Java多态调用子类方法技巧

class Animal {
    void makeSound() {
        System.out.println("Animal makes sound");
    }
}

class Dog extends Animal {
    @Override
    void makeSound() {
        System.out.println("Woof!");
    }
}

class Cat extends Animal {
    @Override
    void makeSound() {
        System.out.println("Meow!");
    }
}

Animal a1 = new Dog();
Animal a2 = new Cat();
a1.makeSound(); // 输出 Woof!
a2.makeSound(); // 输出 Meow!

class Dog extends Animal {
    @Override
    void makeSound() {
        System.out.println("Woof!");
    }

    void fetch() {
        System.out.println("Dog is fetching the ball.");
    }
}

Animal a = new Dog();
a.fetch(); // 编译错误！父类引用看不到子类特有方法

// 正确做法：先判断，再转型
if (a instanceof Dog) {
    Dog d = (Dog) a;
    d.fetch(); // 正常调用
}

interface Drawable {
    void draw();
}

class Circle implements Drawable {
    public void draw() {
        System.out.println("Drawing a circle");
    }
}

class Rectangle implements Drawable {
    public void draw() {
        System.out.println("Drawing a rectangle");
    }
}

Drawable d1 = new Circle();
Drawable d2 = new Rectangle();
d1.draw(); // Drawing a circle
d2.draw(); // Drawing a rectangle