The definitions from the links to the two exceptions above are
IllegalArgumentException: Thrown to indicate that a method has been passed an illegal or inappropriate argument.
NullPointerException: Thrown when an application attempts to use null in a case where an object is required.
The big difference here is the IllegalArgumentException is supposed to be used when checking that an argument to a method is valid. NullPointerException is supposed to be used whenever an object being "used" when it is null.
I hope that helps put the two in perspective.
