第 37 条：用EnumMap代替序数索引

用EnumMap代替序数索引.jpeg

class Plant {
  enum LifeCycle { ANNUAL, PERENNIAL, BIENNIAL }
  final String name;
  final LifeCycle lifeCycle;
  Plant(String name, LifeCycle lifeCycle) { 
    this.name = name;
    this.lifeCycle = lifeCycle;
  }
  @Override
  public String toString() { 
      return name;
  } 
}

// Using ordinal() to index into an array - DON'T DO THIS!
Set<Plant>[] plantsByLifeCycle =(Set<Plant>[]) new Set[Plant.LifeCycle.values().length];
  for (int i = 0; i < plantsByLifeCycle.length; i++)
    plantsByLifeCycle[i] = new HashSet<>(); 
for (Plant p : garden)
  plantsByLifeCycle[p.lifeCycle.ordinal()].add(p);
// Print the results
for (int i = 0; i < plantsByLifeCycle.length; i++) {
  System.out.printf("%s: %s%n", Plant.LifeCycle.values()[i], plantsByLifeCycle[i]);
}

// Using an EnumMap to associate data with an enum
Map<Plant.LifeCycle, Set<Plant>> plantsByLifeCycle = new EnumMap<>(Plant.LifeCycle.class);
for (Plant.LifeCycle lc : Plant.LifeCycle.values())
  plantsByLifeCycle.put(lc, new HashSet<>()); 
for (Plant p : garden)
  plantsByLifeCycle.get(p.lifeCycle).add(p); 
System.out.println(plantsByLifeCycle);

// Naive stream-based approach - unlikely to produce an EnumMap!
System.out.println(Arrays.stream(garden).collect(groupingBy(p -> p.lifeCycle)));

// Using a stream and an EnumMap to associate data with an enum
System.out.println(Arrays.stream(garden)
  .collect(groupingBy(p -> p.lifeCycle,() -> new EnumMap<>(LifeCycle.class), toSet())));

// Using ordinal() to index array of arrays - DON'T DO THIS!
public enum Phase {
    SOLID, LIQUID, GAS;

    public enum Transition {
        MELT, FREEZE, BOIL, CONDENSE, SUBLIME, DEPOSIT;
        // Rows indexed by from-ordinal, cols by to-ordinal
        private static final Transition[][] TRANSITIONS = {
                {null, MELT, SUBLIME},
                {FREEZE, null, BOIL},
                {DEPOSIT, CONDENSE, null}
        };

        // Returns the phase transition from one phase to another
        public static Transition from(Phase from, Phase to) {
            return TRANSITIONS[from.ordinal()][to.ordinal()];
        }
    }
}

// Using a nested EnumMap to associate data with enum pairs
public enum Phase {
  SOLID, LIQUID, GAS;
  public enum Transition {
    MELT(SOLID, LIQUID), 
    FREEZE(LIQUID, SOLID), 
    BOIL(LIQUID, GAS), 
    CONDENSE(GAS, LIQUID), 
    SUBLIME(SOLID, GAS), 
    DEPOSIT(GAS, SOLID);
    private final Phase from; private final Phase to;
    Transition(Phase from, Phase to) { 
      this.from = from; 
      this.to = to;
    }
    // Initialize the phase transition map
    private static final Map<Phase, Map<Phase, Transition>> m = Stream.of(values())
  .collect(groupingBy(t -> t.from, () -> new EnumMap<>(Phase.class), toMap(t -> t.to, t -> t,
(x, y) -> y, () -> new EnumMap<>(Phase.class))));
    public static Transition from(Phase from, Phase to) { 
      return m.get(from).get(to);
    }
  }
}