Java 1.5新特性Enum的用法

Enum是enumeration(列举)的简写形式,包含在java.lang包中.

public enum Season { winter, spring, summer, fall }

 

一个enum是定义一组值的对象,它可以包括零个或多个值成员.它是属于enum类型的,一个enum对象中不可有两个或多个相同的属性或值.在次之前的java程序员一般是 用接口的方法实现列举的,

public interface Season {

   static winter = 0;

   static spring = 1; //etc..

}

 

引入了enum的java的列举的编写方便了许多,只须定义一个enum型的对象.enum对象的值都回自动获得一个数字值,从0开始,依次递增.看一个比较简单的enum实现的例子:

//EnumDemo.java

/**
 * We can loop over the values we put into the enum using the values() method.
 * Note that the enum Seasons is compiled into a separate unit, called
 * EnumDemo$Seasons.class title:
 */
public class EnumDemo {

	/*
	 * declare the enum and add values to it. note that, like in C#, we don't
	 * use a ; to end this statement and we use commas to separate the values
	 */

	private enum Seasons {
		winter, spring, summer, fall
	}

	// list the values

	public static void main(String[] args) {

		for (Seasons s : Seasons.values()) {
			System.out.println(s);
		}
	}

}

 

运行上述代码你回得到 以下结果:

winter

spring

summer

fall

Enum的属性调用:

下面的代码展示了调用enum对象的方法,这也是它通常的用法:

 

/**
 * File: EnumSwitch.java
 * 
 * Purpose: show how to switch against the values in an enum.
 */
public class EnumSwitch {

	private enum Color {
		red, blue, green
	}

	// list the values

	public static void main(String[] args) {
		// refer to the qualified value
		doIt(Color.red);

	}

	/*
	 * note that you switch against the UNQUALIFIED name. that is,
	 * "case Color.red:" is a
	 * 
	 * compiler error
	 */
	private static void doIt(Color c) {
		switch (c) {
		case red:
			System.out.println("value is " + Color.red);
			break;
		case green:
			System.out.println("value is " + Color.green);
			break;
		case blue:
			System.out.println("value is : " + Color.blue);
			break;
		default:
			System.out.println("default");

		}
	}
}

 

为enums添加属性和方法

enums也可以象一般的类一样添加方法和属性,你可以为它添加静态和非静态的属性或方法,这一切都象你在一般的类中做的那样.

/**
 * 在枚举类中加入方法
 * File: EnumWithMethods.java
 * 
 * Purpose: show how to use an enum that also defines its own fields and methods
 */
public class EnumWithMethods {

	// declare the enum and add values to it.
	public enum Season {
		winter, spring, summer, fall;

		private final static String location = "Phoenix";

		public static Season getBest() {
			if (location.equals("Phoenix"))
				return winter;
			else
				return summer;
		}
	}

	public static void main(String[] args) {
		System.out.println(Season.getBest());

	}
}

 

就是这么的简单.但是有一点是需要注意的,那就是enums的值列表必须紧跟在enum声明,不然编译时将会出错.

Enums构造函数:

和类一样enums也可以有自己的构造函数,如下:

 

/**
 * 构造方式，创建枚举 title:
 * 
 */
public class EnumConstructor {

	public static void main(String[] a) {
		// call our enum using the values method
		for (Temp t : Temp.values())
			System.out.println(t + " is : " + t.getValue());
	}

	// make the enum

	public enum Temp {

		absoluteZero(-459), freezing(32), boiling(212), paperBurns(451);

		// constructor here
		Temp(int value) {
			this.value = value;
		}

		// regular field?but make it final,
		// since that is the point, to make constants
		private final int value;

		// regular get method
		public int getValue() {
			return value;
		}
	}

}

 

输出结果是:

absoluteZero is : -459

freezing is : 32

boiling is : 212

paperBurns is : 451

尽管enums有这么多的属性,但并不是用的越多越好,如果那样还不如直接用类来的直接.enums的优势在定义int最终变量仅当这些值有一定特殊含义时.但是如果你需要的是一个类,就定义一个类,而不是enum.