NULL Pointer Exception in Java

Wikitechy | 938 Views | java | 13 Jun 2016

 

Scenario :

  • In java NullPointerException is a RuntimeException.
  • Generally, in java a special null value can be assigned to an object reference. 
  • NullPointerException throws an exception when an application attempts to use an object reference that has null value. 

These exceptions include

  • By Calling an instance method on the object referred by a null reference.
  • Accessing or modifying an instance field of the object referred by a null reference.
  • If the reference type is an array type, taking the length of a null reference.
  • If the reference type is an array type, accessing or modifying the slots of a null reference.
  • If the reference type is a subtype of Throwable, throwing a null reference.
Object Wikitechy_obj = null;
Wikitechy_obj.toString();  // This statement will throw a NullPointerException

Reason:

In java a NullPointerException is a  Runtime Exception and the Javac compiler does not force to use a try-catch block to handle it appropriately.

Fix1:

For example, observe the following 

Here in this example code a null pointer exception occur when the dereference of the variable that’s pointing as null. 

String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown

Fix2:

  • In Java everything will be working in the form of class.
  • If we want to use any object, they have two phases

1. Declare

2. Initialization

Example:

  • Declaration: int a;
  • initialization: a=0;

Similarly, in an array:

  • Declaration: Item i[]=new Item[5];
  • Initialization: i[0]=new Item();

If we not initialize the object (or) variable inside the Initialization section, then the NullpointerException is occur.

Fixes are applicable to the following versions of Java:

  • NullPointerException (Java Platform SE 7 )
  • NullPointerException issues (Java)
  • Java 8 Optional - Avoid Null and NullPointerException

Applies to:

  • J2SE 1.3
  • J2SE 1.4
  • J2SE 5.0
  • Java SE 6
  • Java SE 7
  • Java SE 8

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