### 使用 EnumMap 进行分发 使用 EnumMap 能够实现“真正的”两路分发。EnumMap 是为 enum 专门设计的一种性能非常好的特殊 Map。由于我们的目的是摸索出两种未知的类型，所以可以用一个 EnumMap 的 EnumMap 来实现两路分发： ```java // enums/RoShamBo5.java // Multiple dispatching using an EnumMap of EnumMaps // {java enums.RoShamBo5} package enums; import java.util.*; import static enums.Outcome.*; enum RoShamBo5 implements Competitor<RoShamBo5> { PAPER, SCISSORS, ROCK; static EnumMap<RoShamBo5,EnumMap<RoShamBo5,Outcome>> table = new EnumMap<>(RoShamBo5.class); static { for(RoShamBo5 it : RoShamBo5.values()) table.put(it, new EnumMap<>(RoShamBo5.class)); initRow(PAPER, DRAW, LOSE, WIN); initRow(SCISSORS, WIN, DRAW, LOSE); initRow(ROCK, LOSE, WIN, DRAW); } static void initRow(RoShamBo5 it, Outcome vPAPER, Outcome vSCISSORS, Outcome vROCK) { EnumMap<RoShamBo5,Outcome> row = RoShamBo5.table.get(it); row.put(RoShamBo5.PAPER, vPAPER); row.put(RoShamBo5.SCISSORS, vSCISSORS); row.put(RoShamBo5.ROCK, vROCK); } @Override public Outcome compete(RoShamBo5 it) { return table.get(this).get(it); } public static void main(String[] args) { RoShamBo.play(RoShamBo5.class, 20); } } ``` 输出为： ``` ROCK vs. ROCK: DRAW SCISSORS vs. ROCK: LOSE SCISSORS vs. ROCK: LOSE SCISSORS vs. ROCK: LOSE PAPER vs. SCISSORS: LOSE PAPER vs. PAPER: DRAW PAPER vs. SCISSORS: LOSE ROCK vs. SCISSORS: WIN SCISSORS vs. SCISSORS: DRAW ROCK vs. SCISSORS: WIN SCISSORS vs. PAPER: WIN SCISSORS vs. PAPER: WIN ROCK vs. PAPER: LOSE ROCK vs. SCISSORS: WIN SCISSORS vs. ROCK: LOSE PAPER vs. SCISSORS: LOSE SCISSORS vs. PAPER: WIN SCISSORS vs. PAPER: WIN SCISSORS vs. PAPER: WIN SCISSORS vs. PAPER: WIN ``` 该程序在一个 static 子句中初始化 EnumMap 对象，具体见表格似的 initRow() 方法调用。请注意 compete() 方法，您可以看到，在一行语句中发生了两次分发。