app.java

class app {

  public static void main(String[] args) {
    CreditCardPayment c = new CreditCardPayment();
    System.out.println("need to pay: 50" + c.calcAmount() );
  }
}

CreditCardPayment.java

  public class CreditCardPayment implements Payment {
    int amount;
    CreditCardPayment() {
      this.amount = 500;
    }
    public int calcAmount() {
      return amount - sale.getAmount();
    }
    public boolean authorize() {
       return true;
    }
  }

Payment.java

  public interface Payment {
    public Sale sale = new Sale(50);
    public int calcAmount();
  }

Sale.java

  public class Sale {
    int amount;
    Sale(int a) {
      amount = a;
    }
    int getAmount() {
      return this.amount;
    }
  }