Java Programming

Hello

I have a warning on this code :

class ClassA {
...
   methodA () {
      ObjectA a = new ObjectA();
      methodB(a);
   }

   methodB (ObjectA pa) {
      pa = null;
   }
}

On the ligne "pa=null" the compiler say "the parameter pa should not be assigned" ... why ? ObjectA is not a primitive type !

Thanks!