定义测试所需的类

public class RankDemo {

    public static void main(String[] args) {
        List<UserScore> userScores = initUserScore(10);
        userScores.stream().forEach(System.out::println);
    }

    private static List<UserScore> initUserScore(int count) {
        Random random = new Random();
        int top = 100;
        return IntStream.range(0, count).boxed()
            .map(idx -> new UserScore("haha_" + idx, random.nextInt(top)))
            .sorted(Comparator.comparingInt(UserScore::getScore).reversed())
            .collect(Collectors.toList());
    }

    @Data
    @NoArgsConstructor
    @AllArgsConstructor
    public static class UserScore {
        private String name;
        private int score;
    }

    @Data
    @NoArgsConstructor
    @AllArgsConstructor
    @Builder
    public static class UserRank {
        private String name;
        private int score;
        private int rank;
    }
}

（方法一）使用 for 循环

1）通过 for 循环新增序号

    public static List<UserRank> useFor(List<UserScore> userScores) {
        List<UserRank> result = new ArrayList<>();
        for (int i = 0; i < userScores.size(); ++ i) {
            UserScore userScore = userScores.get(i);
            result.add(UserRank.builder()
                .rank(i + 1)
                .name(userScore.getName())
                .score(userScore.getScore())
                .build());
        }
        return result;
    }

2）测试代码：

    public static void main(String[] args) {
        List<UserScore> userScores = initUserScore(10);
        List<UserRank> userRanks = useFor(userScores);
        userRanks.stream().forEach(System.out::println);
    }

控制台输出：

RankDemo.UserRank(name=haha_1, score=97, rank=1)
RankDemo.UserRank(name=haha_5, score=94, rank=2)
RankDemo.UserRank(name=haha_3, score=90, rank=3)
RankDemo.UserRank(name=haha_6, score=86, rank=4)
RankDemo.UserRank(name=haha_7, score=86, rank=5)
RankDemo.UserRank(name=haha_9, score=79, rank=6)
RankDemo.UserRank(name=haha_0, score=67, rank=7)
RankDemo.UserRank(name=haha_4, score=62, rank=8)
RankDemo.UserRank(name=haha_8, score=24, rank=9)
RankDemo.UserRank(name=haha_2, score=1, rank=10)

（方法二）使用 stream 循环

1）通过 stream() 循环和 AtomicInteger 新增序号

    public static List<UserRank> useStream(List<UserScore> userScores) {
        AtomicInteger rankIndex = new AtomicInteger(1);
        return userScores.stream()
            .map(userScore -> UserRank.builder()
                .rank(rankIndex.getAndIncrement())
                .name(userScore.getName())
                .score(userScore.getScore())
                .build())
            .collect(Collectors.toList());
    }

2）测试代码：

    public static void main(String[] args) {
        List<UserScore> userScores = initUserScore(10);
        List<UserRank> userRanks = useStream(userScores);
        userRanks.stream().forEach(System.out::println);
    }

控制台输出：

RankDemo.UserRank(name=haha_4, score=83, rank=1)
RankDemo.UserRank(name=haha_9, score=75, rank=2)
RankDemo.UserRank(name=haha_0, score=71, rank=3)
RankDemo.UserRank(name=haha_1, score=50, rank=4)
RankDemo.UserRank(name=haha_6, score=48, rank=5)
RankDemo.UserRank(name=haha_5, score=32, rank=6)
RankDemo.UserRank(name=haha_2, score=21, rank=7)
RankDemo.UserRank(name=haha_7, score=14, rank=8)
RankDemo.UserRank(name=haha_8, score=4, rank=9)
RankDemo.UserRank(name=haha_3, score=0, rank=10)