JAVA：如何根据属性值合并来自 arrayList 的对象？

Question

我正在使用 Spring Boot、Java 和 MySQL 构建（或学习如何构建）体育 REST API。我正在构建一种方法，该方法当前从匹配集合中获取每个匹配项，并为完整的匹配项列表返回 TeamStandings 的 ArrayList。

方法如下：

public List<TeamStanding> createStandingsTable(Match[] matches){
        List<TeamStanding> teamStandings = new ArrayList<TeamStanding>();
        for(int i = 0;i < matches.length; i++) {
            TeamStanding firstTeam = new TeamStanding();
            TeamStanding secondTeam = new TeamStanding();

            //set team ids
            firstTeam.setIdTeam(matches[i].getWcmHome());
            secondTeam.setIdTeam(matches[i].getWcmAway());


            //first team stats
            firstTeam.setTeamPlayed((long) 1);
            firstTeam.setTeamGoalsFavor(matches[i].getWcmHomeGoals());
            firstTeam.setTeamGoalsAgainst(matches[i].getWcmAwayGoals());
            firstTeam.setTeamGoalDif(firstTeam.getTeamGoalsFavor() - firstTeam.getTeamGoalsAgainst());

            //second team stats
            secondTeam.setTeamPlayed((long) 1);
            secondTeam.setTeamGoalsFavor(matches[i].getWcmAwayGoals());
            secondTeam.setTeamGoalsAgainst(matches[i].getWcmHomeGoals());
            secondTeam.setTeamGoalDif(secondTeam.getTeamGoalsFavor() - secondTeam.getTeamGoalsAgainst());

            //combined team stats

            if(firstTeam.getTeamGoalsFavor() > secondTeam.getTeamGoalsFavor()) {
                firstTeam.setTeamWins((long) 1);
                firstTeam.setTeamLoses((long) 0);
                firstTeam.setTeamDraws((long) 0);
                firstTeam.setTeamPoints((long) 3);
                secondTeam.setTeamWins((long) 0);
                secondTeam.setTeamLoses((long) 1);
                secondTeam.setTeamDraws((long) 0);
                secondTeam.setTeamPoints((long) 0);
            } else if (firstTeam.getTeamGoalsFavor() == secondTeam.getTeamGoalsFavor()) {
                firstTeam.setTeamWins((long) 0);
                firstTeam.setTeamLoses((long) 0);
                firstTeam.setTeamDraws((long) 1);
                firstTeam.setTeamPoints((long) 1);
                secondTeam.setTeamWins((long) 0);
                secondTeam.setTeamLoses((long) 0);
                secondTeam.setTeamDraws((long) 1);
                secondTeam.setTeamPoints((long) 1);
            } else {
                firstTeam.setTeamWins((long) 0);
                firstTeam.setTeamLoses((long) 1);
                firstTeam.setTeamDraws((long) 0);
                firstTeam.setTeamPoints((long) 0);
                secondTeam.setTeamWins((long) 1);
                secondTeam.setTeamLoses((long) 0);
                secondTeam.setTeamDraws((long) 0);
                secondTeam.setTeamPoints((long) 3);
            }
            teamStandings.add(firstTeam);
            teamStandings.add(secondTeam);
        }
        return teamStandings;
    }

结果是这样的：

[
    {
        "idTeam": 7,
        "teamPoints": 3,
        "teamPlayed": 1,
        "teamWins": 1,
        "teamDraws": 0,
        "teamLoses": 0,
        "teamGoalsFavor": 4,
        "teamGoalsAgainst": 1,
        "teamGoalDif": 3
    },
    {
        "idTeam": 13,
        "teamPoints": 0,
        "teamPlayed": 1,
        "teamWins": 0,
        "teamDraws": 0,
        "teamLoses": 1,
        "teamGoalsFavor": 1,
        "teamGoalsAgainst": 4,
        "teamGoalDif": -3
    },
    {
        "idTeam": 4,
        "teamPoints": 3,
        "teamPlayed": 1,
        "teamWins": 1,
        "teamDraws": 0,
        "teamLoses": 0,
        "teamGoalsFavor": 1,
        "teamGoalsAgainst": 0,
        "teamGoalDif": 1
    },
    {
        "idTeam": 7,
        "teamPoints": 0,
        "teamPlayed": 1,
        "teamWins": 0,
        "teamDraws": 0,
        "teamLoses": 1,
        "teamGoalsFavor": 0,
        "teamGoalsAgainst": 1,
        "teamGoalDif": -1
    }
]

我的问题是如何根据idTeam 合并这些对象？我试图达到的结果是在idTeam 保持不变的情况下将所有其余属性加起来。在给定的示例中，预期的是：

[
        {
            "idTeam": 7,
            "teamPoints": 3,
            "teamPlayed": 2,
            "teamWins": 1,
            "teamDraws": 0,
            "teamLoses": 1,
            "teamGoalsFavor": 4,
            "teamGoalsAgainst": 2,
            "teamGoalDif": 2
        },
        {
            "idTeam": 13,
            "teamPoints": 0,
            "teamPlayed": 1,
            "teamWins": 0,
            "teamDraws": 0,
            "teamLoses": 1,
            "teamGoalsFavor": 1,
            "teamGoalsAgainst": 4,
            "teamGoalDif": -3
        },
        {
            "idTeam": 4,
            "teamPoints": 3,
            "teamPlayed": 1,
            "teamWins": 1,
            "teamDraws": 0,
            "teamLoses": 0,
            "teamGoalsFavor": 1,
            "teamGoalsAgainst": 0,
            "teamGoalDif": 1
        }
    ]

也只是一个细节，我首先构建了TeamStandings 的ArrayList，现在我正在尝试合并它们，但也许我应该将它们作为循环堆叠在匹配数组中，在上述相同的方法中，但是我不确定。

Answer 1

遍历TeamStanding 的列表，注意团队 ID 并执行添加。您可能希望使用 Map 将团队 ID 对保存为键，并将团队本身保存为值，以便于操作。这是截图（我没有测试过，所以你可能需要稍微修改一下）。

List<TeamStanding> list = createStandingsTable(matches);
Map<Integer, TeamStanding> map = new HashMap<>();

for (TeamStanding team: list) {
    int id = team.getIdTeam();
    if (map.containsKey(id)) {
        TeamStanding other = map.get(id);
        other.setTeamPoints(team.getTeamPoints());
        other.setTeamPlayed(team.getTeamPlayed());
        // and so on...
    } else {
        map.put(id, team);
    }
}

List<TeamStanding> merged = new ArrayList<>(map.values());

如果您想直接从Match[] 创建合并的List<TeamStanding>，那么您必须使用相同的想法，但是，将两个迭代组合在一起可能有点复杂。然后我建议你坚持这两个独立的迭代。简洁性、可读性和可维护性优于性能——此外，性能在这里并不是真正的问题。

Answer 2

您可以使用 HashMap。使用“idTeam”作为键，使用对象 TeamStanding 作为值。现在您可以在结果列表上进行迭代，如果您在地图中找到该对象，只需更新其字段，如果您没有找到，则插入该对象。迭代完成后，您可以调用 map.values()，它会给您一个对象集合（TeamStanding），然后您可以使用该集合创建一个新的 ArrayList。

代码如下：

public List<TeamStanding> mergeTeamStandingList(List<TeamStanding> teamStandingList) {
    final Map<Integer, TeamStanding> idTeamVsTeamStandingMap = new HashMap<Integer, TeamStanding>();
    teamStandingList.forEach(teamStanding -> {
        if(idTeamVsTeamStandingMap.containsKey(teamStanding.getIdTeam())) {
            TeamStanding teamStanding1 = idTeamVsTeamStandingMap.get(teamStanding.getIdTeam());
            teamStanding1.setTeamDraws(teamStanding1.getTeamDraws() + teamStanding.getTeamDraws());
            //so on
        } else {
            idTeamVsTeamStandingMap.put(teamStanding.getIdTeam(), teamStanding);
        }
    });

    return new ArrayList<>(idTeamVsTeamStandingMap.values());
}

Answer 3

在您的 Teamstanding 对象上创建一个合并方法。

public TeamStanding merge(TeamStanding other) {
     this.teamPoints += other.getTeamPoints();
     this.teamPlayed += other.getTeamPlayed();
     this.teamWins += other.getTeamWins();
     this.teamDraws += other.getTeamDraws();
     this.teamGoalsFavor += other.getTeamGoalsFavor();
     this.teamLoses += other.getTeamLoses();
     this.teamGoalDif += other.getTeamGoalDif();
     return this;
}

---

然后使用Streams按teamId进行分组，并使用merge的方法减少常用项。

Map<Integer, Optional<TeamStanding>> mapReduced = teamStandings
.stream()
.collect(groupingBy(TeamStanding::getIdTeam, Collectors.reducing(TeamStanding::merge)));