java8 集合 多字段 分组 统计个数代码

1.user实体

package com.demo.dto;
public class User {
  private Integer id;
  private String userName;
  private String password;
  private Integer age;  
  private long c; 

  public User() {
    super();
    // TODO Auto-generated constructor stub
  }

  public User(Integer id, String userName, String password, Integer age) {
    super();
    this.id = id;
    this.userName = userName;
    this.password = password;
    this.age = age;
  }

  public Integer getId() {
    return id;
  }

  public void setId(Integer id) {
    this.id = id;
  }

  public String getUserName() {
    return userName;
  }

  public void setUserName(String userName) {
    this.userName = userName == null ? null : userName.trim();
  }

  public String getPassword() {
    return password;
  }

  public void setPassword(String password) {
    this.password = password == null ? null : password.trim();
  }

  public Integer getAge() {
    return age;
  }

  public void setAge(Integer age) {
    this.age = age;
  }

  public long getC() {
    return c;
  }

  public void setC(long l) {
    this.c = l;
  }

  @Override
  public String toString() {
    return "User [id=" + id + ", userName=" + userName + ", password=" + password + ", age=" + age + ", c=" + c
        + "]";
  }
  public static void main(String[] args) {
    
    User u1=new User(1, "aa", "aap", 23);
    User u2=new User(2, "aa", "aap", 23);
    User u3=new User(3, "bb", "aap", 23);
    User u4=new User(4, "cc", "aap", 23);
    User u5=new User(5, "cc", "aap", 23);
    User u6=new User(6, "cc", "aap", 23);
    User u7=new User(7, "aa", "aap", 24);
    List list=new ArrayList();
    list.add(u1);list.add(u2);list.add(u7);list.add(u3);list.add(u4);list.add(u5);list.add(u6);
//原有list(根据第二个字段:userName和第四个字段:age 统计重复的记录数)
//jdk8的方法统计个数:
Map> map = list.stream().collect(Collectors.groupingBy(User::getUserName,Collectors.groupingBy(User::getAge,Collectors.counting())));
//jdk8以下:
    Map> map=new HashMap>();
    for (User user1 : list) {
        Map value=new HashMap();
        long count=0;
        if(map.containsKey(user1.getUserName())){
          continue;
        }
        for(int i=0;i {
      System.out.println(k+">>>>"+v);
    });
    List list2=new ArrayList();
    list.forEach(user ->{
      map.forEach((k, v) -> {
        if(k==user.getUserName()){
          Long remove = v.remove(user.getAge());
          user.setC(null==remove?0:remove);
        }
      });
      list2.add(user);
    });
    //遍历最后想要的结果(User中c为统计后的个数，方便前台遍历集合时单元格合并行)
    list2.forEach(u ->{
      System.out.println(u);
    });
    
  }
}

备注：运行结果如下

cc>>>>{23=3}
bb>>>>{23=1}
aa>>>>{23=2, 24=1}
User [id=1, userName=aa, password=aap, age=23, c=2]
User [id=2, userName=aa, password=aap, age=23, c=0]
User [id=7, userName=aa, password=aap, age=24, c=1]
User [id=3, userName=bb, password=aap, age=23, c=1]
User [id=4, userName=cc, password=aap, age=23, c=3]
User [id=5, userName=cc, password=aap, age=23, c=0]
User [id=6, userName=cc, password=aap, age=23, c=0]

此处是为了实现如下效果：

补充知识：List集合中元素按照固定数量进行分组

最近做项目，被要求将全国所有省份在页面中展示出来，但是要求

1、字少的三个省份占一行

2、字多的两个省份占一行

想来想去只能想到查询出所有省份，然后分组返回数据，上代码

public Map campus_list_provence(HttpServletRequest request,HttpServletResponse response)throws Exception{
		Map map=new HashMap();
		String sql="查询所有省份的名称，可以先在数据库中将全国所有省份建一个表";
		List list=bd.getSQLList(sql);
    //创建第一个list1，里面存入省份名称少于4个字的
		List list1=new ArrayList();
    //创建第二个list2，里面存入省份名称大于4个字的
		List list2=new ArrayList();
		if(list!=null&&list.size()>0){
			for (String string : list) {
				if(string.length()>4){
					list2.add(string);
				}else{
					list1.add(string);
				}
			}
		}
		int keyToken = 0;
		int toIndex=3;
		List