继承

class Value {
    private int count = 1;

    private Value(int count) {
        this.count = count;
    }

    public static final Value
            v1 = new Value(1),
            v2 = new Value(2),
            v3 = new Value(3);
}

class Person {

    public void eat(Value v) {
        System.out.println("Person.eat()");
    }
}

class Teacher extends Person {
    public void eat(Value v) {
        System.out.println("Teacher.eat()");
    }
}

class Student extends Person {
    public void eat(Value v) {
        System.out.println("Student.eat()");
    }
}

public class UpcastingDemo {
    public static void show(Student s) {
        s.eat(Value.v1);
    }

    public static void show(Teacher t) {
        t.eat(Value.v1);
    }

    public static void show(Person p) {
        p.eat(Value.v1);
    }

    public static void main(String[] args) {
        Student s = new Student();
        Teacher t = new Teacher();
        Person p = new Person();
        show(s);
        show(t);
        show(p);
    }
}

这种做法一个很明显的缺陷就是必须为每一个Person类的衍生类定义与之紧密相关的方法，产生了很多重复的代码。另一方面，对于如果忘记了方法的重载也不会报错。上例中的三个show方法完全可以合并为一个：