Java / OOP / Interface - https://www.minte9.com/java/oop-introduction-interface-1186

interface.java

/** 
 * Define Dog class which 
 * Dog class must implement the correct class
 */
class App {
    public static void main(String[] args) {
        Dog d = new Dog();
        d.bark(); // The dog is barking
    }
}
interface Canine {
    public abstract void bark();
}
interface Feline {
    public abstract void roar();
}

/**
 * SOLUTION
 */
class Dog implements Canine {

    @Override
    public void bark() {
        System.out.println("The dog is barking");
    }
}