Java实现——策略模式

现有一个苹果类，有两个基本属性一个是颜色-color ，一个是重量-weight

public class Apple {

    private String color;
    private long weight;
    
    //省略get/set 方法
}

一、单个参数的需求

现在有一个需求需要获取绿色的苹果

    public static List<Apple> findGreenApple(List<Apple> apples) {

        List<Apple> list = new ArrayList<>();

        for (Apple apple : apples) {
            if ("green".equals(apple.getColor())) {
                list.add(apple);
            }
        }
        return list;
    }

测试方法如下：

    public static void main(String[] args) throws InterruptedException {
        List<Apple> list = Arrays.asList(new Apple("green", 150), new Apple("yellow", 120), new Apple("green", 170));
        List<Apple> greenApples = findGreenApple(list);
        System.out.println(greenApples);
    }

二、两个参数需求

现在不光要获取绿色的苹果，还有可能获取红色的苹果

    public static List<Apple> findApple(List<Apple> apples, String color) {
        List<Apple> list = new ArrayList<>();

        for (Apple apple : apples) {
            if (color.equals(apple.getColor())) {
                list.add(apple);
            }
        }
        return list;
    }

测试方法如下：

    public static void main(String[] args) throws InterruptedException {
        List<Apple> list = Arrays.asList(new Apple("green", 150), new Apple("yellow", 120), new Apple("green", 170));
//        List<Apple> greenApples = findGreenApple(list);

       List<Apple> greenApples = findApple(list, "green");
        System.out.println(greenApples);
    }

这样就可以实现，无论什么颜色的苹果都可以这个方法获取，但是缺点也暴露出来了，需求改变的同时参数在一直改变，如果即根据颜色又根据重量得到苹果呢？那就得增加重量参数

如何满足各种条件又不用更改很多的程序呢？

三、策略模式

定义一个策略接口

    public interface AppleFilter {
        boolean filter(Apple apple);
    }

编写获取苹果的方法，并且传入策略

    public static List<Apple> findApple(List<Apple> apples, AppleFilter appleFilter) {
        List<Apple> list = new ArrayList<>();

        for (Apple apple : apples) {
            if (appleFilter.filter(apple))
                list.add(apple);
        }
        return list;
    }

需要哪种需求，可以对策略接口进行实现

    //重量大于160g的绿色苹果
    public static class GreenAnd160WeightFilter implements AppleFilter {
        @Override
        public boolean filter(Apple apple) {
            return (apple.getColor().equals("green") && apple.getWeight() >= 160);
        }
    }

    //重量小于150g的黄色苹果
    public static class YellowLess150WeightFilter implements AppleFilter {
        @Override
        public boolean filter(Apple apple) {
            return (apple.getColor().equals("yellow") && apple.getWeight() < 150);
        }
    }

测试方法如下：

    public static void main(String[] args) throws InterruptedException {
        List<Apple> list = Arrays.asList(new Apple("green", 150), new Apple("yellow", 120), new Apple("green", 170));
//        List<Apple> greenApples = findGreenApple(list);
//        assert greenApples.size() == 2;

       /* List<Apple> greenApples = findApple(list, "green");
        System.out.println(greenApples);

        List<Apple> redApples = findApple(list, "red");
        System.out.println(redApples);*/
        List<Apple> result = findApple(list, new GreenAnd160WeightFilter());
        System.out.println(result);

方法二、内部类方式

测试方法如下：

    public static void main(String[] args) throws InterruptedException {
        List<Apple> list = Arrays.asList(new Apple("green", 150), new Apple("yellow", 120), new Apple("green", 170));
//        List<Apple> greenApples = findGreenApple(list);
//        assert greenApples.size() == 2;

       /* List<Apple> greenApples = findApple(list, "green");
        System.out.println(greenApples);

        List<Apple> redApples = findApple(list, "red");
        System.out.println(redApples);*/
/*        List<Apple> result = findApple(list, new GreenAnd160WeightFilter());
        System.out.println(result);*/

        List<Apple> yellowList = findApple(list, new AppleFilter() {
            @Override
            public boolean filter(Apple apple) {
                return "yellow".equals(apple.getColor());
            }
        });
    }

end