多态的使用

class Shape {
    public void draw() {

    }
}
class Cycle extends Shape {
    @Override 
    public void draw() {
        System.out.println("○");
    }
}
class Rect extends Shape {
    @Override 
    public void draw() {
        System.out.println("□");
        }
}
class Flower extends Shape {
    @Override 
    public void draw() {
        System.out.println("♣");
    }
}

public class TestDemo1114_1 {
    public static void main(String[] args) {
        Shape shape1 = new Flower();
        Shape shape2 = new Cycle();
        Shape shape3 = new Rect();
        drawShape(shape1);
        drawShape(shape2);
        drawShape(shape3);
    }

    // 打印单个图形 
    public static void drawShape(Shape shape) {
        shape.draw();
    }
}

条件

1.父类应用引用子类对象 2.父类和子类都有同名的覆盖方法 3.通过父类引用这个重写方法的时候

优点

1.类调用者对类的使用成本进—步降低 2.能够降低代码的"圈复杂度"，避免使用大量的if - else 3.可扩展能力更强