Abstract vs concrete classes in Java - Practice Questions

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of abstract class method call

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 Bark

B Animal sound

C Compilation error: cannot instantiate abstract class

D Runtime error

💡 Hint

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

The abstract class Animal cannot be instantiated directly, but Dog extends Animal and provides implementation for the abstract method sound(). The object a is of type Animal but refers to a Dog instance, so Dog's sound() method is called, printing "Bark".

❓ Predict Output

intermediate

What happens when instantiating an abstract class?

What error does the following code produce?

Java

abstract class Vehicle {
    abstract void move();
}

public class Main {
    public static void main(String[] args) {
        Vehicle v = new Vehicle();
        v.move();
    }
}

A Runtime error: NullPointerException

B Compilation error: cannot instantiate abstract class

C No output, program runs successfully

D Compilation error: method move() not implemented

💡 Hint

Can you create an object of an abstract class directly?

Abstract classes cannot be instantiated directly. Trying to create an object of Vehicle causes a compilation error.

❓ Predict Output

advanced

Output with abstract and concrete class methods

What is the output of this Java program?

Java

abstract class Shape {
    void display() {
        System.out.println("Shape display");
    }
    abstract void draw();
}

class Circle extends Shape {
    void draw() {
        System.out.println("Circle draw");
    }
    void display() {
        System.out.println("Circle display");
    }
}

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

A

Circle display
Circle draw

B

Shape display
Shape draw

C

Shape display
Circle draw

D Compilation error: cannot override display()

💡 Hint

Which display() method is called? The one in Shape or Circle?

The object s is of type Shape but refers to a Circle instance. The display() method is overridden in Circle, so Circle's display() runs. The draw() method is abstract in Shape and implemented in Circle, so Circle's draw() runs.

❓ Predict Output

advanced

Which option causes a compilation error?

Given the abstract class and subclass below, which option will cause a compilation error?

Java

abstract class Device {
    abstract void start();
}

class Phone extends Device {
    void start() {
        System.out.println("Phone starting");
    }
}

public class Main {
    public static void main(String[] args) {
        // Options below
    }
}

A Device d = new Phone(); d.start();

B Phone p = new Phone(); p.start();

C Device d = new Device(); d.start();

D Device d; d = new Phone(); d.start();

💡 Hint

Can you create an instance of an abstract class?

Option C tries to instantiate the abstract class Device directly, which is not allowed and causes a compilation error. The other options create instances of Phone, which is concrete.

🧠 Conceptual

expert

Why use abstract classes instead of concrete classes?

Which of the following is the best reason to use an abstract class instead of a concrete class in Java?

A To improve runtime performance by avoiding method calls

B To prevent any subclass from overriding methods

C To allow creating objects directly from the abstract class

D To provide a common template with some methods implemented and others left for subclasses to define

💡 Hint

Think about what abstract classes allow you to do with methods.

Abstract classes let you define some common behavior and force subclasses to implement specific methods. They cannot be instantiated directly. This helps design flexible and reusable code.