方法的匹配顺序

匹配顺序： this.show(O), super.show(O),
this.show(super(O)), super.show(super(O))
1）先确保参数完全匹配O的前提下，依次匹配this与super。
2）再考虑参数用super(O)渐近匹配O，依次匹配this与super
3）当存在子类覆盖父类方法时，根据new子类实例的原则，先调子类方法。

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

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

class C extends B{  }

class D extends B{  }

public class TestDT{  public static void main(String args[]){       A a1 = new A();        A a2 = new B();          B b = new B();        C c = new C();        D d = new D();           System.out.println(a1.show(b)); //  ①  A and A        System.out.println(a1.show(c)); //  ②  A and A        System.out.println(a1.show(d)); //  ③  A and D        System.out.println(a2.show(b)); //  ④  B and A        System.out.println(a2.show(c)); //  ⑤  B and A        System.out.println(a2.show(d)); //  ⑥  A and D        System.out.println(b.show(b));  //  ⑦  B and B        System.out.println(b.show(c));  //  ⑧  B and B        System.out.println(b.show(d));  //  ⑨  A and D  }}