How overcome problem of NullPointerException?

A NullPointerException is thrown when you attempt to access an object reference which is null.

Example:

String str = null;

str.equals("x"); // <- NullPointerException

To fix it, you must read the error message and find out what object is null. Determining how to ensure that this object is not set to null in the future must be made on a case-by-case basis.