Abstract classes in Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of abstract class method call

What is the output of this Java program that uses an abstract class and a subclass?

Java

abstract class Animal {
    abstract void sound();
}

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

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

A Animal

B Runtime error: NullPointerException

C Compilation error: Cannot instantiate abstract class

D Woof

💡 Hint

Look at which class is instantiated and which method is called.

The abstract class Animal cannot be instantiated directly, but the variable 'a' holds a Dog object. Calling a.sound() calls Dog's implementation, printing 'Woof'.

🧠 Conceptual

intermediate

Why use abstract classes?

Which of the following is the main reason to use an abstract class in Java?

A To create objects directly without subclassing

B To allow multiple inheritance of implementation

C To provide a common base with some shared code and force subclasses to implement specific methods

D To improve runtime performance by avoiding method calls

💡 Hint

Think about what abstract classes enforce and what they provide.

Abstract classes let you share code among related classes and require subclasses to implement abstract methods, ensuring a common interface.

🔧 Debug

advanced

Identify the compilation error

What error does this code produce when compiled?

Java

abstract class Vehicle {
    abstract void start();
}

class Car extends Vehicle {
    void start() {
        System.out.println("Car started");
    }
}

class Bike extends Vehicle {
}

public class Test {
    public static void main(String[] args) {
        Vehicle v = new Bike();
        v.start();
    }
}

A Runtime error: NullPointerException

B Compilation error: Bike must implement abstract method start()

C Compilation error: Cannot instantiate abstract class Vehicle

D No error, output: Car started

💡 Hint

Check if all abstract methods are implemented in subclasses.

Bike does not implement the abstract method start(), so it must be declared abstract or implement the method. Otherwise, compilation fails.

📝 Syntax

advanced

Correct abstract method declaration

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

Java

abstract class Shape {
    // method declaration here
}

A abstract void draw();

B void abstract draw();

C abstract void draw() {}

D void draw();

💡 Hint

Abstract methods have no body and use the 'abstract' keyword before the return type.

The correct syntax is 'abstract void draw();' without a method body. Option A has wrong keyword order, C has a body which is not allowed, D lacks 'abstract' keyword.

🚀 Application

expert

Number of objects created

Given the code below, how many objects are created when main runs?

Java

abstract class Fruit {
    Fruit() {
        System.out.println("Fruit created");
    }
}

class Apple extends Fruit {
    Apple() {
        System.out.println("Apple created");
    }
}

public class Main {
    public static void main(String[] args) {
        Fruit f = new Apple();
    }
}

A One object: an Apple instance

B Two objects: one Fruit and one Apple

C No objects, only references

D Compilation error: Cannot instantiate abstract class Fruit

💡 Hint

Remember how constructors work in inheritance and abstract classes.

Only one object is created: an Apple instance. The Fruit constructor runs as part of Apple construction, but no separate Fruit object is created.