如何在Java中定義常量(Constant)

首先看樣本：

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

方法一採用介面(Interface)的中變數預設為static final的特性。

方法二採用了Java 5.0中引入的Enum類型。

方法三採用了在普通類中使用static final修飾變數的方法。

方法四類似方法三，但是通過函數來擷取常量。

 

首先定義全域變數似乎有違Java的物件導向的封裝特性，增加的耦合。所以最佳的方法是避免定義全域變數。如果是參數等，可以寫入設定檔。如果實在是必須的，方法二是最為推薦的。方法三是大家都能想到的，非常的直觀。方法一和方法三本質上一樣。方法四提供了靈活性，具體參考引用【1】。

 

參考

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