Interfaces 

13.2.1 What is an Interface?

An interface in Java is like a pure abstraction.
It’s a contract that says: “Any class that implements me promises to provide these methods.”

Key difference from abstract class:

• An interface has no fields (except static final constants).

• All methods are abstract by default (until Java 8 — newer Java allows default and static methods too).

• A class can implement multiple interfaces, but can extend only one class.

Why use interfaces?

• To design common behaviors that unrelated classes can share.

• To support multiple inheritance of type (Java does not support multiple inheritance of classes, but you can implement many interfaces).

• To keep your code flexible and testable — it’s easy to swap implementations.

13.2.2 How to Declare and Implement an Interface

Declaring an interface:

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

• makeSound() and eat() have no body.

• Any class that implements Animal must define both methods.

Implementing an interface:

java
public class Dog implements Animal {
    @Override
    public void makeSound() {
        System.out.println("Woof! Woof!");
    }

    @Override
    public void eat() {
        System.out.println("Dog is eating.");
    }

    public static void main(String[] args) {
        Dog d = new Dog();
        d.makeSound();
        d.eat();
    }
}

Key points:

• Use implements to connect class to interface.

• Use @Override for each method.

• The compiler checks that you implemented all methods.

13.2.3 Mini Lab: Combining Abstract Class + Interface

In real projects, you often combine both.

Example:

java
public interface Pet {
    void play();
}

public abstract class Animal {
    String name;

    abstract void makeSound();
}

public class Dog extends Animal implements Pet {
    @Override
    void makeSound() {
        System.out.println(name + " says: Woof!");
    }

    @Override
    public void play() {
        System.out.println(name + " is playing fetch!");
    }

    public static void main(String[] args) {
        Dog d = new Dog();
        d.name = "Buddy";
        d.makeSound();
        d.play();
    }
}

How this works:

• Animal defines common blueprint for all animals.

• Pet defines extra behavior for pets.

• Dog combines both — it’s an Animal that is also a Pet.