1# Instance level object reference (non-primitive) implicitly initialize with null so no need to initialize these references.

2# NullPonter occurs when we use variable (reference) before initialize it, So avoid calling before initialize.

3# First check the reference of the variable and then method.
for example :

ArrayList arr = new ArrayList();
/*
* business code here
*/
if(null != arrr && arr.size() > 0)
{//some code}

4# always use null in left side
it will increase your code performance.
Example:

AA aa = new AA();
if(null!= aa)

5# avoid use of size() method in collections just use isEmpty() method.

ArrayList arr = new ArrayList();
/*
* business code here
*/
if(null != arrr && !arr.isEmpty())
{//some code}

6# if you need to check multiple methods
check for each one
Example

if(null != aa && null != aa.method1() && null != aa.method1().method2())
//and so on

String str = null;

try{
System.out.println(str.length());
}
catch(NullPointerException ne)
{
System.out.println(“nullpointerexception”);
}
}
}

//output : nullpointerexception