Static Fields

• A static field belongs to the class, not to any object of the class.  Also called class field

public class BankAccount
 {
    . . .
    private double balance;
    private int accountNumber;
    private static int lastAssignedNumber = 1000;
 }

• If lastAssignedNumber was not static, each instance of
   BankAccount would have its own value of lastAssignedNumber

public BankAccount()
 {
    // Generates next account number to be assigned
    lastAssignedNumber++; // Updates the static field
    // Assigns field to account number of this bank account
    accountNumber = lastAssignedNumber; // Sets the instance field
}

• Minimize the use of static fields (static final fields are ok)

• Three ways to initialize:

  • Do nothing. Field is initialized with 0 (for numbers), false (for boolean values), or null (for objects)

  • Use an explicit initializer, such as

  public class BankAccount
{
   . . .
   private static int lastAssignedNumber = 1000;
      // Executed once,
      // when class is loaded
}

• Use a static initialization block

public class BankAccount
 {
    . . .
    private double balance;
    private int accountNumber;
    private static int lastAssignedNumber;
    static
    {
      lastAssignedNumber =1000;
    }
 }

• Static fields should always be declared as private

• Exception: Static constants, which may be either private or public

public class BankAccount
{
   . . .
   public static final double OVERDRAFT_FEE = 5; //
      Refer to it as
      // BankAccount.OVERDRAFT_FEE
}

Name two static fields of the System class.

   Answer: System.in and System.out.