java8实战：filter的简单使用

《JAVA8实战》中的例子

要实现的功能：通过Apple的color或weight属性，对List<Apple>进行筛选。

1、首先定义com.owl.entity.Apple：

package com.owl.entity;

public class Apple {
    private String color;
    private Integer weight;

    public String getColor() {
        return color;
    }

    public void setColor(String color) {
        this.color = color;
    }

    public Integer getWeight() {
        return weight;
    }

    public void setWeight(Integer weight) {
        this.weight = weight;
    }
}

2、生成一个简单的List<Apple>集合

package com.owl.app;

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

import com.owl.entity.Apple;

public class demo {
    public static void main(String[] args) {
        List<Apple> appleList = new ArrayList<Apple>();

        Apple redApple = new Apple();
        redApple.setColor("red");
        redApple.setWeight(180);
        appleList.add(redApple);

        Apple greenApple = new Apple();
        greenApple.setColor("green");
        greenApple.setWeight(120);
        appleList.add(greenApple);
    }
}

3、在com.owl.entity.Apple中定义筛选条件(绿苹果或者重量大于150的苹果)

    public static boolean isGreenApple(Apple apple) {
        return "green".equals(apple.getColor());
    }

    public static boolean isHeavyApple(Apple apple) {
        return apple.getWeight() > 150;
    }

4、在com.owl.app.demo中定义接口：

    public interface Predicate<T> {
        boolean test(T t);
    }

5、在com.owl.app.demo中定义filter方法：

    static List<Apple> AppleFilter(List<Apple> apples, Predicate<Apple> p) {
        List<Apple> resultApples = new ArrayList<Apple>();
        for (Apple apple:apples) {
            if(p.test(apple)) {
                resultApples.add(apple);
            }
        }
        return resultApples;
    }

6、在main函数中使用filter筛选苹果（需要定义行为类isGreenApple、isHeavyApple）

List<Apple> greenAppleSet = AppleFilter(appleList, Apple::isGreenApple);
List<Apple> heavyAppleSet = AppleFilter(appleList, Apple::isHeavyApple);

System.out.println("=======绿苹果=======");
for (Apple apple:greenAppleSet) {
     System.out.println(apple.getColor());
}

System.out.println("=======大苹果=======");
for (Apple apple:heavyAppleSet) {
     System.out.println(apple.getWeight());
}

结果：

为了实现上述功能，除了需要定义筛选条件之外，仍需要定义Predicate<T>和AppleFilter方法未免太过麻烦，通过lambda表达式有更简单的写法：

List<Apple> greenAppleSet = appleList.stream().filter((Apple apple)->apple.getColor().equals("green")).collect(Collectors.toList());
List<Apple> heavyAppleSet = appleList.stream().filter((Apple apple)->apple.getWeight()>150).collect(Collectors.toList());

System.out.println("=======绿苹果=======");
for (Apple apple:greenAppleSet) {
System.out.println(apple.getColor());
}

System.out.println("=======大苹果=======");
for (Apple apple:heavyAppleSet) {
System.out.println(apple.getWeight());
}

涉及到较大的数据集的时候，可以采用并行处理的方式进行筛选：

List<Apple> greenAppleSet = appleList.parallelStream().filter((Apple apple)->apple.getColor().equals("green")).collect(Collectors.toList());
List<Apple> heavyAppleSet = appleList.parallelStream().filter((Apple apple)->apple.getWeight()>).collect(Collectors.toList());

或者使用匿名函数的形式：

List<Apple> greenAppleSet = AppleFilter(appleList, new Predicate<Apple>() {
    public boolean test(Apple apple) {
        return "green".equals(apple.getColor());
    };
});

List<Apple> heavyAppleSet = AppleFilter(appleList, new Predicate<Apple>() {
    public boolean test(Apple apple) {
        return apple.getWeight() > 150;
    };
});

System.out.println("=======绿苹果=======");
for (Apple apple:greenAppleSet) {
    System.out.println(apple.getColor());
}

System.out.println("=======大苹果=======");
for (Apple apple:heavyAppleSet) {
    System.out.println(apple.getWeight());
}