如何在Java中定义常量(Constant)

首先看示例：

/**
 * Method One
 */
interface ConstantInterface {
	String SUNDAY = "SUNDAY";
	String MONDAY = "MONDAY";
	String TUESDAY = "TUESDAY";
	String WEDNESDAY = "WEDNESDAY";
	String THURSDAY = "THURSDAY";
	String FRIDAY = "FRIDAY";
	String SATURDAY = "SATURDAY";
}
/**
 * Method Two 
 */
enum ConstantEnum {
	SUNDAY, MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY
}
/**
 * Method Three
 */
class ConstantClassField {
	public static final String SUNDAY = "SUNDAY";
	public static final String MONDAY = "MONDAY";
	public static final String TUESDAY = "TUESDAY";
	public static final String WEDNESDAY = "WEDNESDAY";
	public static final String THURSDAY = "THURSDAY";
	public static final String FRIDAY = "FRIDAY";
	public static final String SATURDAY = "SATURDAY";
}
/**
 * Method Four
 * http://www.ibm.com/developerworks/cn/java/l-java-interface/index.html
 */
class ConstantClassFunction {
	private static final String SUNDAY = "SUNDAY";
	private static final String MONDAY = "MONDAY";
	private static final String TUESDAY = "TUESDAY";
	private static final String WEDNESDAY = "WEDNESDAY";
	private static final String THURSDAY = "THURSDAY";
	private static final String FRIDAY = "FRIDAY";
	private static final String SATURDAY = "SATURDAY";
	public static String getSunday() {
		return SUNDAY;
	}
	public static String getMonday() {
		return MONDAY;
	}
	public static String getTuesday() {
		return TUESDAY;
	}
	public static String getWednesday() {
		return WEDNESDAY;
	}
	public static String getThursday() {
		return THURSDAY;
	}
	public static String getFirday() {
		return FRIDAY;
	}
	public static String getSaturday() {
		return SATURDAY;
	}
}
public class TestConstant {
	static final String day = "saturday";
	public static void main(String[] args) {
		System.out.println("Is today Saturday?");
		System.out.println(day.equalsIgnoreCase(ConstantInterface.SATURDAY));
		System.out.println(day.equalsIgnoreCase(ConstantEnum.SATURDAY.name()));
		System.out.println(day.equalsIgnoreCase(ConstantClassField.SATURDAY));
		System.out.println(day.equalsIgnoreCase(ConstantClassFunction
				.getSaturday()));
	}
}

方法一采用接口(Interface)的中变量默认为static final的特性。

方法二采用了Java 5.0中引入的Enum类型。

方法三采用了在普通类中使用static final修饰变量的方法。

方法四类似方法三，但是通过函数来获取常量。

首先定义全局变量似乎有违Java的面向对象的封装特性，增加的耦合。所以最佳的方法是避免定义全局变量。如果是参数等，可以写入配置文件。如果实在是必须的，方法二是最为推荐的。方法三是大家都能想到的，非常的直观。方法一和方法三本质上一样。方法四提供了灵活性，具体参考引用【1】。

参考

【1】http://www.ibm.com/developerworks/cn/java/l-java-interface/index.html