2.7. Numeric Data Types and Operations

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2.6. Constants

The value of a variable may change during the execution of the program, but a constant represents permanent data that never changes. In our ComputeArea program, is a constant. If you use it frequently, you don't want to keep typing 3.14159 ; instead, you can define a constant for Here is the syntax for declaring a constant:

   final   datatype CONSTANTNAME = VALUE; 

A constant must be declared and initialized in the same statement. The word final is a Java keyword which means that the constant cannot be changed. For example, in the ComputeArea program, you could define p as a constant and rewrite the program as follows :

  // ComputeArea.java: Compute the area of a circle    public class   ComputeArea {  /** Main method */    public static void   main(String[] args) {    final double   PI =   3.14159   ;  // Declare a constant    // Assign a radius    double   radius =   20   ; 

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  // Compute area    double   area = radius * radius *  PI  ;  // Display results  System.out.println(   "The area for the circle of radius "   + radius +   " is "   + area); } } 

Caution

By convention, constants are named in uppercase: PI , not pi or Pi .

Note

There are three benefits of using constants: (1) you don't have to repeatedly type the same value; (2) the value can be changed in a single location if necessary; (3) a descriptive name for a constant makes the program easy to read.