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.

Practice

1. Which statement about abstract classes in Java is true?

easy

A. All methods in an abstract class must be abstract.

B. Abstract classes cannot have any methods with code.

C. Abstract classes are the same as interfaces.

D. You cannot create an object directly from an abstract class.

Solution

1. Step 1: Understand abstract class instantiation

   Abstract classes cannot be instantiated directly, meaning you cannot create objects from them using new.

2. Step 2: Check method rules in abstract classes

   Abstract classes can have both abstract methods (without body) and regular methods (with code). So, not all methods must be abstract.

3. Final Answer:

   You cannot create an object directly from an abstract class. -> Option D

4. Quick Check:

   Abstract class instantiation = not allowed [OK]

Hint: Remember: abstract classes can't make objects directly [OK]

Common Mistakes:

• Thinking abstract classes can be instantiated
• Believing all methods must be abstract
• Confusing abstract classes with interfaces

2. Which of the following is the correct way to declare an abstract class in Java?

easy

A. abstract class MyClass {}

B. class abstract MyClass {}

C. abstract MyClass class {}

D. class MyClass abstract {}

Solution

1. Step 1: Recall Java syntax for abstract classes

   The keyword abstract must come before the keyword class in the declaration.

2. Step 2: Check each option's order

   Only abstract class MyClass {} has the correct order: abstract class MyClass {}. Others have incorrect keyword order.

3. Final Answer:

   abstract class MyClass {} -> Option A

4. Quick Check:

   abstract class syntax = 'abstract class' [OK]

Hint: abstract keyword always before class keyword [OK]

Common Mistakes:

• Placing abstract after class
• Mixing keyword order
• Omitting abstract keyword

3. What will be the output of the following code?

abstract class Animal {
    abstract void sound();
    void sleep() {
        System.out.println("Sleeping");
    }
}

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

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

medium

A. Sleeping\nBark

B. Compilation error

C. Bark\nSleeping

D. Runtime error

Solution

1. Step 1: Understand method calls on abstract class reference

   The variable a is of type Animal but refers to a Dog object. Calling sound() calls Dog's implementation, printing "Bark".

2. Step 2: Call the concrete method from abstract class

   Calling sleep() uses the method defined in Animal, printing "Sleeping".

3. Final Answer:

   Bark Sleeping -> Option C

4. Quick Check:

   Dog sound then Animal sleep = Bark then Sleeping [OK]

Hint: Abstract ref calls subclass method, regular method runs as is [OK]

Common Mistakes:

• Expecting compilation error for abstract class reference
• Confusing method call order
• Thinking abstract class methods can't be called

4. Identify the error in the following code:

abstract class Shape {
    abstract void draw();
}

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

class Square extends Shape {
}

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

medium

A. Square class must implement the abstract method draw()

B. Cannot create object of abstract class Shape

C. Method draw() in Circle should be abstract

D. No error, code runs fine

Solution

1. Step 1: Check abstract method implementation in subclasses

   The abstract method draw() in Shape must be implemented by all non-abstract subclasses.

2. Step 2: Verify Square class implementation

   The Square class does not implement draw() and is not declared abstract, causing a compilation error.

3. Final Answer:

   Square class must implement the abstract method draw() -> Option A

4. Quick Check:

   All abstract methods must be implemented in concrete subclasses [OK]

Hint: All abstract methods must be implemented or class must be abstract [OK]

Common Mistakes:

• Forgetting to implement abstract methods
• Thinking abstract class objects can be created
• Marking implemented methods as abstract

5. You want to design a system where different types of vehicles share a common method startEngine() but each vehicle starts differently. Which approach using abstract classes is best?

hard

A. Make Vehicle a concrete class with startEngine() implemented, subclasses override it if needed.

B. Make an abstract class Vehicle with an abstract method startEngine(), then subclasses implement it.

C. Make Vehicle an interface with startEngine() method, implemented by subclasses.

D. Make Vehicle a final class with startEngine() method.

Solution

1. Step 1: Understand the need for shared method with different implementations

   Since startEngine() must be shared but implemented differently, an abstract method enforces subclasses to provide their own version.

2. Step 2: Choose abstract class with abstract method

   Declaring Vehicle as abstract with abstract startEngine() ensures all subclasses implement it, sharing the concept but customizing behavior.

3. Final Answer:

   Make an abstract class Vehicle with an abstract method startEngine(), then subclasses implement it. -> Option B

4. Quick Check:

   Abstract class with abstract method enforces implementation [OK]

Hint: Use abstract method to force subclass-specific behavior [OK]

Common Mistakes:

• Using concrete method without forcing override
• Confusing interfaces with abstract classes
• Making class final prevents subclassing