Java Lambda filter foreach的簡單使用

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Java在對 List 集合進行按條件查詢的時候，經常會想到使用for循環。

自從 Java 8 引入了 Lambda 后，簡化了循環的使用。

舉例說明 Lambda的 foreach 和 filter 的簡單使用。

 

1、自定義類 Hero

/**
 * Created by Miracle Luna on 2020/3/8
 */
public class Hero {
    private String name;
    private String country;

    public Hero(String name, String country) {
        this.name = name;
        this.country = country;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getCountry() {
        return country;
    }

    public void setCountry(String country) {
        this.country = country;
    }

    @Override
    public String toString() {
        return "Hero{" +
                "name='" + name + '\'' +
                ", country='" + country + '\'' +
                '}';
    }
}

 

2、foreach循環

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

/**
 * Created by Miracle Luna on 2020/3/8
 */
public class LambdaForeachDemo {
    public static void main(String[] args) {
        List<Hero> heroList = new ArrayList<>();
        List<Hero> resultList = new ArrayList<>();

        Hero liubei = new Hero("劉備", "蜀國");
        Hero guanyu = new Hero("關羽", "蜀國");
        Hero zhangfei = new Hero("張飛", "蜀國");
        Hero sunce = new Hero("孫策", "吳國");

        heroList.add(liubei);
        heroList.add(guanyu);
        heroList.add(zhangfei);
        heroList.add(sunce);

        // 查詢不是吳國的英雄
        heroList.forEach(hero -> {
            if(!"吳國".equals(hero.getCountry())) {
                resultList.add(hero);
            }
        });

        // 打印結果集合
        resultList.forEach(hero -> System.out.println(hero.toString()));
    }
}

 

3、filter過濾

import java.util.ArrayList;
import java.util.List;
import java.util.stream.Collectors;

/**
 * Created by Miracle Luna on 2020/3/8
 */
public class LambdaFilterDemo {
    public static void main(String[] args) {
        List<Hero> heroList = new ArrayList<>();
        List<Hero> resultList = new ArrayList<>();

        Hero liubei = new Hero("劉備", "蜀國");
        Hero guanyu = new Hero("關羽", "蜀國");
        Hero zhangfei = new Hero("張飛", "蜀國");
        Hero sunce = new Hero("孫策", "吳國");

        heroList.add(liubei);
        heroList.add(guanyu);
        heroList.add(zhangfei);
        heroList.add(sunce);

        // 查詢不是吳國的英雄
        resultList = heroList.stream()
                .filter(hero -> !"吳國".equals(hero.getCountry()))
                .collect(Collectors.toList());

        // 打印結果集合
        resultList.forEach(hero -> System.out.println(hero.toString()));
    }
}

 

4、運行結果如下：

Hero{name='劉備', country='蜀國'}
Hero{name='關羽', country='蜀國'}
Hero{name='張飛', country='蜀國'}