Constants

   

Java™ 2 Primer Plus
By Steven Haines, Steve Potts

Table of Contents
Chapter 2.  Keywords, Data Types, and Variables


Now that you have data types that represent memory in a specific way and variables as a convention to refer to that memory, how would you represent values that do not change, for example pi? You could define a variable named pi as follows:

 double pi = 3.14285 

When you need to use it, you could simply reference it in an equation:

 double r = 10;  double area = pi * r * r; 

Variables can, as their name implies, contain variable data. But because variables such as pi really cannot change, you must prevent the user from doing the following:

 pi = pi / 2; 

This is perfectly legal in Java, but violates the property of pi.

The answer is that we can declare a variable to be a constant, or not changing. Java uses the keyword final to denote that a variable is in fact a constant. Now you can preface your declaration of pi with the keyword final and ensure that its value cannot change:

 final double pi = 3.14285 

When you try to compile a statement that modifies a constant value, such as

 final double pi = 3.14285  pi = pi / 2; 

The compiler generates an error saying that you cannot assign a value to a final variable.

By convention, programmers usually capitalize all the letters in a constant's name so that it can be found with only a casual glance of the source code. Pi, therefore, would be declared as follows:

 final double PI = 3.14285; 

       
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    Java 2 Primer Plus
    Java 2 Primer Plus
    ISBN: 0672324156
    EAN: 2147483647
    Year: 2001
    Pages: 332

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