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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 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 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
Q6. 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.
Q7.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.
Q8.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.
Q9.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.
Q10.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.
Q11.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.
Q12.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.
Q13.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.
Q14.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 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.
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.
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.
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.
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.
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.
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
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.
