Summary - Interface

Combine interface and default methods to write a payment system.

Summary: Payment system

In this summary you'll design a payment system. The PaymentSystem interface defines the pay method contract and a default method that prints a receipt. The CreditCard class implements this contract.

First you'll define the PaymentSystem interface: a void pay(double amount) signature and a default void receipt(double amount) body. receipt prints the amount. Then the CreditCard class writes pay via implements PaymentSystem. This structure shows how the interface and default method work together in a real scenario.

Exercise Define the PaymentSystem interface

Define an interface PaymentSystem. Add the void pay(double amount) signature. Add a default void receipt(double amount) method; its body runs System.out.println("Receipt: " + amount);. Do not define an interface variable in main; only write the interface. Then in class Main with a main method run System.out.println("Ready");.

Show example solution

interface PaymentSystem {
    void pay(double amount);
    default void receipt(double amount) {
        System.out.println("Receipt: " + amount);
    }
}
class Main {
    public static void main(String[] args) {
        System.out.println("Ready");
    }
}

Exercise Implement the CreditCard class

Write both the PaymentSystem interface and the CreditCard class in this file. The interface has a void pay(double amount) signature and a default void receipt(double amount) method; the receipt body runs System.out.println("Receipt: " + amount);. Write class CreditCard implements PaymentSystem; write the public void pay(double amount) method with a body that runs System.out.println("Paid by credit card: " + amount);. In main create CreditCard card = new CreditCard(); then call card.pay(150) and card.receipt(150).

Show example solution

interface PaymentSystem {
    void pay(double amount);
    default void receipt(double amount) {
        System.out.println("Receipt: " + amount);
    }
}
class CreditCard implements PaymentSystem {
    public void pay(double amount) {
        System.out.println("Paid by credit card: " + amount);
    }
}
class Main {
    public static void main(String[] args) {
        CreditCard card = new CreditCard();
        card.pay(150);
        card.receipt(150);
    }
}

Test yourself

How should the receipt method be in the PaymentSystem interface?
- abstract method
- default method
- private method
- static method
Show answer

default method — receipt is a default method with a body.
Match the payment system parts

PaymentSystem

Contract
CreditCard

Implementing class
pay

Abstract method
receipt

default method

Complete the implements line

class CreditCard ____ PaymentSystem {
    public void pay(double amount) { }
}

Show answer

class CreditCard implements PaymentSystem {
    public void pay(double amount) { }
}

Next lesson: abstract class

We Are Enhancing Your Experience

Cookie Preferences

Strictly Necessary Cookies

Analytical Cookies