Java8 stream collect groupBy分组例子
Posted rhyme
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实体类People,有个返回list的buildPeopleList方法,方便测试。
import lombok.AllArgsConstructor; import lombok.Builder; import lombok.Data; import lombok.NoArgsConstructor; import java.util.ArrayList; import java.util.Arrays; import java.util.List; /** * @author: rhyme * @date: 2019-09-26 17:31 * @topic: "主题" * @description: "描述" */ @Data @NoArgsConstructor @AllArgsConstructor @Builder public class People { private String name; private String character; private String sex; private int age; public static People buildDefault() { return People.builder().name("LuoTianyan").character("optimistic").sex("female").age(24).build(); } public static List<People> buildPeopleList() { People p1 = People.builder().name("LuoTianyan").character("optimistic").sex("female").age(24).build(); People p2 = People.builder().name("Zhang").character("optimistic").sex("female").age(23).build(); People p3 = People.builder().name("Hu").character("shy").sex("female").age(25).build(); People p4 = People.builder().name("Y").character("Righteous").sex("male").age(24).build(); return new ArrayList<>(Arrays.asList(p1, p2, p3, p4)); } }
测试类GroupByTest,分别根据性别分组,统计各个性别人数,以及各个性别映射姓名。
import org.junit.After; import org.junit.Before; import org.junit.Test; import org.springframework.util.CollectionUtils; import java.util.List; import java.util.Map; import java.util.function.BiConsumer; import java.util.stream.Collectors; /** * @author: rhyme * @date: 2019-10-14 15:59 * @topic: "主题" * @description: "描述" */ public class GroupByTest { private List<People> people; private BiConsumer soutKV; private Map result; @Before public void init() { people = People.buildPeopleList(); soutKV = (k, v) -> System.out.println(k + ":" + v); } @After public void soutResult() { if (!CollectionUtils.isEmpty(result)) { result.forEach(soutKV); } } /** * 根据性别分组 */ @Test public void groupBySex() { result = people.stream().collect( Collectors.groupingBy(People::getSex) ); } /** * 分别统计不同性别各自的人数 */ @Test public void groupBySexCount() { result = people.stream().collect( Collectors.groupingBy(People::getSex, Collectors.counting() )); } /** * 分别统计不同性别各自的name列表 */ @Test public void groupBySexName() { result = people.stream().collect( Collectors.groupingBy( People::getSex, Collectors.mapping(People::getName, Collectors.toSet()) )); } }
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