策略模式

策略模式定义了算法族(也可以说是一组行为)，分别封装起来，让它们之间可以相互交替，此模式让算法的变化独立于使用算法的客户。

示例—不同鸭子的表现

设计几种鸭子的类，比如绿头鸭、红头鸭等。鸭子都由不同的样子，有些会飞，有些不会飞（比如橡皮鸭），有些会叫，有些不会叫（比如橡皮鸭）。

UML图表示

鸭子的策略模式设计

代码演示

1. 由于每个鸭子会飞，但有些会飞，有些不会飞，所以设计一个飞行行为接口，用不同实现表示

interface FlyBehavior{
    void fly();
}

class FlyWithWings implements FlyBehavior{
    @Override
    public void fly() {
        System.out.println("I'm flying!!");
    }
}

class FlyNoWay implements FlyBehavior{
    @Override
    public void fly() {
        System.out.println("I can't fly");
    }
}

2. 大部分鸭子会叫，也有不会叫的鸭子，而且叫的声音也不同。也采用接口+实现的方式

interface QuackBehavior{
    void quack();
}

class Quack implements QuackBehavior{
    @Override
    public void quack() {
        System.out.println("Quack");
    }
}

class MuteQuack implements QuackBehavior{
    @Override
    public void quack() {
        System.out.println("<< Silence >>");
    }
}

class Squeak implements QuackBehavior{
    @Override
    public void quack() {
        System.out.println("Squeak");
    }
}

3. 设计鸭子的基类，总结鸭子的行为有：飞、叫、游泳、显示的外观。

public abstract class Duck {
    private FlyBehavior flyBehavior;
    private QuackBehavior quackBehavior;

    public void swim(){
        System.out.println("I can swim");
    }

    public abstract void display();

    public void performFly(){
        flyBehavior.fly();
    }

    public void performQuack(){
        quackBehavior.quack();
    }

    public void setFlyBehavior(FlyBehavior flyBehavior){
        this.flyBehavior = flyBehavior;
    }

    public void setQuackBehavior(QuackBehavior quackBehavior){
        this.quackBehavior = quackBehavior;
    }
}

4. 四种鸭子的实现：绿头鸭、红头鸭、橡皮鸭、诱饵鸭。

class MallardDuck extends Duck{

    MallardDuck(){
        setFlyBehavior(new FlyWithWings());
        setQuackBehavior(new Quack());
    }

    @Override
    public void display() {
        System.out.println("I'm a green head duck");
    }
}

class RedHeadDuck extends Duck{

    RedHeadDuck(){
        setFlyBehavior(new FlyWithWings());
        setQuackBehavior(new Quack());
    }

    @Override
    public void display() {
        System.out.println("I'm a red head duck");
    }
}

class RubberDuck extends Duck{

    RubberDuck(){
        setFlyBehavior(new FlyNoWay());
        setQuackBehavior(new MuteQuack());
    }

    @Override
    public void display() {
        System.out.println("I'm a model duck");
    }
}

class DecoyDuck extends Duck{

    DecoyDuck(){
        setFlyBehavior(new FlyNoWay());
        setQuackBehavior(new Squeak());
    }

    @Override
    public void display() {
        System.out.println("I'm a decoy duck");
    }
}

5. 添加测试代码

 public static void main(String[] args) {
        MallardDuck mallardDuck = new MallardDuck();
        mallardDuck.swim();
        mallardDuck.display();
        mallardDuck.performFly();
        mallardDuck.performQuack();

        System.out.println("~~~~~~~~~~~~~~~~~~~");

        RedHeadDuck redHeadDuck = new RedHeadDuck();
        redHeadDuck.swim();
        redHeadDuck.display();
        redHeadDuck.performFly();
        redHeadDuck.performQuack();

        System.out.println("~~~~~~~~~~~~~~~~~~~");

        RubberDuck rubberDuck = new RubberDuck();
        rubberDuck.swim();
        rubberDuck.display();
        rubberDuck.performFly();
        rubberDuck.performQuack();

        System.out.println("~~~~~~~~~~~~~~~~~~~");

        DecoyDuck decoyDuck = new DecoyDuck();
        decoyDuck.swim();
        decoyDuck.display();
        decoyDuck.performFly();
        decoyDuck.performQuack();
    }

6. 测试结果

I can swim
I'm a green head duck
I'm flying!!
Quack
~~~~~~~~~~~~~~~~~~~
I can swim
I'm a red head duck
I'm flying!!
Quack
~~~~~~~~~~~~~~~~~~~
I can swim
I'm a model duck
I can't fly
<< Silence >>
~~~~~~~~~~~~~~~~~~~
I can swim
I'm a decoy duck
I can't fly
Squeak