What causes a NullPointerException?

As you should know, Java types are divided into primitive types (boolean, int etc) and reference types. Reference types in Java allow you to use the special value null which is the Java way of saying "no object".

A NullPointerException is thrown at runtime whenever your program attempts to use a null as if it was a real reference. For example, if you write this:

    public class Test {
        public static void main(String[] args) {
            String foo = null;
            int length = foo.length();   // HERE
        }
    }

the statement labelled "HERE" is going to attempt to run the length() method on a null reference, and this will throw a NullPointerException.

There are many ways that you could use a null value that will result in a NullPointerException. If fact, the only things that you can do with a null without causing an NPE are:

• assign it to a reference variable or read it from a reference variable,
• assign it to an array element or read it from an array element (provided that array reference itself is non-null!),
• pass it as a parameter or return it as a result, or
• test it using the == or != operators.