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 function is the special one that's provided to allow us to make a string value of the object.  As you notice, it's called toString.  That name is fixed; you have to use that name.  It cannot take any parameters and it must return a String value.  But that's part of the convention of overriding what's provided in Java's Object class. 
       As a side note, remember up at the top, we just said 'public class BankAccount'. We didn't specify any class that we inherited from.  Just remember that all Java classes inherit from Java's Object if it's not specified.  And that's Object, with a capital 'O.' 
       So, let's look in a little deeper at the toString method.  In there is a return super.toString. The word 'super' is a keyword in the Java language, and it says that the super class' toString is going to be invoked.  Well, what is BankAccount's superclass?  It's Object, and so it's going to invoke Object's toString method and then concatenate with it (notice the plus signs) a string that has the id and another string that has the balance in it.  When we get done, we'll have some indicator specifying the object itself, followed by these two values.  If you remember when we executed it, you saw the word BankAccount@ and a hexadecimal number.  Object actually returns the class name, @, and then the address of the object.  That's Object's default behavior for its toString. You don�t have to use super.toString if you don't want to; it's there if you need it."