如何在Java中定義常量(Constant)

首先看示例：

1. /**
2. * Method One
3. */
4. interface ConstantInterface {
5. String SUNDAY = "SUNDAY";
6. String MONDAY = "MONDAY";
7. String TUESDAY = "TUESDAY";
8. String WEDNESDAY = "WEDNESDAY";
9. String THURSDAY = "THURSDAY";
10. String FRIDAY = "FRIDAY";
11. String SATURDAY = "SATURDAY";
12. }
13. /**
14. * Method Two
15. */
16. enum ConstantEnum {
17. SUNDAY, MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY
18. }
19. /**
20. * Method Three
21. */
22. class ConstantClassField {
23. public static final String SUNDAY = "SUNDAY";
24. public static final String MONDAY = "MONDAY";
25. public static final String TUESDAY = "TUESDAY";
26. public static final String WEDNESDAY = "WEDNESDAY";
27. public static final String THURSDAY = "THURSDAY";
28. public static final String FRIDAY = "FRIDAY";
29. public static final String SATURDAY = "SATURDAY";
30. }
31. /**
32. * Method Four
33. * http://www.ibm.com/developerworks/cn/java/l-java-interface/index.html
34. */
35. class ConstantClassFunction {
36. private static final String SUNDAY = "SUNDAY";
37. private static final String MONDAY = "MONDAY";
38. private static final String TUESDAY = "TUESDAY";
39. private static final String WEDNESDAY = "WEDNESDAY";
40. private static final String THURSDAY = "THURSDAY";
41. private static final String FRIDAY = "FRIDAY";
42. private static final String SATURDAY = "SATURDAY";
43. public static String getSunday() {
44. return SUNDAY;
45. }
46. public static String getMonday() {
47. return MONDAY;
48. }
49. public static String getTuesday() {
50. return TUESDAY;
51. }
52. public static String getWednesday() {
53. return WEDNESDAY;
54. }
55. public static String getThursday() {
56. return THURSDAY;
57. }
58. public static String getFirday() {
59. return FRIDAY;
60. }
61. public static String getSaturday() {
62. return SATURDAY;
63. }
64. }
65. public class TestConstant {
66. static final String day = "saturday";
67. public static void main(String[] args) {
68. System.out.println("Is today Saturday?");
69. System.out.println(day.equalsIgnoreCase(ConstantInterface.SATURDAY));
70. System.out.println(day.equalsIgnoreCase(ConstantEnum.SATURDAY.name()));
71. System.out.println(day.equalsIgnoreCase(ConstantClassField.SATURDAY));
72. System.out.println(day.equalsIgnoreCase(ConstantClassFunction
73. .getSaturday()));
74. }
75. }

方法一采用接口(Interface)的中變量默認為static final的特性。

方法二采用了Java 5.0中引入的Enum類型。

方法三采用了在普通類中使用static final修飾變量的方法。

方法四類似方法三，但是通過函數來獲取常量。

首先定義全局變量似乎有違Java的面向對象的封裝特性，增加的耦合。所以最佳的方法是避免定義全局變量。如果是參數等，可以寫入配置文件。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