Java8新特性系列一：行为参数化

1. 什么是行为参数化

 行为参数化主要是为了应对需求的变化，减少代码的变动冗余、变动，应对行为标准建模，方式可以为定义接口，下面以一个例子说明行为参数化的必要性

2. 需求一

 有一个Apple类，属性包括颜色、重量等，现在用户需求为：筛选出苹果重量>100的苹果，这个需求很简单，很容易想到的实现如下：

package lambdasinaction.chap1;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

/**
 * @Author oxygen
 * @Date 2020/7/20 0020
 * @Version V1.0
 **/
public class MyFilterApple {

    public static void main(String[] args) {
        List<Apple> inventory = Arrays.asList(new Apple(100, "red"), new Apple(180, "green"));
        List<Apple> apple = findWeightGH100Apple(inventory);
    }

    private static List<Apple> findWeightGH100Apple(List<Apple> inventory) {
        List<Apple> result = new ArrayList<>();
        for (Apple apple : inventory) {
            if (apple.getWeight() > 100) {
                result.add(apple);
            }
        }
        return result;
    }

    public static class Apple {
        private int weight = 0;
        private String color = "";

        public Apple(int weight, String color) {
            this.weight = weight;
            this.color = color;
        }
　　　　public void setMethod&getMethod(...)
    }
}

3. 需求二

 用户改变增加需求：

 筛选出重量>100而且颜色为red的苹果，很容易想到的实现为增加一个实现方法，但是随着需求的增加，会增加N多筛选的方法，同时冗余代码

 private static List<Apple> findWeightGH100Apple(List<Apple> inventory) {
        List<Apple> result = new ArrayList<>();
        for (Apple apple : inventory) {
            if (apple.getWeight()>100 && "green".equals(apple.getColor())){
                result.add(apple);
            }
        }
    }

4. 改进一

4.1 定义接口传递行为

   public interface ApplePredicate {
        boolean test(Apple apple);
    }

4.2 多个类实现具体的逻辑

   /**
     *过滤重量大于100而且颜色是红色的苹果
     */
    public class AppleColorPredicate implements ApplePredicate {
        @Override
        public boolean test(Apple apple) {
            return "red".equals(apple.getColor()) && apple.getWeight() > 100;
        }
    }

    /**
     * 过滤重量>150的苹果
     */
    public class AppleHeavyWeightPredicate implements ApplePredicate {
        @Override
        public boolean test(Apple apple) {
            return apple.getWeight() > 150;
        }
    }

4.3 定义实现方法

List<Apple> inventory = Arrays.asList(new Apple(100, "red"), new Apple(180, "green"));
MyFilterApple.AppleColorPredicate p = new MyFilterApple().new AppleColorPredicate();
List<Apple> apples = filterApples(inventory, p);

MyFilterApple.AppleHeavyWeightPredicate p2 = new MyFilterApple().new AppleHeavyWeightPredicate();
List<Apple> apples1 = filterApples(inventory, p2);

private static List<Apple> filterApples(List<Apple> inventory, ApplePredicate p) {
    List<Apple> result = new ArrayList<>();
    for (Apple apple : inventory) {
        if (p.test(apple)) {
            result.add(apple);
        }
    }
    return result；
}

5. 改进二

 filterApples方法只能接受对象，所以必须把代码包裹在ApplePredicate对象里，因为通过一个实现了test方法的对象来传递布尔表达式

 可用匿名内部类和Lambd改进

List<Apple> apples3 = filterApples(inventory, new ApplePredicate() {//匿名内部类优化
　　@Override
　　public boolean test(Apple apple) {
　　　　return apple.getWeight() > 150;
       }
    });
List<Apple> apples3 = filterApples(inventory, apple -> apple.getWeight() > 100);//Lambda表达式优化