What is a NullPointerException? How do I fix it?

Question:

What are Null Pointer Exceptions (java.lang.NullPointerException)? How do I determine the cause? I want to stop the exception from causing the program to terminate prematurely.

Answer:

NullPointerException occurs when you use a reference that points to no location in memory (null) and treat it as though it was referencing an object.

Calling a 1. method on a null reference or  2.trying to access a field of a null reference will trigger it.

There are also other ways are listed on the NullPointerException javadoc page.

An example code is:

public class Example {

    public static void main(String[] args) {
        Object obj = null;
        obj.hashCode();
    }

}

On the first line inside main, the setting the Object reference obj equal to null. This means there is a reference, but it is not pointing to anobject. After that, I try to treat the reference as though it points to an object by calling a method on it. This results in a NullPointerException because there is no code to execute in the location that the reference is pointing.