为什么会输出这样的结果?

public class A {

    public String show(D obj) {
        return ("A and D" );
    }

    public String show(A obj) {
        return ("A and A" );
    }
}

public class B extends A  {
    
        public String show(B obj) {
            return ("B and B");
        }
    
        public String show(A obj) {
            return ("B and A");
        }
    }


public class C extends B {}
public class D extends B {}

     	A a1=new A();
        A a2=new B();
        B b=new B();
        C c=new C();
        D d=new D();
public class Test {
    public static void main(String[] args) {
        A a2  = new B();
        B b = new B();
        System.out.println(a2.show(b));

    }
}

执行结果:
B and A