app.java

class Sale {
  int amount;
  
  Sale(int a) {
    amount = a;
  }

  int getAmount() {
    return this.amount;
  }
}

interface Payment {
  public int calcAmount();
}

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

// not done
class CashPayment {
  int amountTendered;
  boolean changeGiven;

  public int calcAmount() {
    return 100;
  }
}

class app {
  public static void main(String[] args) {
    CreditCardPayment c = new CreditCardPayment();
    System.out.println( c.calcAmount(new Sale(40)));
  }
}