集合中为对象，根据对象中的某一属性值分组
 

例子

1、实体类：构造方法、get/set方法、toString方法省略

public class User {
    private String id;
    
    private String name;
    
    private String age;
    
    .......
    .......
}

2、测试类：

public static void main(String[] args){
    List<User> list = new ArrayList<>();
    User user1 = new User("1", "张三1", "10");
    User user2 = new User("2", "张三2", "10");
    User user3 = new User("2", "张三3", "10");
    User user4 = new User("4", "张三4", "11");
    User user5 = new User("5", "张三5", "11");
    User user6 = new User("6", "张三6", "11");
    User user7 = new User("7", "张三7", "12");
    User user8 = new User("8", "张三8", "12");
    User user9 = new User("9", "张三9", "12");
    list.add(user1);
    list.add(user2);
    list.add(user3);
    list.add(user4);
    list.add(user5);
    list.add(user6);
    list.add(user7);
    list.add(user8);
    list.add(user9);
    List<List<User>> groupList = new ArrayList<>();
    list.stream().collect(Collectors.groupingBy(User::getAge,Collectors.toList()))
                  .forEach((age,fooListByAge)->{
                    groupList.add(fooListByAge);
                  });
    
    groupList.forEach((userList)->{
      System.out.println(userList.size());
      userList.forEach((user)->System.out.println(user));
    });
  }

结果为：

3
User [id=4, name=张三4, age=11]
User [id=5, name=张三5, age=11]
User [id=6, name=张三6, age=11]
3
User [id=7, name=张三7, age=12]
User [id=8, name=张三8, age=12]
User [id=9, name=张三9, age=12]
3
User [id=1, name=张三1, age=10]
User [id=2, name=张三2, age=10]
User [id=2, name=张三3, age=10]
 

注意：

list.stream().collect(Collectors.groupingBy(User::getAge,Collectors.toList()))
                    .forEach((age,fooListByName)->{
                        groupList.add(fooListByName);
                    });

此句中的User::getAge 

getAge 为对象中某属性的get方法