Thursday, June 24, 2010

Nitin Gudle



Java Interview Questions






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Question:


Does
Java provide any construct to find out the
size of an object?








Question:

Give a simplest way to find out the time
a method takes for execution without using
any profiling tool? 







Question:

What are wrapper classes?







Question:

Why do we need wrapper classes?
 







Question:

What are checked exceptions?







Question:

What are runtime exceptions?







Question:

What is the difference between error and
an exception??







Question:

How to create custom exceptions?







Question:

If I want an object of my class to be thrown
as an exception object, what should I do?







Question:

If my class already extends from some other
class what should I do if I want an instance
of my class to be thrown as an exception
object?








Question:

How does an exception permeate through the
code?







Question:

What are the different ways to handle exceptions?







Question:

What
is the basic difference between the 2 approaches
to exception handling...1> try catch
block and 2> specifying the candidate
exceptions in the throws clause?


When should you use which approach?



Question:

Is
it necessary that each try block must be
followed by a catch block?







Question:

If
I write return at the end of the try block,
will the finally block still execute?







Question:

If I write System.exit (0); at the end of
the try block, will the finally block still
execute?







 
 









Q:

Does
Java provide any construct to find out
the size of an object?
A:
No there is not sizeof operator in Java.
So there is not direct way to determine
the size of an object directly in Java.
 



Q:

Give a simplest way to find out the time a method takes for execution without using any profiling tool?
A:
Read the system time just before the method
is invoked and immediately after method
returns. Take the time difference, which
will give you the time taken by a method
for execution.

To
put it in code...


long
start = System.currentTimeMillis ();

method ();

long end = System.currentTimeMillis ();


System.out.println
("Time taken for execution is "
+ (end - start));


Remember
that if the time taken for execution is
too small, it might show that it is taking
zero milliseconds for execution. Try it
on a method which is big enough, in the
sense the one which is doing considerable
amout of processing.
 



Q:

What
are wrapper classes?
A:
Java provides specialized classes corresponding
to each of the primitive data types. These
are called wrapper classes. They are e.g.
Integer, Character, Double etc.
 



Q:

Why
do we need wrapper classes?
A:
It is sometimes easier to deal with primitives
as objects. Moreover most of the collection
classes store objects and not primitive
data types. And also the wrapper classes
provide many utility methods also. Because
of these resons we need wrapper classes.
And since we create instances of these classes
we can store them in any of the collection
classes and pass them around as a collection.
Also we can pass them around as method parameters
where a method expects an object.
 



Q:

What
are checked exceptions?
A:
Checked exception are those which the Java
compiler forces you to catch. e.g. IOException
are checked Exceptions.
 



Q:

What
are runtime exceptions?
A:
Runtime exceptions are those exceptions
that are thrown at runtime because of either
wrong input data or because of wrong business
logic etc. These are not checked by the
compiler at compile time.
 



Q:

What
is the difference between error and an
exception?
A:
An error is an irrecoverable condition occurring
at runtime. Such as OutOfMemory error. These
JVM errors and you can not repair them at
runtime. While exceptions are conditions
that occur because of bad input etc. e.g.
FileNotFoundException will be thrown if
the specified file does not exist. Or a
NullPointerException will take place if
you try using a null reference. In most
of the cases it is possible to recover from
an exception (probably by giving user a
feedback for entering proper values etc.).
 



Q:

How
to create custom exceptions?
A:
Your class should extend class Exception,
or some more specific type thereof.
 



Q:

If
I want an object of my class to be thrown
as an exception object, what should I
do?
A:
The class should extend from Exception class.
Or you can extend your class from some more
precise exception type also.
 



Q:

If
my class already extends from some other
class what should I do if I want an instance
of my class to be thrown as an exception
object?
A:
One can not do anytihng in this scenarion.
Because Java does not allow multiple inheritance
and does not provide any exception interface
as well.
 



Q:

How
does an exception permeate through the
code?
A:
An unhandled exception moves up the method
stack in search of a matching When an exception
is thrown from a code which is wrapped in
a try block followed by one or more catch
blocks, a search is made for matching catch
block. If a matching type is found then
that block will be invoked. If a matching
type is not found then the exception moves
up the method stack and reaches the caller
method. Same procedure is repeated if the
caller method is included in a try catch
block. This process continues until a catch
block handling the appropriate type of exception
is found. If it does not find such a block
then finally the program terminates.
 



Q:

What
are the different ways to handle exceptions?
A:
There are two ways to handle exceptions,


1. By wrapping the desired code in a try
block followed by a catch block to catch
the exceptions. and

2. List the desired exceptions in the throws
clause of the method and let the caller
of the method hadle those exceptions.
 



Q:


What is the basic difference between
the 2 approaches to exception handling.

1>
try catch block and

2> specifying the
candidate exceptions in the throws clause?

When should you use which approach?
A:
In the first approach as a programmer of
the method, you urself are dealing with
the exception. This is fine if you are in
a best position to decide should be done
in case of an exception. Whereas if it is
not the responsibility of the method to
deal with it's own exceptions, then do not
use this approach. In this case use the
second approach. In the second approach
we are forcing the caller of the method
to catch the exceptions, that the method
is likely to throw. This is often the approach
library creators use. They list the exception
in the throws clause and we must catch them.
You will find the same approach throughout
the java libraries we use.
 



Q:

Is
it necessary that each try block must
be followed by a catch block?
A:
It is not necessary that each try block
must be followed by a catch block. It should
be followed by either a catch block OR a
finally block. And whatever exceptions are
likely to be thrown should be declared in
the throws clause of the method.
 



Q:

If
I write return at the end of the try block,
will the finally block still execute?
A:
Yes even if you write return as the last
statement in the try block and no exception
occurs, the finally block will execute.
The finally block will execute and then
the control return.
 



Q:


If I write System.exit (0); at the end
of the try block, will the finally block
still execute?
A:
No in this case the finally block will not
execute because when you say System.exit
(0); the control immediately goes out of
the program, and thus finally never executes.
 



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