interface IBank {
  void changeBalance( int amount );
  int  getBalance();
}

class Factory {
  static IBank makeBank() { return new BankImpl(); }
}

public class Client {
  public static void main( String[] args ) {
    IBank bank = Factory.makeBank();
    int[] nums = { 10, 0, 20, 0, -15, 0 };
    for (int i=0; i < nums.length; i++)
      if (nums[i] != 0) bank.changeBalance( nums[i] );
      else System.out.println( "balance is " + bank.getBalance() );
} }


class BankImpl implements IBank {
  private int balance;

  public BankImpl() { balance = 0; }

  public void changeBalance( int amount ) {
    balance += amount;
    // System.out.println( "balance is " + balance ); 
  }

  public int getBalance() { return balance; }
}