Hi Friends Java Interview questions has been designed for you to use to the Java Programming Interview questions and answers.
Java is Robust programming language , its intended is to 'write once and run anywhere'
Java is object oriented programming language . 90 % of the application are build in Java as per recent surveys.
Most of the applications are run on java and or they need java support to be executed .
Java has been vastly in demand in all the MNC's . If you are an expert of Java you will never have to sit back empty handed you can easily get jobs .
Java have several stages we have basic Java which is called core java , we have J2ee , we have Struts , Spring , Java beans and many more . Its a ocean which have many things beneath itself .
Java Interview questions will provide you a very good overview whats is asked in your java interview in companies.
Thanks and all the best for your job interview.
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?
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 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.
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() method is used to set the position and size of a component.
22. 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.
23. Which java.util classes and interfaces support event handling?
The EventObject class and the EventListener interface support event processing.
24. Is sizeof a keyword?
The sizeof operator is not a keyword.
25. What are wrapped classes?
Wrapped classes are classes that allow primitive types to be accessed as objects.
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.
27. 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).
28. 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.
29. What is the immediate superclass of the Applet class?
Panel.
30. 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.
31. Name three Component subclasses that support painting.
The Canvas, Frame, Panel, and Applet classes support painting.
32. 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.
33. What is the immediate superclass of the Dialog class?
Window.
34. What is clipping?
Clipping is the process of confining paint operations to a limited area or shape.
35. What is a native method?
A native method is a method that is implemented in a language other than Java.
36. Can a for statement loop indefinitely?
Yes, a for statement can loop indefinitely. For example, consider the following:
for(;;) ;
37. 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.
A thread enters the waiting state when it blocks on I/O.
39. To what value is a variable of the String type automatically initialized?
The default value of an String type is null.
40. 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.
41. 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.
42. 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.
43. 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.
44. 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.
45. 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.
46. What is the immediate superclass of Menu?
MenuItem.
47. 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.
48. Which class is the immediate superclass of the MenuComponent class?
Object.
49. 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'srun() method when the thread is initially executed.
50. What is the difference between the Boolean & operator and the && operator?
If an expression involving the Boolean & operator is evaluated, both operands are evaluated.
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.
51. Name three subclasses of the Component class.
Box.Filler, Button, Canvas, Checkbox, Choice, Container, Label, List, Scrollbar,
or TextComponent.
52. What is the GregorianCalendar class?
The GregorianCalendar class provides support for traditional Western calendars.
53. Which Container method is used to cause a container to be laid out and redisplayed?
validate() method is used to cause a container to be laid out and redisplayed.
54. What is the purpose of the Runtime class?
The purpose of the Runtime class is to provide access to the Java runtime system.
55. 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.
56. 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.
57. What is the argument type of a program's main() method?
A program's main() method takes an argument of the String[] type.
58. Which Java operator is right associative?
The = operator is right associative.
59. Can a double value be cast to a byte?
Yes, a double value can be cast to a byte.
60. 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.
61. 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.
62. Name two subclasses of the TextComponent class.
TextField and TextArea.
Frame.
64. 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 forstatement.
65. 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..
66. What is an abstract method?
An abstract method is a method whose implementation is deferred to a subclass.
67. 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.
68. 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.
69. What are the high-level thread states?
The high-level thread states are ready, running, waiting, and dead.
70. 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.
71. Can a Byte object be cast to a double value?
No. An object cannot be cast to a primitive value.
72. 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.
73. What is the difference between the String and StringBuffer classes?
String objects are constants. StringBuffer objects are not constants.
74. 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.
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.
76. What is the Dictionary class?
The Dictionary class provides the capability to store key-value pairs.
77. 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.
78. 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.
79. 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.
80. 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.
81. Which class is extended by all other classes?
The Object class is extended by all other classes.
82. 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..
83. Is the ternary operator written x : y ? z or x ? y : z ?
It is written x ? y : z.
84. 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 aFont object.
85. How is rounding performed under integer division?
The fractional part of the result is truncated. This is known as rounding toward zero.
86. 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.
InputStream/OutputStream class hierarchy?
The Reader/Writer class hierarchy is character-oriented, and the InputStream/OutputStream class
hierarchy is byte-oriented.
88. 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.
89. 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.
90. What is the SimpleTimeZone class?
The SimpleTimeZone class provides support for a Gregorian calendar.
91. What is the Map interface?
The Map interface replaces the JDK 1.1 Dictionary class and is used associate keys with values.
92. Does a class inherit the constructors of its superclass?
A class does not inherit constructors from any of its superclasses.
93. 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 orcontinue statement.
94. What is the purpose of the System class?
The purpose of the System class is to provide access to system resources.
95. Which TextComponent method is used to set a TextComponent to the read-only state?
setEditable().
96. 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.
97. Is &&= a valid Java operator?
No. It is not a valid java operator.
98. Name the eight primitive Java types.
The eight primitive types are byte, char, short, int, long, float, double, and boolean.
99. 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.
When a window is repainted by the AWT painting thread, it sets the clipping regions to the area
of the window that requires repainting.
101. Is "abc" a primitive value?
The String literal "abc" is not a primitive value. It is a String object.
102. 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.
103. 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.
104. What modifiers may be used with an interface declaration?
An interface may be declared as public or abstract.
105. Is a class a subclass of itself?
106. 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.
107. What event results from the clicking of a button?
The ActionEvent event is generated as the result of the clicking of a button.
108. 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.
109. 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.
110. 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.
The Collection interface provides support for the implementation of a mathematical bag - an
unordered collection of objects that may contain duplicates.
112. What modifiers can be used with a local inner class?
A local inner class may be final or abstract.
113. 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.
Non-static variables take on unique values with each object instance.
114. 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
causepaint() to be invoked by the AWT painting thread.
115. 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.
116. Can an exception be rethrown?
Yes, an exception can be rethrown.
117. Which Math method is used to calculate the absolute value of a number?
The abs() method is used to calculate absolute values.
118. 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.
119. 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.
120. 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.
121. Which class is the immediate superclass of the Container class?
Component.
122. 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.
123. How can the Checkbox class be used to create a radio button?
By associating Checkbox objects with a CheckboxGroup.
identifier?
The non-Unicode letter characters $ and _ may appear as the first character of an identifier
125. What restrictions are placed on method overloading?
Two methods may not have the same name and argument list but different return types.
126. 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.
127. 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 arraytype reference.
128. What is the return type of a program's main() method?
A program's main() method has a void return type.
129. Name four Container classes.
Window, Frame, Dialog, FileDialog, Panel, Applet, or ScrollPane.
130. 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.
131. What class of exceptions are generated by the Java run-time system?
The Java runtime system generates RuntimeException and Error exceptions.
132. What class allows you to read objects directly from a stream?
The ObjectInputStream class supports the reading of objects from input streams.
133. 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.
collector?
The garbage collector invokes an object's finalize() method when it detects that the object has
become unreachable.
135. 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.
136. 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.
137. 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.
138. Why are the methods of the Math class static?
So they can be invoked as if they are a mathematical code library.
139. What Checkbox method allows you to tell if a Checkbox is checked?
getState().
140. What state is a thread in when it is executing?
An executing thread is in the running state.
141. 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 arraytype.
142. How are the elements of a GridBagLayout organized?
The elements of a GridBagLayout are of equal size and are laid out using the squares of a grid.
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.
144. 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.
145. 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.
146. 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.
147. What are E and PI?
E is the base of the natural logarithm and PI is mathematical value pi.
148. Are true and false keywords?
The values true and false are not keywords.
149. What is a void return type?
A void return type indicates that a method does not return a value.
150. 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.
