Java exception : NullPointerException

So suppose you are coding and this "NullPointerException" on the way so what should understand or derive from the exception or moreover how you should proceed for debugging, or getting rid of the exception, to do this lets understand due to what all problems it may cause :

Thrown when an application attempts to use null in a case where an object is required. These include:

1. Calling the instance method of a null object.
2. Accessing or modifying the field of a null object.
3. Taking the length of null as if it were an array.
4. Accessing or modifying the slots of null as if it were an array.
5. Throwing null as if it were a Throwable value.

So just check in your code, and if you are trying to access, or perform some operation on it, which is still null :)

Some examples are :

1. call to Stack's "pop" method for an empty stack
2. call to "factorial" function with a negative number
3. call to List's nextElement method when hasMoreElements is false