java8中predicate的用法介绍(code)

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

但是接下来，有人可能想要选出重的苹果，比如超过150克，于是你心情沉重地写了下面这
个方法，甚至用了复制粘贴：

public static List filterHeavyApples(List inventory){List result = new ArrayList();for (Apple apple: inventory){if (apple.getWeight() > 150) {result.add(apple); } }return result;}

我们都知道软件工程中复制粘贴的危险——给一个做了更新和修正，却忘了另一个。嘿，这
两个方法只有一行不同： if 里面高亮的那行条件。如果这两个高亮的方法之间的差异仅仅是接受
的重量范围不同，那么你只要把接受的重量上下限作为参数传递给 filter 就行了，比如指定
(150, 1000) 来选出重的苹果（超过150克），或者指定 (0, 80) 来选出轻的苹果（低于80克）。
但是，我们前面提过了，Java 8会把条件代码作为参数传递进去，这样可以避免 filter 方法
出现重复的代码。现在你可以写：

public static boolean isGreenApple(Apple apple) { return "green".equals(apple.getColor());}public static boolean isHeavyApple(Apple apple) { return apple.getWeight() > 150;}static List filterApples(List inventory, Predicate p) { List result = new ArrayList(); for (Apple apple: inventory){ if (p.test(apple)) { result.add(apple); } } return result;}

要用它的话，你可以写：
filterApples(inventory, Apple::isGreenApple);
或者
filterApples(inventory, Apple::isHeavyApple);
什么是谓词？

从传递方法到 Lambda

把方法作为值来传递显然很有用，但要是为类似于 isHeavyApple 和 isGreenApple 这种可
能只用一两次的短方法写一堆定义有点儿烦人。不过Java 8也解决了这个问题，它引入了一套新
记法（匿名函数或Lambda），让你可以写
filterApples(inventory, (Apple a) -> “green”.equals(a.getColor()) );
或者
filterApples(inventory, (Apple a) -> a.getWeight() > 150 );
甚至
filterApples(inventory, (Apple a) -> a.getWeight() <80 ||
“brown”.equals(a.getColor()) );
完整的代码为：

public class FilteringApples1 { public static void main(String[] args) { List inventory = Arrays.asList(new FilteringApples1.Apple(80, "green"),new FilteringApples1.Apple(155, "green"),new FilteringApples1.Apple(120, "red")); List greenApples2 = filterApples(inventory, (FilteringApples1.Apple a) -> "green".equals(a.getColor())); System.out.println(greenApples2); // [Apple{color='green', weight=155}] List heavyApples2 = filterApples(inventory, (FilteringApples1.Apple a) -> a.getWeight() > 150); System.out.println(heavyApples2); // [] List weirdApples = filterApples(inventory, (FilteringApples1.Apple a) -> a.getWeight() <80 ||"brown".equals(a.getColor())); System.out.println(weirdApples); } public static List filterApples(List inventory, Predicate p) { List result = new ArrayList(); for (FilteringApples1.Apple apple : inventory) { if (p.test(apple)) {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 Integer getWeight() { return weight; } public void setWeight(Integer weight) { this.weight = weight; } public String getColor() { return color; } public void setColor(String color) { this.color = color; } public String toString() { return "Apple{" + "color='" + color + '\'' + ", weight=" + weight + '}'; } }}

java8中内置filter函数

static Collection filter(Collection c, Predicate p);

这样你甚至都不需要写 filterApples 了，因为比如先前的调用

filterApples(inventory, (Apple a) -> a.getWeight() > 150 );

就可以直接调用库方法 filter ：

filter(inventory, (Apple a) -> a.getWeight() > 150 );