Interfaces

Relax. We'll talk through this in plain words — no textbook voice.

Interface is like a contract for what a class must be able to do. It declares method signatures, and when a class implements it, that class has to write those methods. It's like the interface saying, “If you're going to deliver, you must have a deliver() method.”

java

interface Notifier {
  void send(String message);
}

class EmailNotifier implements Notifier {
  public void send(String message) {
    System.out.println("Email sent: " + message);
  }
}

class SmsNotifier implements Notifier {
  public void send(String message) {
    System.out.println("SMS sent: " + message);
  }
}

public class Main {
  public static void main(String[] args) {
    Notifier notifier = new EmailNotifier();
    notifier.send("Welcome to Java!");
  }
}

Notifier interface requires a send method. EmailNotifier and SmsNotifier implement the interface, each writing its own version of the send behavior.

You should see

Email sent: Welcome to Java!

Real-World Use

Using interfaces for interchangeable systems like Email/SMS/Push notifications, Stripe/PayPal payments, or local/cloud storage providers keeps your architecture clean.

Easy traps

When you implement an interface method, the access modifier has to be public — otherwise you'll hit a weaker-access-privileges error.