做应用系统时,我们往往假设用户是小白,那么为了保证系统的正常,我们往往会对用户的参数做限制,并且前后端都要对用户的参数做验证。那我们在设计的时候是否可以提前预防这种问题呢?其中的一种方式就是:用枚举enum替代int常量。
枚举的好处:
1. 类型安全性
2.使用方便性
示例1
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()返回一个数组,这里就是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; }
示例2:
EnumMap是专门为枚举类型量身定做的Map实现。虽然使用其它的Map实现(如HashMap)也能完成枚举类型实例到值得映射,但是使用EnumMap会更加高效:它只能接收同一枚举类型的实例作为键值,并且由于枚举类型实例的数量相对固定并且有限,所以EnumMap使用数组来存放与枚举类型对应的值。这使得EnumMap的效率非常高。
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)); }}