[goldhouse]

This may be the first exception one find in his code , Best coding practices will make this exception disappear from your code.
This exception simply say ,that used object is null

Code:

String f = null ;
System.out.println("this will cause null pointer exception  "+ f.length());

to avoid that always use a null checking

Code:

String f = null ;
if(f!==null)
System.out.println("this will cause null pointer exception  "+ f.length());