1 package welcome;
 2 
 3 public class Constants {
 4     public static void main(String[] args){
 5         final double CM_PER_INCH = 2.54; // final表示变量只能被赋值一次，一旦被赋值之后，就不能再更改了
 6         double paperWidth = 8.5;
 7         double paperHeight = 11;
 8         System.out.println("Paper in centimeters: " 
 9                 + paperWidth * CM_PER_INCH + " by " + paperHeight * CM_PER_INCH);
10     }
11 }

 1 package welcome;
 2 
 3 public class Constants2 {
 4     // 设置一个类常量，让同一个类的每个方法都可以使用。若一个常量被声明为public，则其他类的方法也可以使用这个常量
 5     public static final double CM_PER_INCH = 2.54;  
 6     
 7     public static void main(String[] args){
 8         double paperWidth = 8.5;
 9         double paperHeight = 11;
10         
11         System.out.println("Paper in centimeters: "
12                 + paperWidth * CM_PER_INCH + " by " + paperHeight * CM_PER_INCH);
13     }
14 }