使用collect（）而不是filter（）筛选流

Question

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

我想把这个列表收集到两张地图中（如果可能的话，也可以在一张地图中）

一个映射应该将user_id映射到用户玩过的游戏数量--基本上是具有finished状态的游戏

user_id | finished_games
   1            1
   2            1

另一个映射应该将user_id存储到所有游戏计数。它看起来像这样

user_id  | all_games
   1            2
   2            2

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;
        }
    }
}

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

有没有一种方法可以在collection块内实现，而不是使用filter？

Answer 1

如果您需要同时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())));

或者您可能希望按battle status本身分组：

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

我甚至会创建一个gettergetBattleStatus()，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())));