继承与多态实例

class Person {
    public String name;
    public int age;
    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }
    public void greet(Person other) {
        System.out.println("Hello, " + other.name);
    }
}
class Grandma extends Person {
    public Grandma(String name, int age) {
        super(name, age);
    }
    @Override
    public void greet(Person other) {
        System.out.println("Hello, young whippersnapper");
    }
    public void greet(Grandma other) {
        System.out.println("How was bingo, " + other.name + "?");
    }
}
public class testPeople {
    public static void main(String[] args) {
        Person n = new Person("Neil", 12);
        Person a = new Grandma("Ada", 60);
        Grandma v = new Grandma("Vidya", 80);
        // Grandma al = new Person("Alex", 70); // Compile time error
        n.greet(a); // "Hello Ada"
        n.greet(v); // "Hello Vidya"
        v.greet(a); // "Hello, young whippersnapper"
        v.greet((Grandma) a); // "How was bingo, Ada?"
        a.greet(n); // "Hello, young whippersnapper"
        a.greet(v); // "Hello, young whippersnapper"
        ((Grandma) a).greet(v); // "How was bingo, Vidya?"
        ((Grandma) n).greet(v); // Runtime error
    }
}

:::danger 特别注意第42行的输出！！！ :::