Error meaning

[Johanis]

java.lang.NullPointerException

What does this error mean?

[Tolls]

The JVM is attempting to call a method/access an attribute on a reference that is null.

public SomeClass {
   private String someAttribute;
   public SomeClass () {
      System.out.println(someAttribute.length());
   }
}

In the above the someAttribute reference is never initialised to point to a String object, so it is defaulted to null.
The attempt in the constructor to access the length() method will result in a null pointer exception.

[Johanis]

Thank you :)