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 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));
  }
}

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;
  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())
        ));
  }


}