Abstract methods in Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of abstract method implementation

What is the output of the following Java code?

Java

abstract class Animal {
    abstract void sound();
}

class Dog extends Animal {
    void sound() {
        System.out.println("Bark");
    }
}

public class Main {
    public static void main(String[] args) {
        Animal a = new Dog();
        a.sound();
    }
}

A Animal sound

B Bark

C Compilation error: Cannot instantiate abstract class

D Runtime error

💡 Hint

Look at which class implements the abstract method and what method is called.

The abstract method sound() is implemented in Dog class. The object a is of type Dog, so calling a.sound() prints "Bark".

🧠 Conceptual

intermediate

Purpose of abstract methods

What is the main purpose of declaring a method as abstract in Java?

A To force subclasses to provide their own implementation of the method

B To provide a method implementation that cannot be overridden

C To allow the method to be called without creating an object

D To make the method private and inaccessible outside the class

💡 Hint

Think about what happens if a subclass does not implement an abstract method.

Abstract methods have no body and require subclasses to implement them, ensuring specific behavior is defined in subclasses.

🔧 Debug

advanced

Identify the compilation error

What error will this code produce?

Java

abstract class Shape {
    abstract void draw();
}

class Circle extends Shape {
    // No draw() method implemented
}

public class Main {
    public static void main(String[] args) {
        Shape s = new Circle();
        s.draw();
    }
}

A Compilation error: Cannot instantiate abstract class Shape

B Runtime error: draw() method not found

C Compilation error: Circle must implement abstract method draw()

D No error, prints nothing

💡 Hint

Check if all abstract methods are implemented in subclasses.

Circle does not implement the abstract method draw(), so it must be declared abstract or implement draw(). This causes a compilation error.

📝 Syntax

advanced

Correct syntax for abstract method declaration

Which option shows the correct way to declare an abstract method in Java?

A public abstract void run() {}

B void abstract run();

C public void abstract run();

D abstract void run();

💡 Hint

Abstract methods have no body and must end with a semicolon.

The correct syntax is to use 'abstract' before the return type and end with a semicolon without a method body.

🚀 Application

expert

Number of abstract methods in a class

Given the following code, how many abstract methods does class B have?

Java

abstract class A {
    abstract void m1();
    abstract void m2();
}

abstract class B extends A {
    void m1() {
        System.out.println("Implemented m1");
    }
}

A 1

B 2

C 0

D Cannot determine

💡 Hint

Count abstract methods not implemented in class B.

Class B implements m1(), so only m2() remains abstract. Therefore, B has 1 abstract method.