dan new collections

[tony kanvas]

  
class A {}
class B extends A {}
class C extends B {
  static void m(A x, A y) {System.out.print("AA");}
  static void m(A x, B y) {System.out.print("AB");}
  static void m(B x, A y) {System.out.print("BA");}
  static void m(B x, B y) {System.out.print("BB");}
  public static void main(String[] args) {
    A a;
    B b;
    m(null,null);
    m(a=null,b=null);
    m(b, a);
  }
}

dan said The first method call, m(null,null), calls method m with two null literals that have type null. Both null literals are promoted to type B because B is more specific than type A
but if you see A its same as B why compiler take B

[John Paverd]

Imagine the following class hierarchy:
class Animal {}
class Mammal extends Animal {}
class Horse extends Mammal {}
This makes it a bit easier to see that a sub class is more specific than a super class. Any horse is an animal, but not every animal is a horse.
JLS section 15.12.2.2 has more information about choosing the most specific method:

The informal intuition is that one method declaration is more specific than another if any invocation handled by the first method could be passed on to the other one without a compile-time type error.

[Abu Yoosuf]

Originally posted by tony kanvas:

  
class C extends B {
  //add this method to the C class and test out the output
  static void m(C x, C y) {
      System.out.print("CC");
  }
  //...
}