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Java Interview Questions and Answers
Give a simplest way to find out the time a
method takes for execution without using any profiling tool?
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 amount of processing.
Why do we need wrapper classes?
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 reasons 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.
What are checked exceptions?
Checked exception are those which the Java compiler forces you to catch.
e.g. IOException are checked Exceptions.
What are runtime exceptions?
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.
What is the difference between error and an exception?
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.).
How to create custom exceptions?
Your class should extend class Exception, or some more specific type
thereof.
If I want an object of my class to be thrown as an exception object,
what should I do?
The class should extend from Exception class. Or you can extend your
class from some more precise exception type also.
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?
One can not do anything in this scenario. Because Java does not allow
multiple inheritance and does not provide any exception interface as
well.
How does an exception permeate through the code?
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.
What are the different ways to handle exceptions?
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 handle those exceptions.
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?
In the first approach as a programmer of the method, you yourself 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.
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