編寫程序有時會用到有序常量。一個常見的情況是對命令行參數(shù)進(jìn)行處理，如下的代碼片段：

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就是我這里說的有序產(chǎn)量，這樣的常量通常被賦予一段順序的值。但如果像上面這段代碼的寫法，將會在程序維護(hù)時帶來一些小麻煩。比如增加一個新的產(chǎn)量PARAM_5，除了需要定義型的常量以外，還需要調(diào)整PARAMS_COUNT的值。如果需要在原序列中插入一個新的常量，那就更麻煩了，不僅需要調(diào)整PARAMS_COUNT的值，還需要調(diào)整插入位置之后的常量的值。如在上面的代碼中插入一個常量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

            …….

其實(shí)我們只需要引入一個靜態(tài)變量就可以比較好的解決這個問題。下面是修改后的代碼：

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;

            …….

}

在新的代碼中，我們引入了一個變量PARAM_INDEX，并賦予一個初始值（這里是零），借助于對這個變量，我們消除了硬編碼常量的值帶來的那些弊端。同樣插入一個新的常量，現(xiàn)在我們只需要增加新的內(nèi)容，而不需要改變原有的代碼了。代碼如下：

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;

            …….

}