java - 计算列表列表中对象属性的总和(带流)

Question

我使用的是 Java 8，我想使用 List.stream() 功能解决我的问题。

这是我的问题:

这是 Score 类(Player 类无关紧要):

public class Score {

    private Player player;
    private int score;

    public Player getPlayer() {
        return player;
    }

    public void setPlayer(Player player) {
        this.player = player;
    }

    public int getScore() {
        return score;
    }

    public void setScore(int score) {
        this.score = score;
    }
}

在我的程序开始时，我有一个玩家列表:

List<Player> players; // Populated elsewhere

然后我的程序执行它必须执行的操作，并且我得到一个这种类型的变量:

List<List<Score>> scores; // Populated elsewhere

现在我想为每个玩家打印这样的格式化输出:

Player.getName() + " : " + averagedScore;

其中 averagedScore 是一个 Player 的 score.getScore() 值的总和除以 Score 的出现次数这个玩家在我的 scores 变量中。

---

现在，我到目前为止所做的事情:

for (Players player : players) {
    scores.stream().filter(x -> x.stream().anyMatch(y -> y.getPlayer().equals(player)));
}

但我不知道如何计算子列表中包含的并由 element.getPlayer() 过滤的 element.getScore() 的总和

Answer 1

在“平面映射”您的列表列表后，您可以结合使用 groupingBy 和 averagingInt 收集器:

Map<Player, Double> scoreByPlayer = scores.stream()
                .flatMap(List::stream)
                .collect(groupingBy(Score::getPlayer, averagingInt(Score::getScore)));

scoreByPlayer.forEach((p, s) -> System.out.println(p + " : " + s));

_{注意:需要以下静态导入:}

import static java.util.stream.Collectors.averagingInt;
import static java.util.stream.Collectors.groupingBy;

---

供引用，完整示例:

public static void main(String[] args) {
  Player adam = new Player("Adam");
  Player pat = new Player("Pat");
  Player susie = new Player("Susie");
  List<List<Score>> scores = new ArrayList<> ();
  scores.add(Arrays.asList(s(adam, 1), s(pat, 2), s(susie, 3)));
  scores.add(Arrays.asList(s(adam, 2), s(pat, 4), s(susie, 6)));
  scores.add(Arrays.asList(s(adam, 3), s(pat, 6), s(susie, 9)));

  Map<Player, Double> scoreByPlayer = scores.stream()
                  .flatMap(List::stream)
                  .collect(groupingBy(Score::getPlayer, averagingInt(Score::getScore)));

  scoreByPlayer.forEach((p, s) -> System.out.println(p + " : " + s));
}

private static Score s(Player p, int score) { return new Score(p, score); }

public static class Score {
  private final Player player;
  private final int score;
  public Score(Player player, int score) {
    this.player = player;
    this.score = score;
  }

  public Player getPlayer() { return player; }
  public int getScore() { return score; }
}

public static class Player {
  private final String name;
  public Player(String name) { this.name = name; }
  @Override public String toString() { return name; }
}