Most Commonly Asked Interview Questions
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 non abstract 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 the 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 vice versa.
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.
Q21.What is a
transient variable?
A transient variable is a variable that may not be
serialized.
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.
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.
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 super class 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 super class 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(;;) ;
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.
Associativity 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 a 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
super class, 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
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-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.
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.
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 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 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..
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.
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 class hierarchy?
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.
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 super class?
A class does not inherit constructors from any of its super classes.
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?
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 event delegation 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 accommodate
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.
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-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.
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 super class 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
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.
Q164. 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.
Q165. What is the
difference between the JDK 1.02 event model and the eventdelegation 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.
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?
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. TheenableEvents() 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.
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 isthrown 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 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 nested.
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 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.
