The accepted practice if to use the IllegalArgumentException( String message ) to declare a parameter to be invalid and give as much detail as possible... So to say that a parameters was found to be null while exception non-null, you would do something like this:
if( variable == null )
throw new IllegalArgumentException("The object 'variable' cannot be null");
You have virtually no reason to implicitly use the "NullPointerException". The NullPointerException is an exception thrown by the Java Virtual Machine when you try to execute code on null reference (Like toString()).
