Summary - Interface
- Java course
- Intermediate
- Lesson 52/116
- 5 min
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.
Practice 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");.
Starter code
// Define the PaymentSystem interface
// Add pay signature and default receipt method
// In Main print a ready messageSample 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");
}
}Expected output
Ready
Practice 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).
Starter code
// Define the PaymentSystem interface
// Write CreditCard implements PaymentSystem
// Write the pay method with a body
// Call pay and receipt on the card in mainSample 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);
}
}Expected output
Paid by credit card: 150 Receipt: 150
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 How should the receipt method be in the PaymentSystem interface?
- abstract method
- default method Correct answer
- private method
- static method
Explanation: receipt is a default method with a body.
2 Match the payment system parts
| Term | Meaning |
|---|---|
PaymentSystem |
Contract |
CreditCard |
Implementing class |
pay |
Abstract method |
receipt |
default method |
3 Complete the implements line
class CreditCard ____ PaymentSystem {
public void pay(double amount) { }
}class CreditCard implements PaymentSystem {
public void pay(double amount) { }
}Explanation: CreditCard implements PaymentSystem.