize a sequence of statements as a single statement group.

Q205. What is a Java package and how is it used? A Java package is a naming context for classes and interfaces. A package is used to create a separate name space for groups of classes and interfaces. Packages are also used to organize related classes and interfaces into a single API unit and to control accessibility to these classes and interfaces.

Q206. What modifiers may be used with a top-level class? A top-level class may be public, abstract, or final.

Q207. What are the Object and Class classes used for? The Object class is the highest-level class in the Java class hierarchy. The Class class is used to represent the classes and interfaces that are loaded by a Java program..

Q208. How does a try statement determine which catch clause should be used to handle an exception? When an exception is thrown within the body of a try statement, the catch clauses of the try statement are examined in the order in which they appear. The first

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catch clause that is capable of handling the exception is executed. The remaining catch clauses are ignored.

Q209. Can an unreachable object become reachable again? An unreachable object may become reachable again. This can happen when the object's finalize() method is invoked and the object performs an operation which causes it to become accessible to reachable objects.

Q210. When is an object subject to garbage collection? An object is subject to garbage collection when it becomes unreachable to the program in which it is used.

Q211. What method must be implemented by all threads? All tasks must implement the run() method, whether they are a subclass of Thread or implement the Runnable interface.

Q212. What methods are used to get and set the text label displayed by a Button object? getLabel() and setLabel()

Q213. Which Component subclass is used for drawing and painting? Canvas

Q214. What are synchronized methods and synchronized statements? Synchronized methods are methods that are used to control access to an object. A thread only executes a synchronized method after it has acquired the lock for the method's object or class. Synchronized statements are similar to synchronized methods. A synchronized statement can only be executed after a thread has acquired the lock for the object or class referenced in the synchronized statement.

Q215. What are the two basic ways in which classes that can be run as threads may be defined? A thread class may be declared as a subclass of Thread, or it may implement the Runnable interface.

Q216. What are the problems faced by Java programmers who don't use layout managers? Without layout managers, Java programmers are faced with determining how their GUI will be displayed across multiple windowing systems and finding a common sizing and positioning that will work within the constraints imposed by each windowing system.

Q217. What is the difference between an if statement and a switch statement? The if statement is used to select among two alternatives. It uses a boolean expression to decide which alternative should be executed. The switch statement is used to select among multiple alternatives. It uses an int expression to determine which alternative should be executed.

Q218. What happens when you add a double value to a String? The result is a String object.

Q219. What is the List interface? The List interface provides support for ordered collections of objects.

XXX

Q1. What are limitations of object Arrays?

The main limitations of Object arrays are

 These are fixed in size ie once we created an array object there is no chance of increasing or decreasing size based on our requirement. Hence If we don’t know size in advance , arrays are not recommended to use

 Arrays can hold only homogeneous elements.

 There is no underlying data structure for arrays and hence no readymade method support for arrays. Hence for every requirement programmer has to code explicitly

To over come these problems collections are recommended to use

Q2. What are differences between arrays and collections?

Arrays

Collections

1. Arrays r fixed in size and hence once we created an array we are not allowed to increase or decrease the size based on our requirement.

1. Collections are growable in nature and hence based on our requirement we can increase or decrease the size.

2. Memory point of view arrays are not recommended to use

2. Memory point of view collections are recommended to use.

3. Performance point of view arrays are recommended to use

3. Performance point of view collections are not recommended to use.

4. Arrays can hold only homogeneous elements

4. Collections can hold both homogeneous and heterogeneous elements.

5. Arrays can hold both primitives as well as objects

5. Collections can hold only objects.

6. For any requirement, there is no ready method support compulsory programmer has to code explicitly.

6. For every requirement ready made method support is available. Being a programmer we have to know how to use those methods and we are not responsible to implement those.

Q3. what are differences between arrays and ArrayList? Refer the answer of Q2

Q4. What are differences between arrays and Vector? Refer the answer of Q2

Q5. What is Collection API ?

It defines set of classes and interfaces which can be used for representing a group of objects as single entity

Q6. What is Collection framework?

It defines set of classes and inter faces which can be used for representing a group of objects as single entity

Q7. What is difference between Collections and Collection?

Collection is an interface which can be used for representing a group of individual objects as single entity and it acts as root interface of collection frame work.

Collections is an utility class to define several utility methods for Collection implemented class objects.

Q8. Explain about Collection interface?

 This interface can be used to represent a group of objects as a single entity.

 It acts as root interface for entire collection framework.

 It defines the most commonly used methods which can be applicable for any collection implemented class object

Q9. Explain about List interface?

List interface is a child interface of Collection interface. This can be used to represent group of individual objects in as a single entity where

 Duplicates are allowed

 Insertion order is preserved

Q10. Explain about Set interface?

Set is a child interface of Collection interface. it can be used to represent a group of individual objects as a single entity where

 Duplicate objects are not allowed.

 Insertion order is not preserved

Q11. Explain about SortedSet interface?

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it is child interface of Set interface. it can be used to represent a group of individual objects in to a single entity where

 All the objects are arranged in some sorting order (Can be natural sorting order or customizede).

 Duplicates are not allowed.

Q12. Explain about NavigableSet ?

It is child interface of SortedSet and provides several utility methods for navigation purposes

 It doesn’t allows duplicates

 Insertion order is preserved

 It is introduced in 1.6 version

Q13. Explain about Queue interface?

If we want to represent a group of individual objects prior to processing, then we should go for Queue interface. It is child interface of Collection interface.

It has introduced in 1.5 version.

Q14. Explain about Map interface?

Remember it is not a child Interface of Collection Interface and hence Map and Collection Interfaces doesn’t have any relationship.

 It can be used for representing a group of Objects as key, value pairs.

 Both keys and values should be objects

 Keys can t be duplicated but values can be duplicated.

 it has introduced in 1.2 version

Q15. Explain about SortedMap ?

 If we want to represent a group of objects as key value pairs where all the entries are arranged according some sorting order of keys then we should go for SortedMap.

 It is child interface of Map.

 It has introduced in 1.2 version

Q16. Explain about NavigableMap?

 It is child interface of SortedMap and defines several method for navigation purpose

 It is introduced in 1.6 version

Q17. Explain about ArrayList class?

ArrayList is a Collection which can be used to represent a group of objects as a single entity.

 it is a implemented class for List interface

 Introduced in 1.2 version

 The underlying data structure is resizable or growable array.

 Insertion order is preserved

 Duplicates are allowed

 Heterogeneous objects are allowed

 null insertion is possible

 This class implements RandomAccess , Serializable , Cloneable interfaces

 Best choice for retrieval purpose and worst if our frequent operation is insertion or deletion in the middle

Q18. What is RandomAccess Interface?

 If a collection class implements RandomAccess interface then we can access any of its element with the same speed.

 RandomAccess interface is marker interface and it dosent contains any methods.

 ArrayList and vector classes implements this interface.

Q19. Explain about LinkedList class?

LinkedList is a Collection implemented class which can be used for representing a group of objects as a single entity.

 LinkedList is the implemetation class for List interface

 Introduced in 1.2 version

 Underlying data Structure is DoubleLinkedList

 Allows duplicates

 Insertion order is preserved

 Allows heterogeneous objects

 null insertion is possible

 LinkedList class implements Seriallizable and Cloneable interface but not RandomAccess interface

 Best choice if frequent operation is insertion or deletion an objects in middle but worst choice if frequent operation is retrieval.

Q20. Explain about Vector class?

Vector is a legacy collection class which can be used to represent a group of objects.

 Introduced in 1.0 version. it is legacy class

 The underlying data structure is resizable or growable array.

 Insertion order is preserved

 Duplicates are allowed

 Heterogeneous objects are allowed

 It is a implemented class for List interface

 null insertion is possible

 Vector class implements RandomAccess ,Serializable,Cloneable interfaces

 Best Choice if frequent operation is retrieval and worst choice if frequent operation is insertion or deletion in the middle.

 All methods present in Vector class are synchronized hence Vector class object is thread safe.

Q21. What is difference between ArrayList and Vector?

ArrayList

Vector

1. No method is synchronized in the ArrayList class

1. All methods in Vector are synchronized.

2. ArrayList object is not thread safe.

2. Vector is thread safe.

3. Relatively performance is high

3. Relatively performance is low

4. Introduced in 1.2 version and it is non legacy

4. Introduced in 1.0 version and it is legacy

Q22. How we can get synchronized version of ArrayList?

Collections class contains synchronizedList() method for this

Public static List synchronizedList(List l) EX ArrayList l= new ArrayList(); List l2=Collections.synchronizedList(l);

Similarly we can get synchronized versions of Set and Map objects by the following methods.

Public static List synchronizedSet(Set s) Public static List synchronizedMap(Map m)

Q23. What is difference between size and capacity of a Collection Object?

size means number of objects present where as capacity means no of objects it can accommodate.

Q24. What is difference between ArrayList and Linked List?

ArrayList

LinkedList

1. The underlying data structure is resizable or growable array.

1. The underlying data structure is Double Linked List.

2. This is Best choice if frequent operation is retrieval and worst choice if frequent operation is insertion or deletion in the

2. This is Best choice if frequent operation is insertion or deletion in the middle and worst choice if frequent operation is retrieval .

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middle.

3. This class implements Serializable , Cloneable and RandomAccess interfaces.

3. This class implements Serializable , Cloneable but not RandomAccess interface.

Q25. What are legacy classes and interfaces present in Collections framework ?

 Enumeration ---Interface

 Dictonary ------Abstract class

 Hashtable -----Concrete class

 Properties -----Concrete class

 Vector -----Concrete class

 Stack -----Concrete class

Q26. what is difference Enumeration and Iterator?

Enumeration

Iterator

1. It is legacy interface and introduced in 1.0 version

1 It is non-legacy and introduced in 1.2 version

2Applicable only for legacy classes and it is not universal cursor

2Applicable for any Collection implemented class object.

3While iterating the elements we are not allowed to remove the objects just we can perform only read operation

3While iterating we can perform removal also in addition to read operation.

4By using elements() method we can get Enumeration object

4. By using iterator() method we can get Iterator object

Q27. What are limitations of Enumeration?

 While iterating the elements we are not allowed to perform removal operation

 It is applicable only for legacy classes and it is not a universal cursor.

 It can retrieve the elements only in forward direction

Q28. What is difference between enum and Enumeration?

An enum can be used to define a group of named constants .It has introduced in 1.5 version

Ex Class Beer{ KO,KF,RC,FO }

Enumeration is cursor to retrieve Objects one by one from Collection objects.

Q29. What is difference between Iterator and ListIterator?

o ListIterator is the child interface of the Iterator

o Iterator is the single direction cursor where as ListIterator is bidirectional cursor.

o While iterating the elements by Iterator we can perform only read and remove operations. But by using ListIterator we can perform read,removal, replace and addition of new objects also.

o Iterator is applicable for every Collecton implemented class object but ListIterator is applicable only for List implemented class objects.

o Iterator can be get by using iterator() of Collection interface where as ListIterator can be get by using listIterator() method of List interface

o both are introduced in 1.2 version

Q30. What is relation between ListIterator and Iterator? ListIterator is child interface of Iterator

Q31. Explain about HashSet class?

 The underlying data structure is Hashtable

 null values are accepted

 duplicates are not allowed

 insertion order is based on hashcode of the object hence insertion order is not preserved

 best suitable if frequent operation is search operations

 HashSet class implements Serializable and Cloneable

 it is implementation class for Set interface

 heterogeneous objects are allowed

 it is introduced in 1.2 version

Q32. If we are trying to insert duplicate values in Set what will happen?

If we are trying to insert duplicate objects to the HashSet , we wont get any compile time or run time errors just the add(Object o) returns false and it doesn’t add that object.

Q33. What is LinkedHashSet? It is the child class of HashSet. The main difference between HashSet and LinkedHashSet is:

In the case of HashSet insertion order is not preserved , but in the case of LinkedHashSet insertion will be preserved.

Q34. Differences between HashSet and LinkedHashSet?

HashSet

LinkedHashSet

1The Underlying datastructure is Hashtable

1The underlying datastructure is combination of LinkedList and Hashtable

2Insertion Order is not preserved

2 Insertion order is preserved.

3Introduced in 1.2 version

3 Introduced in 1.4 version

Q35. What are major enhancements in 1.4 version of collection frame work?

LinkedHashSet LinkedHashMap IdentityHashMap

Q36. Explain about TreeSet? It is Collection object which can be used to represent a group of objects according to some sorting order.

 The underlying datastructure is Balanced tree

 Duplicates are not allowed

 All objects are stored according to some sorting order hence insertion order is not preserved

 Heterogeneous objects are not allowed violation leads to ClassCastException

 For an Empty TreeSet as firs element null value can be inserted but after inserting that first value if we are trying to insert any other objects then we will get NullPointerException

 For an non empty TreeSet if we are trying to inser null value at run time u will get NullPointerException

Q37. What are differences between List and Set interfaces?

List

Set

1Insertion Order is preserved

1Insertion Order is not preserved

2Duplicate Objects are allowed

2 Duplicate Objects are not allowed

3The implemented classes are ArrayList,LinkedList , Vector and Stack classes

3 The implemented classes are HashSet, LinkedHashSet and Tree

Q38. What is Comparable interface?

 This interface can be used for defining natural sorting order of the objects.

 It is present in java.lang package

 It contains a method public int compareTo(Object obj1)

Q39. What is Comparator interface?

 This interface can be used for implementing customized sorting order.

 It is present in java.util package

 It contains two methods

o public int compare(Object ,Object)

o public boolean equals(Object)

Q40. What are differences between Comparable and Comparator?

Comparable

Comparator

1This can be used for natural sorting order

1This can be used for implementing customized sorting

2This interface present in java.lang package

2 This is present in java.util package

3Contains only one method:

public int compareTo(Object obj1)

3 It contains two methods. public int compare(Object ,Object) public Boolean equals(Object)

4 It is marker interface

4 It is not a marker interface.

Q41. What is difference between HashSet and TreeSet?

HashSet

TreeSet

1The underlying data structure is Hashtable

1The underlying data structure is balanced tree

2Heterogeneous objects are allowed

2 Heterogeneous objects are not allowed bydefalut

3Insertion order is not preserved and it is based on hashcode of the objects

3 Insertion order is not preserved and all the objects are inserted according to some sorting order.

4null insertion is possible

4 As the first element only null insertion is possible and in all other cases we will get NullPointerException

Q42. What is Entry interface?

It is inner interface of Map. In the Map each key value pair is considered as Entry object. interface Map{ //more code here interface Entry{ Object getKey() Object getValue() Object setValue(Object new) } }

Q43. Explain about HashMap? It is a Map Object which can be used used to represent a group of objects as key-value pairs.

 The underlying data structure is Hashtable

 Duplicaes keys are not allowed duplicate values are allowed

 Insertion order is not preserved because insertion is based on hashcode of keys.

 Heterogeneous objects are allowed for both keys and values

 null key is allowed only once

 null values are allowed multiple times

 Introduced in 1.2 version

Q44. Explain about LinkedHashMap?

It is child class of HashMap. It is exactly same as HashMap except the following difference.

In the case of HashMap the insertion order is not preserved but in the case of LinkedHashMap insertion order is preserved. Introduced in 1.4 version

Q45. Differences between HashMap and LinkedHashMap ?

HashMap

LinkedHashMap

1.The underlying data structure is Hashtable

1.The underlying data structure is a combination of Hashtable and linkedlist

2.Insertion order is not preserved and it is based on hashcode of keys

2 Insertion order is preserved

3.Introduced in 1.2 version

3 Introduced in 1.4 version.

Q46. Differences between HashMap and Hashtable?

HashMap

Hashtable

1.The underlying data structure is Hashtable

1.The underlying data structure of Hashtable

2.No method is synchronized and hence HashMap object is not thread safe

2 .All methods are synchronized and hence it is thread safe

3.Performance is high

3. Performance is low

4.null insertion is possible for both keys and values

4. null insertion is not possible for both key and value violation leads to NullPointerException

5.Introduced in 1.2 version and it is non legacy

5. Introduced in 1.0 version and it is legacy

Q47. What is IdentityHashMap?

It is exactly same as HashMap except the following difference.

In the HashMap JVM uses equals() method to identify duplicate keys but in the case of IdentityHashMap JVM uses == operator for this.

Q48. What is difference between HashMap and IdentityHashMap? Refer Q47 for the answer.

Q49. What is WeakHashMap? It is exactly same as HashMap except the following difference.

In case of HashMap an Object is not eligible for garbage collection if it is associated with HashMap even though it dosent have any external references. ie HashMap dominates garbage collector.

But in case of WeakHashMap , if an Object is not having any external references then it is always eligible for garabage collectoion even though it is associated with weakHashMap. ie garbage collector dominates WeakHashMap

Q50. What is difference between HashMap and WeakHashMap? Refer Q49 for the answer.

Q51. What is TreeMap?

TreeMap can be used to store a group of objects as key-value pairs where all the entries are arranged according to some sorting order of keys.

 The underlying data structure is RED-BLACK Tree

 Duplicates keys are not allowed but values can be duplicated.

 Insertion order is not preserved because insertion is based on some sorting order

 If we are depending on Natural sorting order then keys should be homogeneous(violation leads to ClassCastException) but values need not homogeneous

 In case of customized sorting order we can insert heterogeneous keys and values

 For empty TreeMap as first entry with null values are allowed but after inserting that entry if we are trying to insert any other entry we will get NullPointerException

 For non empty TreeMap if we are trying to insert null keys we will get NullPointerException

 There are no restrictions for null values.

Q52. What is Hashtable Hashtable is a legacy Map and can be used to store objects as key value pairs.

 The underlying data sturucture is Hashtabe

 Duplicates keys are not allowed but duplicate values are allowed

 null insertion is not possible for both keys and values

 all methods are synchronized

 insertion order is not preserved because it is based on hashcode of keys

 heterogeneous Objects are allowed for both keys and values

 introduced in 1.0 version it is legacy class

Q53. What is PriorityQueue?

It represents a data structure to hold group of individual objects prior to processing based on some priority .it can be natural sorting order and it can be customized sorting order described by Comparator. It is the implementation class of Queue interface.

 Insertion order is not preserved because here insertion is done based on some sorting order

 Duplicates are not allowed

 null insertion is not possible even as first element also

 If we are depending on natural sorting order Objects should be homogeneous violation leads to ClassCastException

 If we are depending on customized sorting order Objects can be heterogeneous also.

Q54. What is Arrays class?

 It is utility class for arrays.

 It defines several utility methods for arrays like sorting an array or searching an element in array

 present in java.util package

Q55. We are planning to do an indexed search in a list of objects. Which of the two Java collections should you use: ArrayList or LinkedList? ArrayList Q56. Why ArrayList is faster than Vector?

All methods present in the Vector are synchronized and hence any method can be executed by only one thread at a time. It slows down the execution.

But in ArrayList, no method is synchronized and hence multiple thread are allowed execute simultaneously which speed up the execution.

XXX

Q1.What is an Exception? Ans.An unwanted, unexpected event that disturbs normal flow of the program is called Exception.Example: FileNotFondException.

Q2.What is the purpose of Exception Handling? Ans.The main purpose of Exception Handling is for graceful termination of the program. Q3.What is the meaning of Exception Handling? Ans. Exception Handling doesn’t mean repairing an Exception, we have to define alternative way to continue rest of the code normally. Example: If our programming requirement is to read the data from the file locating at London but at Runtime if London file is not available then we have to use local file alternatively to continue rest of program normally. This is nothing but Exception Handling. Q4.Explain Default Exception Handling Mechanism in java? Ans.If an exception raised, the method in which it’s raised is responsible for the creation of Exceptions object by including the following information:

 Name of the Exception

 Description of the Exception

 Stack Trace

 After creating Exception object the method handover it to the JVM.

 JVM checks for Exception Handling code in that method.

 If the method doesn’t contain any Exception handling code then JVM terminates the method abnormally and removes the corresponding entry from the stack.

 JVM identify the caller method and checks for Exception Handling code in that method. If the caller doesn’t contain any exception handling code then JVM terminates that method abnormally and removes the corresponding entry from the stack.

 This process will be continue until main() method.

 If the main() method also doesn’t contain exception handling code the JVM terminates that main() method and removes the corresponding entry from the stack.

 Just before terminating the program abnormally JVM handovers the responsibility of exception handling to the Default Exception Handler which is the component of JVM.

 Default Exception Handler just print exception information to the consol in the following format

Name of Exception: Description Stack Trace (Location of the Exception)

Q5.What is the purpose of try? Ans We should maintain all risky code inside the try block. Q6. What is the purpose of catch block? Ans.We have to maintain all Exception Handling code inside the catch block. Q7. Is try with multiple catch block is possible? Ans. The way of handling an exception is varied from exception to exception compulsory we have to write a separate catch block for every exception. Hence try will multiple catch block is possible and it is recommended to use. Example:

try{ //Risky code } catch(IOException e) { //Hndling code for IOException } catch(ArithmeticException e) { //handling code for AE } catch(NullPointerExcetpion e) { // handling code for NPE } catch(Exception e) { //default exception handling code }

Q8. If try with multiple catch block present is order of catch blocks important in which order we have to take? Ans. If try with multiple catch block present then the order of catch block is very important it should be from child to parent but not from parent to child.

Q9. What are various methods to print Exception information? and differentiate them.

Ans.

Throwable class defines the following method to print exception or error information . 1. printStackTrace() :- This method print exception information in the following format.

Name of the Exception: Description StackTrace

2.toString():- This method print exception information in the following format.

Name of the Exception: Description

3.getMessage():- This method prints only description of the exception.

Description

Q10.If an exception rised inside catch block then what will happen? Ans. If an exception raised inside catch block and it is not part of any try block then it is always abnormal termination. Q11. Is it possible to take try, catch inside try block? Ans. Yes, It is possible to take try, catch inside try block. That is nesting of try catch is possible. Q12.Is it possible to take try, catch inside catch block? Ans. Yes, It is possible to take try, catch inside catch block.

Q13. Is it possible to take try without catch? Ans. Yes, it is possible to take try without catch but compulsory finally block should be available. 

Q14. What is the purpose of finally block? 

Ans. The main purpose of finally block is, to maintain the cleanup code. This block will execute always. 

Q15. Is finally block will be execute always? 

Ans. Yes finally block will be executed always irrespective of whether exception raised or not raised whether exceptions are handled or not handle. There is one situation where the finally block won’t be executed if the JVM is going to be shutdown.

 Q16. In which situation finally block will not executed?

 Ans. There is one situation where the finally block won’t be executed if we are using system.exit(0) explicitly then JVM itself will be shutdown and there is no chance of executing finally block.

Q17. If return statement present inside try is finally block will be executed? 

Ans. Yes, if return statement present inside try, then also finally block will be executed. finally block will dominate return statement also.

 Q18. What is the difference between final, finally and finalize()?

 Ans. final:- final is a modifier applicable for variables, methods and classes. final variable means constant and reassignment is not possible. final method means implementation is final in the child classes we can’t override. final classmeans it won’t participate in inheritance and child class creation is not possible.

 finally:- It is a block associated with try catch to maintain cleanup code. Finally block will be executed always irrespective of whether exception is raised or not raised or whether the exception is handle or not handle. 

finalize():- It is a method, Garbage collector always calls this method just before destroying any object to perform cleanup activities.

Q19. Is it possible to write any statement between try-catch and finally? 

Ans. No, it is not possible to write any statement between try catch and finally. If we will try to write any statement between them then we will get compile time error

. Q20. Is it possible to take two finally blocks for the same try?

Ans. No, it is not possible to take two finally blocks for the same try. If we try to take then we will get compile time error.

Q21. Is syntax try-finally-catch is valid ?

 Ans. No, this syntax is not valid. It should be like try-catch-finally then only code will compile. Q22. What is the purpose of throw? 

Ans. Sometimes we can create Exception object explicitly and we can handover that exception object to the JVM explicitly by throw keyword. The purpose of throw keyword is to handover our created exception object explicitly to the JVM. Example1: class Test{ public static void main(String[] args){ System.out.println(10/0); } } In this case ArithmeticException object created implicitly and handover to the JVM automatically by the main method.

Example2: Class Test{ Public static void main(String[] args){ Throw new ArithmeticException(“/by Zero”); } } In this case creation of an exception object and handover to the JVM explicitly by the programmer.

Q23. Is it possible to throw an Error?

 Ans. Yes, It is possible to throw any Throwable type including Error.

Q24. Is it possible to throw any java object? 

Ans. No, we can use throw keyword only for throwable objects otherwise we will get compile time error saying incompatible type.

Q25. After throw is it allow to take any statement directly? 

Ans. After throw statement we are not allow to place any statement directly violation leads to compile time error saying Unreachable Statement

. Q26. What is the purpose of throws? 

Ans. The main purpose of throws keyword is to delegate the responsibilities of exception handling to the caller. It requires in the case of checked exception

. Q27. What is the difference between throw and throws? 

Ans. Sometimes we can create Exception object explicitly and we can handover that exception object to the JVM explicitly by throw keyword.The main purpose of throw keyword is to handover our created exception object explicitly to the JVM. The main purpose of throws keyword is to delegate the responsibilities of exception handling to the caller. It requires in the case of checked exception.

Q28. What is the difference between throw and thrown? 

Ans. There is no terminology of thrown in java.

Q29. Is it possible to use throws keyword for any java class? 

Ans. No, we can use throws keyword only for Throwable classes. Otherwise we will get compile time error saying Incompatible types. 

Q30. If we are taking catch block for an exception but there is no chance of rising that exception in try then what will happen? 

Ans. If there is no chance of raising an exception in try then we are not allow to write catch block for that exception violation leads to compile time error. But this rule is applicable only for fully checked exception. 

Q31. Explain Exception Handling keyword? 

Ans. Exception Handling keyword: Try :- To maintain Risky code. Catch:- To maintain Exception Handling code. Finally:- To maintain the clean up code. Throw:- To handover our created exception object to the JVM explicitly. Throws:- To delegate the responsibilities of Exception Handling to the caller.

Q32. Which class act as root for entire java Exception hierarchy?

 Ans. Throwable class act as root for entire java Exception hierarchy.

Q33. What is the difference between Error and Exception?

 Ans. Throwable class contain two child classes. Exception:- These are mostly caused by our program and are recoverable. Error:- These are not caused by our program, mostly caused by lake of system resources. These are non recoverable.

Q34. What is difference between checked exception and unchecked exception?

 Ans. The exceptions which are checked by the compiler for smooth execution of the program at Runtime is called checked exception. Example: IOException, InterruptedException.The exceptions which are not checked by the compiler are called unchecked exception. Example: ArithmeticException,RuntimeException.

 Q35.What is difference between partially checked and fully checked Exception?

 Ans. A checked exception is said to be fully checked if and only if all the child classes also checked otherwise it is called partially checked exception. Example: IOException:- fully checked exception Exception:- partially checked exception Throwable:- partially checked exception RuntimeException:- unchecked exception

Q36. What is a customized Exception? 

Ans. Sometimes based on our programming requirement we have to create our own exception such type of exception are called customize Exception. Example: TooYoungException TooOldException InsufficientFundException

Q37. Explain the process of creating the customized Exception. 

Ans. Creating customized Exception: Class TooYoungException extends RuntimeException{ TooYoungExcetpion(String desc){ Super(desc); } } Class TooOldException extends RuntimeException { TooOldException(String desc){ super(desc); } } Class custExcepiton{ Public static void main(String[] args){ Int age=Integer.parseInt(args[0]); If(age>60) { Throw new TooYoungException(“Please wait some more time, definitely you will get best match”); } Else if(age<18) { Throw new TooOldException(“Your age is already crossed of marriage, no chance to getting marriage”); } Else { System.out.println(“Congratulation! You will get match details soon by your email”); } }

 Q38. Explain control flow in try, catch, finally.

 Ans. Try{ Statement1; Statement2; Statement3; } Catch(X e){ Statement4; } Finally{ Statement5; } Statement6; Case1: If there is no Exception then output is Statement1 Statement2

Statement3 Statement5 Statement6 Normal termination Case2: If an exception raised at statement2 and corresponding catch block has matched then output is Statement1 Statement4 Statement5 Statement5 Normal termination Case3: An exception raised at statement2 and corresponding catch has not matched then output is Statement1 Statement5 Abnormal termination Case4: An exception occurs at statement4 it always Abnormal termination but before that finally block will be executed and output is Statement1 Statement2 Statement5 Abnormal termination Case5: If an exception raised at statement5 or statement6, it is always abnormal termination.

 Q39. Can you give the most common occurred exception in your previous project. 

Ans. NullPointerException, ArrayIndexOutofBoundException, StackOverFlowError, ClassCastException, NoClassDefFoundError, ExceptionInitilizerError, IllegalArgumentException, NumberFormatException, IllegalStateException, AssertionError. 

Q40. Explain the cases where you used Exception Handling in your previous project?