初始化有序常量的一点小技巧

编写程序有时会用到有序常量。一个常见的情况是对命令行参数进行处理，如下的代码片段：

public class A {

            …….

            private static final int PARAM_1 = 0;

            private static final int PARAM_2 = 1;

            private static final int PARAM_3 = 2;

            private static final int PARAM_4 = 3;

            private static final int PARAMS_COUNT = 4;

            …….

            public static void main(String[] args) {

                        if (args.length != PARAMS_COUNT) {

                                    System.out.println("Usage: program <Param_1> <Param_2> <Param_3> <Param_3> <Param_4>");

                                    System.exit(1);

                        }

                        callFun1(args[PARAM_1], args[PARAM_2]);

                        callFun2(args[PARAM_1], args[PARAM_3], args[PARAM_4]);

            }

}

PARAM_1到PARAM_4就是我这里说的有序产量，这样的常量通常被赋予一段顺序的值。但如果像上面这段代码的写法，将会在程序维护时带来一些小麻烦。比如增加一个新的产量PARAM_5，除了需要定义型的常量以外，还需要调整PARAMS_COUNT的值。如果需要在原序列中插入一个新的常量，那就更麻烦了，不仅需要调整PARAMS_COUNT的值，还需要调整插入位置之后的常量的值。如在上面的代码中插入一个常量PARAM_NEW到PARAM_3的位置：

            …….

            private static final int PARAM_1 = 0;

            private static final int PARAM_2 = 1;

            private static final int PARAM_NEW = 2          // New Constant

            private static final int PARAM_3 = 3;                // Need Changed

            private static final int PARAM_4 = 4;                // Need Changed

            private static final int PARAMS_COUNT = 5;  // Need Changed

            …….

其实我们只需要引入一个静态变量就可以比较好的解决这个问题。下面是修改后的代码：

public class A {

            …….

            private static int PARAM_INDEX = 0;

            private static final int PARAM_1 = PARAM_INDEX++;

            private static final int PARAM_2 = PARAM_INDEX++;

            private static final int PARAM_3 = PARAM_INDEX++;

            private static final int PARAM_4 = PARAM_INDEX++;

            private static final int PARAMS_COUNT = PARAM_INDEX;

            …….

}

在新的代码中，我们引入了一个变量PARAM_INDEX，并赋予一个初始值（这里是零），借助于对这个变量，我们消除了硬编码常量的值带来的那些弊端。同样插入一个新的常量，现在我们只需要增加新的内容，而不需要改变原有的代码了。代码如下：

public class A {

            …….

            private static int PARAM_INDEX = 0;

            private static final int PARAM_1 = PARAM_INDEX++;

            private static final int PARAM_2 = PARAM_INDEX++;

            private static final int PARAM_NEW = PARAM_INDEX++;  // New Constant

            private static final int PARAM_3 = PARAM_INDEX++;

            private static final int PARAM_4 = PARAM_INDEX++;

            private static final int PARAMS_COUNT = PARAM_INDEX;

            …….

}