What is the difference between an interface and a class?

Example

Imagine you have different types of payment methods in an application, such as CreditCard, PayPal, and BankTransfer. You can define an interface PaymentMethod that declares a method processPayment(amount). Each payment class implements this interface and provides its own implementation of the processPayment method.

public interface PaymentMethod {
    void processPayment(double amount);
}

public class CreditCard implements PaymentMethod {
    @Override
    public void processPayment(double amount) {
        // Implementation for credit card payment
        System.out.println("Processing credit card payment of $" + amount);
    }
}

public class PayPal implements PaymentMethod {
    @Override
    public void processPayment(double amount) {
        // Implementation for PayPal payment
        System.out.println("Processing PayPal payment of $" + amount);
    }
}

In this example, both CreditCard and PayPal classes implement the PaymentMethod interface, ensuring they both have the processPayment method, but each handles the payment processing differently.

Class

A class is a blueprint for creating objects. It defines both the data (attributes) and the behavior (methods) that the objects created from the class will have. Classes can implement interfaces and can also inherit from other classes, enabling code reuse and the creation of hierarchical relationships.

Example

Continuing with the payment example, here's how you might define a PaymentProcessor class that uses the PaymentMethod interface:

public class PaymentProcessor {
    private PaymentMethod paymentMethod;

    public PaymentProcessor(PaymentMethod paymentMethod) {
        this.paymentMethod = paymentMethod;
    }

    public void executePayment(double amount) {
        paymentMethod.processPayment(amount);
    }
}

// Usage
public class Main {
    public static void main(String[] args) {
        PaymentMethod creditCard = new CreditCard();
        PaymentProcessor processor = new PaymentProcessor(creditCard);
        processor.executePayment(100.0); // Outputs: Processing credit card payment of $100.0
    }
}

Here, the PaymentProcessor class uses a PaymentMethod to execute a payment. This setup allows PaymentProcessor to work with any payment method that implements the PaymentMethod interface, providing flexibility and extensibility.

Key Differences Between Interface and Class

Purpose

• Interface: Defines a contract with method declarations without implementations. It specifies what a class should do.
• Class: Defines both data and methods, including implementations. It specifies how something should be done.

Implementation

• Interface: Cannot hold any state (no instance variables) and cannot provide method implementations (except default methods in some languages like Java).
• Class: Can have state through instance variables and provide concrete implementations of methods.

Inheritance

• Interface: Supports multiple inheritances, allowing a class to implement multiple interfaces.
• Class: Typically supports single inheritance, meaning a class can inherit from only one superclass.

Usage

• Interface: Used to define capabilities that can be shared across different classes, promoting loose coupling.
• Class: Used to create objects with specific attributes and behaviors, encapsulating data and functionality.

Recommended Courses

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