### 使用 EnumMap 進行分發 使用 EnumMap 能夠實現“真正的”兩路分發。EnumMap 是為 enum 專門設計的一種性能非常好的特殊 Map。由于我們的目的是摸索出兩種未知的類型，所以可以用一個 EnumMap 的 EnumMap 來實現兩路分發： ```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); } } ``` 輸出為： ``` 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 ``` 該程序在一個 static 子句中初始化 EnumMap 對象，具體見表格似的 initRow() 方法調用。請注意 compete() 方法，您可以看到，在一行語句中發生了兩次分發。