These challenges are cool and all, but in this particular one - you don't really practice inheritance, do you? The "inheritance" part is pretty much done for you and you're basicallly practicing writing a simple method.

Still, I love HackerRank and this is just a minor issue.

"then modify the main method accordingly" would you be so kind and remove that statement ? or... let us modify the main method then....

void sing() { System.out.println("I am singing"); }

class Bird extends Animal implements asd{ void fly(){ System.out.println("I am flying"); } public void sing() { System.out.println("I am singing"); } } interface asd{ void sing(); }