Implementing Interfaces

Part of the Object Oriented Programming section of Coddy's Java journey — lesson 35 of 87.

To use an interface, a class must implement it using the implements keyword. The class then provides concrete implementations for all methods declared in the interface:

public interface Drawable {
    void draw();
}

public class Circle implements Drawable {
    private double radius;
    
    public Circle(double radius) {
        this.radius = radius;
    }
    
    @Override
    public void draw() {
        System.out.println("Drawing a circle with radius " + radius);
    }
}

If a class implements an interface but doesn't provide implementations for all its methods, the compiler will throw an error. The @Override annotation is optional but recommended as it helps catch mistakes.

Just like with abstract classes, you can use an interface as a reference type. This enables polymorphism where different classes implementing the same interface can be treated uniformly:

Drawable shape1 = new Circle(5.0);
Drawable shape2 = new Rectangle(4.0, 3.0);

shape1.draw();  // Drawing a circle with radius 5.0
shape2.draw();  // Drawing a rectangle 4.0 x 3.0

This is powerful because code that works with Drawable references doesn't need to know the specific class type. It only cares that the object can be drawn.

In the next lesson, you'll see how a single class can implement multiple interfaces simultaneously.

Challenge

Easy

Let's build a speaker system that demonstrates how classes implement interfaces. You'll create an interface that defines what any speaker can do, then build two different speaker types that each implement this contract in their own way.

You'll organize your code across four files:

Speaker.java: Define an interface called Speaker that declares what any speaker must be able to do. Every speaker should have a speak() method that returns a String—this is the contract that all implementing classes must fulfill.
Human.java: Create a class that implements the Speaker interface. A Human has a name field (String). Include a constructor to initialize the name, and implement the speak() method to return: [name] says: Hello everyone!
Robot.java: Create another class that implements the Speaker interface. A Robot has a model field (String). Include a constructor to initialize the model, and implement the speak() method to return: Robot [model] says: Beep boop!
Main.java: Bring everything together using interface-based polymorphism. You'll receive two inputs: a human name and a robot model. Create a Human and a Robot, then store them in an array of Speaker references. Loop through the array and print the result of calling speak() on each speaker.

You will receive two inputs: the human's name (String) and the robot's model (String).

Your output should show two lines—one for each speaker. Notice how both the Human and Robot can be treated uniformly through the Speaker interface, even though they speak in completely different ways!

Cheat sheet

A class uses the implements keyword to implement an interface and must provide concrete implementations for all methods declared in the interface:

public interface Drawable {
    void draw();
}

public class Circle implements Drawable {
    private double radius;
    
    public Circle(double radius) {
        this.radius = radius;
    }
    
    @Override
    public void draw() {
        System.out.println("Drawing a circle with radius " + radius);
    }
}

The @Override annotation is optional but recommended as it helps catch mistakes.

Interfaces can be used as reference types, enabling polymorphism where different classes implementing the same interface can be treated uniformly:

Drawable shape1 = new Circle(5.0);
Drawable shape2 = new Rectangle(4.0, 3.0);

shape1.draw();  // Drawing a circle with radius 5.0
shape2.draw();  // Drawing a rectangle 4.0 x 3.0

Try it yourself

import java.util.Scanner;

class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        
        // Read inputs
        String humanName = scanner.nextLine();
        String robotModel = scanner.nextLine();
        
        // TODO: Create a Human object with the given name
        
        // TODO: Create a Robot object with the given model
        
        // TODO: Create an array of Speaker references containing both objects
        
        // TODO: Loop through the array and print the result of speak() for each speaker
    }
}