lamda list 分组_Java Lambda 方式将List按照对象属性值分组成Map
Java Lambda 方式将List按照对象属性值分组成Map
有时候，需要对一个List结果集进行分组处理(按照对象中的某一个属性值进行分组)
例如：使用三国英雄的所属国家，进行分组英雄。
1、英雄实体类(Hero)
public classHero {privateString name;privateString country;publicHero(String name, String country) {this.name =name;this.country =country;
}publicString getName() {returnname;
}public voidsetName(String name) {this.name =name;
}publicString getCountry() {returncountry;
}public voidsetCountry(String country) {this.country =country;
}
@OverridepublicString toString() {return "Hero{" +
"name='" + name + '\'' +
", country='" + country + '\'' +
'}';
}
}
2、代码实现(Lambda方式)进行分组
packagecom.miracle.luna.lambda;importcom.alibaba.fastjson.JSON;import java.util.*;importjava.util.stream.Collectors;/*** Created by Miracle Luna on 2020/5/17*/
public classThreeCountryLambda {public static voidmain(String[] args) {
List heroList = new ArrayList<>();
Map> heroListMap = new HashMap>();
Hero liubei= new Hero("刘备", "蜀国");
Hero zhugeliang= new Hero("诸葛亮", "蜀国");
Hero sunce= new Hero("孙策", "吴国");
Hero zhouyu= new Hero("周瑜", "吴国");
Hero caocao= new Hero("曹操", "魏国");
Hero guojia= new Hero("郭嘉", "魏国");
heroList.add(liubei);
heroList.add(zhugeliang);
heroList.add(sunce);
heroList.add(zhouyu);
heroList.add(caocao);
heroList.add(guojia);//按照所属国家分组
heroListMap = heroList.stream().collect(Collectors.groupingBy(hero ->hero.getCountry()));
System.out.println(JSON.toJSONString(heroListMap));
}
}
3、运行结果
{"吴国":[{"country":"吴国","name":"孙策"},{"country":"吴国","name":"周瑜"}],"魏国":[{"country":"魏国","name":"曹操"},{"country":"魏国","name":"郭嘉"}],"蜀国":[{"country":"蜀国","name":"刘备"},{"country":"蜀国","name":"诸葛亮"}]}
使用JSON在线解析工具(https://www.json.cn/)，查看结果如下：
{"吴国":[
{"country":"吴国","name":"孙策"},
{"country":"吴国","name":"周瑜"}
],"魏国":[
{"country":"魏国","name":"曹操"},
{"country":"魏国","name":"郭嘉"}
],"蜀国":[
{"country":"蜀国","name":"刘备"},
{"country":"蜀国","name":"诸葛亮"}
]
}
PS：
上述场景，使用传统方式分组的话，需要遍历List，用对象中的所属国家，依次匹配Map中的key。
匹配上，则归类，加到子集中，作为Map的value。
1、代码实现(传统方式)进行分组：
packagecom.miracle.luna.lambda;importcom.alibaba.fastjson.JSON;importjava.util.ArrayList;importjava.util.HashMap;importjava.util.List;importjava.util.Map;/*** Created by Miracle Luna on 2020/5/17*/
public classThreeCountryTradition {public static voidmain(String[] args) {
List heroList = new ArrayList<>();
Map> heroListMap = new HashMap>();
Hero liubei= new Hero("刘备", "蜀国");
Hero zhugeliang= new Hero("诸葛亮", "蜀国");
Hero sunce= new Hero("孙策", "吴国");
Hero zhouyu= new Hero("周瑜", "吴国");
Hero caocao= new Hero("曹操", "魏国");
Hero guojia= new Hero("郭嘉", "魏国");
heroList.add(liubei);
heroList.add(zhugeliang);
heroList.add(sunce);
heroList.add(zhouyu);
heroList.add(caocao);
heroList.add(guojia);//按照所属国家分组
for(Hero hero : heroList) {if (!heroListMap.containsKey(hero.getCountry())) {
List valueList = new ArrayList<>();
valueList.add(hero);
heroListMap.put(hero.getCountry(), valueList);
}else{
heroListMap.get(hero.getCountry()).add(hero);
}
}
System.out.println(JSON.toJSONString(heroListMap));
}
}
2、运行结果：
{"吴国":[{"country":"吴国","name":"孙策"},{"country":"吴国","name":"周瑜"}],"蜀国":[{"country":"蜀国","name":"刘备"},{"country":"蜀国","name":"诸葛亮"}],"魏国":[{"country":"魏国","name":"曹操"},{"country":"魏国","name":"郭嘉"}]}
{"吴国":[
{"country":"吴国","name":"孙策"},
{"country":"吴国","name":"周瑜"}
],"蜀国":[
{"country":"蜀国","name":"刘备"},
{"country":"蜀国","name":"诸葛亮"}
],"魏国":[
{"country":"魏国","name":"曹操"},
{"country":"魏国","name":"郭嘉"}
]
}
总结：
通过两种实现方式的代码对比，可以看出：Lambda方式的逻辑清晰，代码更加简洁。
强烈推荐大家使用！！！
希望能帮到需要的小伙伴，谢谢~