Lambdas 演进

演示Lambdas，以过滤苹果为例

List<Apple> inventory = Arrays.asList(new Apple(80,"green"),
                                              new Apple(155, "green"),
                                              new Apple(120, "red"));

原始代码

public static List<Apple> filterGreenApples(List<Apple> inventory){
    List<Apple> result = new ArrayList<>();
    for (Apple apple: inventory){
        if ("green".equals(apple.getColor())) {
            result.add(apple);
        }
    }
    return result;
}

演进代码

定义一个接口

interface ApplePredicate {
    boolean test(Apple a);
}

接口作为一个参数传递方法

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

• 方式1

接口的实现，过滤green苹果

public class AppleColorPredicate implements ApplePredicate {
   public boolean test(Apple apple){
      return "green".equals(apple.getColor());
   }
}

调用过滤方法

List<Apple> greenApples = filter(inventory, new AppleColorPredicate());

• 方式2

匿名类

List<Apple> redApples = filter(inventory, new ApplePredicate() {
   public boolean test(Apple a){
      return a.getColor().equals("red"); 
   }
});

等同于：Java8写法

List<Apple> redApples = filter(inventory, a -> a.getColor().equals("red"));

Lambdas

static <T> Collection<T> filter(Collection<T> c, Predicate<T> p); 

So you wouldn’t even have to write methods like filterApples because, for example, the previous call

filterApples(inventory, (Apple a) -> a.getColor().equals("red") );

could simply be written as a call to the library method filter:

filter(inventory, (Apple a) -> a.getColor().equals("red") );