请写出下面程序的运行结果。 class EXA { int add(int x,int y) { return x+y; } } class EXB extends EXA { int add(int x,int y) { return x-y; } } public class ExtendsExercise { public static void main(String args[]) { EXA a=new EXA(); System.out.println(a.add(80,20)); a=new EXB (); System.out.println(a.add(80,20)); } }