Lambda(一)lambda表达式初体验

Lambda(一)lambda表达式初体验

Lambda引入 ：

随着需求的不断改变，代码也需要随之变化

需求一：有一个农场主要从一堆苹果中挑选出绿色的苹果

解决方案：常规做法，source code如下

public static void main(String[] args){
        List<Apple> apples = Arrays.asList(
                new Apple("green", 120),
                new Apple("red", 150),
                new Apple("green", 170),
                new Apple("green", 150),
                new Apple("yellow", 100));

        List<Apple> filterApple = filterGreenApple(apples);
        System.out.println(filterApple);
}

public static List<Apple> filterGreenApple(List<Apple> apples){
        List<Apple> list = new ArrayList<>();
        for(Apple a: apples){
            if(a.getColor().equals("green")){
                list.add(a);
            }
        }
        return list;
}            

假如现在需求发生改变：需要挑选红色、黄色等其他颜色

解决方案：基于上面解决方案，多加一个color参数，source code如下

public static List<Apple> findApple(List<Apple> apples,String color){
        List<Apple> list = new ArrayList<>();
        for(Apple a: apples){
            if(a.getColor().equals(color)){
                list.add(a);
            }
        }
        return list;
}

为了应对更复杂的需求，这里我们使用策略模式，source code如下

@FunctionalInterface
public interface FindApple{
        boolean filter(Apple apple);
    }

public static List<Apple> findApple(List<Apple> apples,FindApple findApple){
        List<Apple> list = new ArrayList<>();
        for(Apple apple:apples){
            if(findApple.filter(apple)){
                list.add(apple);
            }
        }
        return list;
}

public static void main(String[] args){
        //匿名类
        List<Apple> yellowApple = findApple(apples, new FindApple() {
            @Override
            public boolean filter(Apple apple) {
                return apple.getColor().equals("yellow") && apple.getWeight() >= 100;
            }
        });
        System.out.println(yellowApple);

        List<Apple> complexApple = findApple(apples, new greenAndGreater150WeightApple ());
        System.out.println(complexApple);
}

//过滤绿色且重量大于150g的Apple
public static class greenAndGreater150WeightApple implements FindApple{
        @Override
        public boolean filter(Apple apple) {
            return apple.getColor().equals("green")&&apple.getWeight()>=150;
        }
}

匿名类的方式比较臃肿，容易发生混淆，故这里引入Lambda表达式，source code如下

//基于上面的source code
List<Apple> lambdaresult = findApple(apples, apple -> apple.getColor().equals("green"));
System.out.println(lambdaresult);