用枚舉enum替代int常量

枚舉的好處：

1. 類型安全性

2.使用方便性

 

public class EnumDemo {     
    enum Color{     
        RED(3),BLUE(5),BLACK(8),YELLOW(13),GREEN(28);          
        private int colorValue;     
        private Color(int rv){     
         this.colorValue=rv;       
        }   
        private int getColorValue(){
            return colorValue;
        }   
        
        private int value(){
            return ordinal()+1;
        }
}          
    public static void main(String[] args) { 
        for(Color s : Color.values()) {     
            //enum的values()返回一個數組，這裏就是Seasons[]     
            System.out.println(s.value()+":"+s.name()+"="+s.getColorValue());     
        }     
    }     
}   

output：

1:RED=3
2:BLUE=5
3:BLACK=8
4:YELLOW=13
5:GREEN=28

其中，

    /**
     * Returns the ordinal of this enumeration constant (its position
     * in its enum declaration, where the initial constant is assigned
     * an ordinal of zero).
     *
     * Most programmers will have no use for this method.  It is
     * designed for use by sophisticated enum-based data structures, such
     * as {@link java.util.EnumSet} and {@link java.util.EnumMap}.
     *
     * @return the ordinal of this enumeration constant
     */
    public final int ordinal() {
        return ordinal;
    }

 EnumMap是專門爲枚舉類型量身定作的Map實現。雖然使用其它的Map實現（如HashMap）也能完成枚舉類型實例到值得映射，可是使用EnumMap會更加高效：它只能接收同一枚舉類型的實例做爲鍵值，而且因爲枚舉類型實例的數量相對固定而且有限，因此EnumMap使用數組來存放與枚舉類型對應的值。這使得EnumMap的效率很是高。

import java.util.*;

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 src;
        private final Phase dst;

        Transition(Phase src, Phase dst) {
            this.src = src;
            this.dst = dst;
        }

        private static final Map<Phase, Map<Phase, Transition>> m = new EnumMap<Phase, Map<Phase, Transition>>(
                Phase.class);
        static {
            for (Phase p : Phase.values())
                m.put(p, new EnumMap<Phase, Transition>(Phase.class));
            for (Transition trans : Transition.values())
                m.get(trans.src).put(trans.dst, trans);
        }

        public static Transition from(Phase src, Phase dst) {
            return m.get(src).get(dst);
        }
    }

    public static void main(String[] args) {
        for (Phase src : Phase.values())
            for (Phase dst : Phase.values())
                if (src != dst)
                    System.out.printf("%s to %s : %s %n", src, dst,
                            Transition.from(src, dst));
    }
}