Java Interfaces

What is an Interface?

An interface is like a contract. It says, "If you use me, you must do these things." It defines what should be done, not how to do it.

Daily Life Example

Think of a TV remote. It has buttons: power, volume, channel, etc. Every brand has a remote. But the buttons are always there. This is like an interface.

The remote says: "You must give me a turnOn() and changeChannel() function." It doesn't care how the TV turns on. Each brand does it in its own way.

Why Use Interfaces?

• To create flexible code
• To allow different classes to work together
• To make your code easier to change later

How to Create an Interface

interface Animal {
  void makeSound();
  void eat();
}

This says: "Any animal must make a sound and eat."

How to Use It

class Dog implements Animal {
  public void makeSound() {
    System.out.println("Woof!");
  }
  public void eat() {
    System.out.println("Dog eats bones.");
  }
}

The Dog class agrees to the contract. It gives details for makeSound() and eat().

Another Example: Payments

You shop online. You can pay with credit card, PayPal, or a gift card. Each one works differently. But they all have one job: pay().

interface PaymentMethod {
  void pay(double amount);
}

class CreditCard implements PaymentMethod {
  public void pay(double amount) {
    System.out.println("Paid with credit card: $" + amount);
  }
}

class PayPal implements PaymentMethod {
  public void pay(double amount) {
    System.out.println("Paid with PayPal: $" + amount);
  }
}

Interface vs Class

• Class: Tells what to do AND how to do it
• Interface: Only tells what to do
• A class can implement many interfaces

Java 8 and Later

In Java 8 and later, interfaces can have default methods too.

interface Greeting {
  default void sayHello() {
    System.out.println("Hello!");
  }
}

Summary

• Interfaces define rules
• Classes follow these rules by implements keyword
• Interfaces help us write flexible code