public class BankAccount {

    public BankAccount () { this(0.0) ; } 
   
    public BankAccount( double initBal )
       { balance_=initBal; id_=newID(); }

    public double balance() { return balance_; }
    public int   id()       { return id_; }

    public void withdraw(double amt) { balance_ -=amt; }
    public void  deposit(double amt) { balance_ +=amt; }

    public String toString()    
       { return super.toString() + "(id:" + id_ + ", bal:" + balance_ + ")" ; }

    // Instance variables
    private double balance_;
    private int id_;

    // Class variable and class method
    public static int nextID_=100;
    private static int newID() { return nextID_++; }

    // Another "special" class method
    public static void main(String args[]) {

    BankAccount a=new BankAccount(15.25);
    BankAccount b=new BankAccount();

    System.out.println("a=" + a.toString() );
    System.out.println("b=" + b.toString() );

    a.withdraw(5.50);
    b.deposit(125.99);

    System.out.println("a=" + a);
    System.out.println("b=" + b);

    System.exit(0);     
    } // no semi-colon
}

Detailed Description:

"The next things are two instance variables: id_ and balance_. One is a double and one's an integer (int), and they have their trailing underscores to represent the fact that they're instance variables. They're also private, which says that they�re in the center of the doughnut -- they're encapsulated. Anything that is public is considered on the outside. It's generally a good idea to make data private, but there are cases where it's appropriate not to make it private."