集合中為物件，根據物件中的某一屬性值分組
 

例子

1、實體類：構造方法、get/set方法、toString方法省略

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

2、測試類：

public static void main(String[] args){
		List<User> list = new ArrayList<>();
		User user1 = new User("1", "張三1", "10");
		User user2 = new User("2", "張三2", "10");
		User user3 = new User("2", "張三3", "10");
		User user4 = new User("4", "張三4", "11");
		User user5 = new User("5", "張三5", "11");
		User user6 = new User("6", "張三6", "11");
		User user7 = new User("7", "張三7", "12");
		User user8 = new User("8", "張三8", "12");
		User user9 = new User("9", "張三9", "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));
		});
	}
 

結果為：

3
User [id=4, name=張三4, age=11]
User [id=5, name=張三5, age=11]
User [id=6, name=張三6, age=11]
3
User [id=7, name=張三7, age=12]
User [id=8, name=張三8, age=12]
User [id=9, name=張三9, age=12]
3
User [id=1, name=張三1, age=10]
User [id=2, name=張三2, age=10]
User [id=2, name=張三3, age=10]
 

注意：

list.stream().collect(Collectors.groupingBy(User::getAge

此句中的User::getAge 

getAge 為物件中某屬性的get方法