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", "10");
		User user2 = new User("2", "張三", "11");
		User user3 = new User("2", "張三", "12");
		User user4 = new User("4", "李四", "13");
		User user5 = new User("5", "李四", "14");
		User user6 = new User("6", "李四", "15");
		User user7 = new User("7", "王五", "16");
		User user8 = new User("8", "王五", "17");
		User user9 = new User("9", "王五", "18");
		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);
		Map<String,List<User>> userGroupMap = list.stream().
				collect(Collectors.groupingBy(User::getName));
		Set<String> keySet = userGroupMap.keySet();
		keySet.forEach((key)->{
			System.out.println(key);
			userGroupMap.get(key).forEach((user)->System.out.println(user));
		});
	}
 

結果為：

李四
User [id=4, name=李四, age=13]
User [id=5, name=李四, age=14]
User [id=6, name=李四, age=15]
張三
User [id=1, name=張三, age=10]
User [id=2, name=張三, age=11]
User [id=2, name=張三, age=12]
王五
User [id=7, name=王五, age=16]
User [id=8, name=王五, age=17]
User [id=9, name=王五, age=18]

、

注意：

        Map<String,List<User>> userGroupMap = list.stream().
                collect(Collectors.groupingBy(User::getName

此句中的User::getName

getName 為物件中某屬性的get方法