[Java] java里的enum，

[Java] java里的enum，

enum自jdk1.5引入，其实是一个特殊的class，写法简单，JVM自动帮你封装了很多东西而已

package scjp; public class Test { public static void main(String args[]) { System.out.println(CoffeeSize.BIG == CoffeeSize.BIG); } } enum CoffeeSize { // 8, 10 & 16 are passed to the constructor BIG(8), HUGE(10), OVERWHELMING(16); CoffeeSize(int ounces) { // constructor this.ounces = ounces; System.out.println(ounces); } private int ounces; // an instance variable public int getOunces() { return ounces; } }

 

 

编译后的enum（上面是CoffeeSize）也会对应生成一个CoffeeSize.class

通过类汇编反编译 javap -c CoffeeSize 得到如下代码

Compiled from "Test.java" final class scjp.CoffeeSize extends java.lang.Enum{ public static final scjp.CoffeeSize BIG; public static final scjp.CoffeeSize HUGE; public static final scjp.CoffeeSize OVERWHELMING; static {}; Code: 0: new #1; //class scjp/CoffeeSize 3: dup 4: ldc #16; //String BIG 6: iconst_0 7: bipush 8 9: invokespecial #17; //Method "<init>":(Ljava/lang/String;II)V 12: putstatic #21; //Field BIG:Lscjp/CoffeeSize; 15: new #1; //class scjp/CoffeeSize 18: dup 19: ldc #23; //String HUGE 21: iconst_1 22: bipush 10 24: invokespecial #17; //Method "<init>":(Ljava/lang/String;II)V 27: putstatic #24; //Field HUGE:Lscjp/CoffeeSize; 30: new #1; //class scjp/CoffeeSize 33: dup 34: ldc #26; //String OVERWHELMING 36: iconst_2 37: bipush 16 39: invokespecial #17; //Method "<init>":(Ljava/lang/String;II)V 42: putstatic #27; //Field OVERWHELMING:Lscjp/CoffeeSize; 45: iconst_3 46: anewarray #1; //class scjp/CoffeeSize 49: dup 50: iconst_0 51: getstatic #21; //Field BIG:Lscjp/CoffeeSize; 54: aastore 55: dup 56: iconst_1 57: getstatic #24; //Field HUGE:Lscjp/CoffeeSize; 60: aastore 61: dup 62: iconst_2 63: getstatic #27; //Field OVERWHELMING:Lscjp/CoffeeSize; 66: aastore 67: putstatic #29; //Field ENUM$VALUES:[Lscjp/CoffeeSize; 70: return public int getOunces(); Code: 0: aload_0 1: getfield #36; //Field ounces:I 4: ireturn public static scjp.CoffeeSize[] values(); Code: 0: getstatic #29; //Field ENUM$VALUES:[Lscjp/CoffeeSize; 3: dup 4: astore_0 5: iconst_0 6: aload_0 7: arraylength 8: dup 9: istore_1 10: anewarray #1; //class scjp/CoffeeSize 13: dup 14: astore_2 15: iconst_0 16: iload_1 17: invokestatic #55; //Method java/lang/System.arraycopy:(Ljava/lang/Object;ILjava/lang/Object;II)V 20: aload_2 21: areturn public static scjp.CoffeeSize valueOf(java.lang.String); Code: 0: ldc #1; //class scjp/CoffeeSize 2: aload_0 3: invokestatic #61; //Method java/lang/Enum.valueOf:(Ljava/lang/Class;Ljava/lang/String;)Ljava/lang/Enum; 6: checkcast #1; //class scjp/CoffeeSize 9: areturn }

 

可以看到各个element其实是enum类的public static final 的instance，

还有包括：

public static scjp.CoffeeSize[] values();

public static scjp.CoffeeSize valueOf(java.lang.String);

等方法。

所以在一些ide（如eclipse）里面 .后面会自动提示这些方法或属性。

也可以用反射再次验证一下类汇编代码。

 

 

另外，enum的constructor只允许为private。

equals方法被声明为final，所以不能override，以保证不同元素之间的比较返回false，相同元素返回true。

 

 

enum的override

public class Demo { public static void main(String ...args) {// <--> String args[] MyEnum.B.method(); // output: // B:method() } }; enum MyEnum{ A(1), B(2){//seems enum elements must be placed ahead of other fieds and methods //override public void method(){ System.out.println("B:method()"); } }; private int value; MyEnum(int value){ this.value = value; } // a common method public void method(){ System.out.println("MyEnum:method()"); } };

 

 

总结： enum可以看作一个特殊的class，语法上作了要求，比如constructor不能为public， 元素的声明必须放在类的最前面。element为一些public static final的instance， enum不能在method里声明，等。