1、用户对象

@Getter
@Setter
@AllArgsConstructor
public class UserTest {
    // 名称
    private String userName;
    // 年龄
    private string age;
   // 分数（这个无所谓啊）
    private double score;

}

2、准备数据

List<UserTest> userList = new ArrayList<>();
userList.add(new UserTest("user", "1",3.55));
userList.add(new UserTest("user1", "3",4.66));
userList.add(new UserTest("user2", "2",90.66));
userList.add(new UserTest("user3", "2",7.999));

3、根据年龄分组

 Map<Integer, List<UserTest>> groupData = userList.stream()
                .collect(Collectors.groupingBy(UserTest::getAge));

结果：

{
	"1": [{
		"age": "1",
		"score": 3.55,
		"userName": "user"
	}],
	"2": [{
		"age": "2",
		"score": 90.66,
		"userName": "user2"
	}, {
		"age": "2",
		"score": 7.999,
		"userName": "user3"
	}],
	"3": [{
		"age": "3",
		"score": 4.66,
		"userName": "user1"
	}]
}

4、根据年龄分组、拼按年龄倒序

Map<String, List<UserTest>> groupData = userList.stream()
                .collect(
                        // 根据年龄分组
                        Collectors.groupingBy(UserTest::getAge,
                                // 年龄倒序（key）
                                () -> new TreeMap<>((k1, k2)->  - k1.compareTo(k2)),
                                Collectors.toList())

                );



        System.out.println(JSON.toJSONString(groupData));

结果

{
	"3": [{
		"age": "3",
		"score": 4.66,
		"userName": "user1"
	}],
	"2": [{
		"age": "2",
		"score": 90.66,
		"userName": "user2"
	}, {
		"age": "2",
		"score": 7.999,
		"userName": "user3"
	}],
	"1": [{
		"age": "1",
		"score": 3.55,
		"userName": "user"
	}]
}

5、根据年龄分组、拼按年龄倒序、value按分数倒序

Map<String, List<UserTest>> groupData = userList.stream()
                .collect(
                   // 根据年龄分组
                   Collectors.groupingBy(UserTest::getAge,
                        // 年龄倒序（key）
                        () -> new TreeMap<>((k1, k2) -> -(k1.compareTo(k2))),
                            // 分数倒序（value）
                            Collectors.collectingAndThen(
                                    Collectors.toCollection(
                                            ()-> new TreeSet<>(Comparator.comparing(UserTest::getScore,Comparator.reverseOrder())))
                                            ,ArrayList::new
                            )
                        ));
        System.out.println(JSON.toJSONString(groupData));

结果：

{
	"3": [{
		"age": "3",
		"score": 4.66,
		"userName": "user1"
	}],
	"2": [{
		"age": "2",
		"score": 90.66,
		"userName": "user2"
	}, {
		"age": "2",
		"score": 7.999,
		"userName": "user3"
	}],
	"1": [{
		"age": "1",
		"score": 3.55,
		"userName": "user"
	}]
}