Default methods in Java - Mini Project: Build & Apply

Using Default Methods in Java Interfaces

📖 Scenario: Imagine you are building a simple system for different types of vehicles. Each vehicle can describe itself. You want to provide a common description method that all vehicles can use, but also allow each vehicle to add its own details.

🎯 Goal: You will create an interface with a default method that provides a basic description. Then, you will create a class that implements this interface and uses the default method. Finally, you will print the description of the vehicle.

📋 What You'll Learn

Create an interface called Vehicle with a default method describe() that returns a String.

The default describe() method should return the String "This is a vehicle."

Create a class called Car that implements the Vehicle interface.

In the Car class, do not override the describe() method.

Create a main method to create an instance of Car and print the result of calling describe().

💡 Why This Matters

🌍 Real World

Default methods allow adding new functionality to interfaces without breaking existing code. This is useful when evolving APIs or libraries.

💼 Career

Understanding default methods is important for Java developers to maintain and extend interfaces safely in large projects.

1

Create the Vehicle interface with a default method

Create an interface called Vehicle with a default method named describe() that returns the String "This is a vehicle."

Java

// Your code here

Need a hint?

Use the default keyword inside the interface to create a method with a body.

2

Create the Car class implementing Vehicle

Create a class called Car that implements the Vehicle interface. Do not override the describe() method.

Java

public interface Vehicle {
    default String describe() {
        return "This is a vehicle.";
    }
}

// Your code here

Need a hint?

Just declare the class and implement the interface. No need to write the describe method again.

3

Add the main method to create Car and call describe

Add a main method inside the Car class. Inside main, create an instance of Car named myCar and call myCar.describe(), storing the result in a variable called description.

Java

public interface Vehicle {
    default String describe() {
        return "This is a vehicle.";
    }
}

public class Car implements Vehicle {
    // No override of describe()

    // Your code here
}

Need a hint?

Remember the syntax for the main method and how to create an object and call a method.

4

Print the description

Inside the main method of the Car class, add a line to print the description variable using System.out.println().

Java

public interface Vehicle {
    default String describe() {
        return "This is a vehicle.";
    }
}

public class Car implements Vehicle {
    // No override of describe()

    public static void main(String[] args) {
        Car myCar = new Car();
        String description = myCar.describe();
        // Your code here
    }
}

Need a hint?

Use System.out.println(description); to print the text.