Classes - Multimedia Introduction to Programming Using Java - page 122

approximation to the value of π, the ratio of the circumference of a circle to its
diameter.
A constant is usually made static because there is no need for more than one
copy of it. Placing one copy of the variable in each instance of a class would
waste space. Making it static means that there is exactly one copy, in the file
drawer of the class. Further, it can be referenced even if no instance of the class
has been created. Just use the class name followed by a period followed by the
constant name, e.g. Math.PI .

Providing communication among instances of a class
A bank account generally has an account number. The bank gives each
account a different, unique, number —it would reek havoc on the system to have
two different accounts with the same number.
Consider designing a class BankAccount , each instance of which maintains
one bank account, with an owner, account number, and balance. We need a way
to assign a new account number to each instance as it is created. To do this, we
use a static variable nextAccountNumber , which always contains the next
account number to generate. We show how the account numbers are created in
Fig. 3.7, which contains part of class BankAccount .

/** An instance is a bank account */
public class BankAccount {
/** The next account number to assign —numbers 1000..nextAccountNumber-1
have already been assigned */
private static int nextAccountNumber= 1000;

private String person; // The account owner
private int number; // The account number
private double balance; // The balance in the account

/** Constructor: an account for person p with initial balance b*/
public BankAccount(String p, double b) {
person= p
balance= b;
number= nextAccountNumber;
nextAccountNumber= nextAccountNumber + 1;

}

/** = the bank account number */
public int getNumber() { return number; }

/** = the number of accounts created thus far */
public static int numberOfAccounts()
{ return nextAccountNumber - 1000; }

}
Figure 3.7: Assigning bank account numbers (only three methods are shown)