這篇文章主要介紹了JAVA8 STREAM COLLECT GROUPBY分組例項解析,文中通過示例程式碼介紹的非常詳細，對大家的學習或者工作具有一定的參考學習價值,需要的朋友可以參考下

實體類People，有個返回list的buildPeopleList方法，方便測試。

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: "主題"
 * @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));
  }
}

測試類GroupByTest，分別根據性別分組，統計各個性別人數，以及各個性別對映姓名。

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: "主題"
 * @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);
    }
  }

  /**
   * 根據性別分組
   */
  @Test
  public void groupBySex() {
    result = people.stream().collect(
        Collectors.groupingBy(People::getSex)
    );
  }

  /**
   * 分別統計不同性別各自的人數
   */
  @Test
  public void groupBySexCount() {
    result = people.stream().collect(
        Collectors.groupingBy(People::getSex,Collectors.counting()
        ));
  }

  /**
   * 分別統計不同性別各自的name列表
   */
  @Test
  public void groupBySexName() {
    result = people.stream().collect(
        Collectors.groupingBy(
            People::getSex,Collectors.mapping(People::getName,Collectors.toSet())
        ));
  }


}

以上就是本文的全部內容，希望對大家的學習有所幫助，也希望大家多多支援我們。