使用 collect() 而不是 filter() 过滤流

Question

我的列表包含如下所示的数据

game_id | user_id | status
   1         1       STARTED
   2         1       FINISHED
   1         2       STARTED
   2         2       FINISHED

我想将此列表收集到两个地图中（或者如果可能的话，在单个地图中）

一张地图应该将user_id 映射到用户玩过的游戏数量——基本上是finished 状态的游戏

user_id | finished_games
   1            1
   2            1

另一张地图应将user_id 存储到所有游戏计数中。它看起来像这样

user_id  | all_games
   1            2
   2            2

我这样做（我删除了game_id 字段，因为它实际上并不重要）

import java.util.*;
import java.util.stream.*;

public class MyClass {
    public static void main(String args[]) {
        List<Game> games = Arrays.asList(
            create("user-1", "STARTED"),
            create("user-2", "STARTED"),
            create("user-1", "FINISHED"),
            create("user-2", "FINISHED"),
            create("user-1", "FINISHED"),
            create("user-2", "FINISHED")
        );

        // expect user-1: all_games (3), finished_games (2)

        Map<String, Long> allGames = games
            .stream()
            .collect(Collectors.groupingBy(Game::getUserId, Collectors.counting()));
        System.out.println(allGames);

        Map<String, Long> finishedGames = games
            .stream()
            .filter(game -> game.battleStatus.equals("FINISHED"))
            .collect(Collectors.groupingBy(Game::getUserId, Collectors.counting()));
        System.out.println(finishedGames);

    }

    private static Game create(String id, String status) {
        Game game = new Game();
        game.userId = id;
        game.battleStatus = status;
        return game;
    }

    private static class Game {
        String userId;
        String battleStatus;

        public String getUserId() {
            return userId;
        }
    }
}

它似乎工作正常。但是当我计算带有状态的游戏时，我会进行过滤以便只留下带有FINISHED 状态的项目。

Map<String, Long> finishedGames = games
    .stream()
    .filter(game -> game.battleStatus.equals("FINISHED"))
    .collect(Collectors.groupingBy(Game::getUserId, Collectors.counting()));

有没有办法在 collect 块中实现这一点，而不是使用 filter？

Answer 1

您可以使用多个groupingBy 来获取此信息，例如：

Map<String, Map<String, Long>> gameStatuses = games
   .stream()
   .collect(Collectors.groupingBy(Game::getUserId, 
   Collectors.groupingBy(Game::getBattleStatus, Collectors.counting())));
System.out.println(gameStatuses);

为此，您需要为battleStatus 的Game 类添加一个getter，例如：

private static class Game {
    String userId;
    String battleStatus;

    public String getUserId() {
        return userId;
    }

    public String getBattleStatus() {
        return battleStatus;
    }
}

这将为您提供以下输出：

{user-1={STARTED=1, FINISHED=2}, user-2={STARTED=1, FINISHED=2}}

您可以使用嵌套的map.get 来获取任何用户和任何状态的计数。

Answer 2

如果您需要计算已完成和未完成的游戏，那么另一个groupingBy 可以帮助您：

Function<Game, Boolean> isFinished = game -> "FINISHED".equals(game.battleStatus);

Map<String, Map<Boolean, Long>> groupedGames = 
    games.stream()
         .collect(groupingBy(Game::getUserId, 
                      groupingBy(isFinished, counting())));

或者您可能想按battleStatus 本身进行分组：

Map<String, Map<String, Long>> groupedGames = 
    games.stream()
         .collect(groupingBy(Game::getUserId, 
                      groupingBy(game -> game.battleStatus, counting())));

我什至会创建一个getter getBattleStatus()，然后将game -> game.battleStatus 替换为方法引用Game::getBattleStatus：

Map<String, Map<String, Long>> groupedGames = 
    games.stream()
         .collect(groupingBy(Game::getUserId, 
                      groupingBy(Game::getBattleStatus, counting())));