讓你的系統更安全——用枚舉enum替代int常量

作應用系統時，咱們每每假設用戶是小白，那麼爲了保證系統的正常，咱們每每會對用戶的參數作限制，而且先後端都要對用戶的參數作驗證。那咱們在設計的時候是否能夠提早預防這種問題呢？其中的一種方式就是：用枚舉enum替代int常量。

枚舉的好處：

作應用系統時，咱們每每假設用戶是小白，那麼爲了保證系統的正常，咱們每每會對用戶的參數作限制，而且先後端都要對用戶的參數作驗證。那咱們在設計的時候是否能夠提早預防這種問題呢？其中的一種方式就是：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

示例1

<pre spellcheck="false" style="box-sizing: border-box; margin: 5px 0px; padding: 5px 10px; border: 0px; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-variant-numeric: inherit; font-variant-east-asian: inherit; font-weight: 400; font-stretch: inherit; font-size: 16px; line-height: inherit; font-family: inherit; vertical-align: baseline; cursor: text; counter-reset: list-1 0 list-2 0 list-3 0 list-4 0 list-5 0 list-6 0 list-7 0 list-8 0 list-9 0; background-color: rgb(240, 240, 240); border-radius: 3px; white-space: pre-wrap; color: rgb(34, 34, 34); letter-spacing: normal; orphans: 2; text-align: left; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">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());     
        }     
    }     
} 
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output：

1:RED=3

2:BLUE=5

3:BLACK=8

4:YELLOW=13

5:GREEN=28

其中

<pre spellcheck="false" style="box-sizing: border-box; margin: 5px 0px; padding: 5px 10px; border: 0px; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-variant-numeric: inherit; font-variant-east-asian: inherit; font-weight: 400; font-stretch: inherit; font-size: 16px; line-height: inherit; font-family: inherit; vertical-align: baseline; cursor: text; counter-reset: list-1 0 list-2 0 list-3 0 list-4 0 list-5 0 list-6 0 list-7 0 list-8 0 list-9 0; background-color: rgb(240, 240, 240); border-radius: 3px; white-space: pre-wrap; color: rgb(34, 34, 34); letter-spacing: normal; orphans: 2; text-align: left; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">    /** * 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;
    }
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示例2：

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。

<pre spellcheck="false" style="box-sizing: border-box; margin: 5px 0px; padding: 5px 10px; border: 0px; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-variant-numeric: inherit; font-variant-east-asian: inherit; font-weight: 400; font-stretch: inherit; font-size: 16px; line-height: inherit; font-family: inherit; vertical-align: baseline; cursor: text; counter-reset: list-1 0 list-2 0 list-3 0 list-4 0 list-5 0 list-6 0 list-7 0 list-8 0 list-9 0; background-color: rgb(240, 240, 240); border-radius: 3px; white-space: pre-wrap; color: rgb(34, 34, 34); letter-spacing: normal; orphans: 2; text-align: left; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">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));
    }
}
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