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.

Practice

1. Which statement about abstract methods in Java is correct?

easy

A. Abstract methods have no body and must be implemented by subclasses.

B. Abstract methods can have a body and be called directly.

C. Abstract methods are only used in interfaces, not classes.

D. Abstract methods can be private and final.

Solution

1. Step 1: Understand abstract method definition

   Abstract methods declare a method signature without a body, forcing subclasses to provide implementation.

2. Step 2: Check each option against the definition

   Abstract methods have no body and must be implemented by subclasses. correctly states abstract methods have no body and must be implemented by subclasses. Options A, B, and D are incorrect because abstract methods cannot have bodies, are used in abstract classes (not only interfaces), and cannot be private or final.

3. Final Answer:

   Abstract methods have no body and must be implemented by subclasses. -> Option A

4. Quick Check:

   Abstract method = no body, must implement [OK]

Hint: Abstract methods have no body and require subclass implementation [OK]

Common Mistakes:

• Thinking abstract methods can have a body
• Confusing abstract methods with interface methods
• Believing abstract methods can be private or final

2. Which of the following is the correct syntax to declare an abstract method in Java?

easy

A. public void abstract display();

B. void abstract display();

C. abstract public void display() {}

D. public abstract void display();

Solution

1. Step 1: Recall abstract method syntax

   Abstract methods must be declared with the keyword abstract, have no body, and specify the return type and method name.

2. Step 2: Evaluate each option

   public abstract void display(); correctly declares an abstract method with no body. public void abstract display(); incorrectly places abstract after void. abstract public void display() {} incorrectly provides a method body ({}). void abstract display(); misses the access modifier and places abstract incorrectly.

3. Final Answer:

   public abstract void display(); -> Option D

4. Quick Check:

   abstract method = abstract + no body [OK]

Hint: Abstract method syntax: 'abstract' before return type, no body [OK]

Common Mistakes:

• Adding method body {} to abstract methods
• Placing 'abstract' keyword incorrectly
• Omitting access modifiers (though optional)

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

abstract class Animal {
    abstract void sound();
}

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();
    }
}

medium

A. Animal sound

B. Bark

C. Compilation error due to abstract method

D. Runtime error

Solution

1. Step 1: Understand class hierarchy and method implementation

   Class Animal is abstract with an abstract method sound(). Class Dog extends Animal and provides implementation for sound() that prints "Bark".

2. Step 2: Analyze main method execution

   In main, an Animal reference points to a Dog object. Calling sound() invokes Dog's implementation, printing "Bark".

3. Final Answer:

   Bark -> Option B

4. Quick Check:

   Abstract method implemented in subclass = prints subclass output [OK]

Hint: Abstract method calls subclass implementation at runtime [OK]

Common Mistakes:

• Expecting abstract method to run without implementation
• Confusing abstract class instantiation
• Thinking abstract method prints default text

4. Identify the error in the following Java 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. Method draw() in Circle should be abstract

B. Cannot create object of abstract class Shape

C. Class Square must implement the abstract method draw()

D. No error, code runs fine

Solution

1. Step 1: Check abstract method implementation in subclasses

   Class Shape has abstract method draw(). Circle correctly implements it. Square does not implement draw().

2. Step 2: Understand consequences of missing implementation

   Since Square does not implement the abstract method, it must be declared abstract or implement the method. Otherwise, compilation error occurs.

3. Final Answer:

   Class Square must implement the abstract method draw() -> Option C

4. Quick Check:

   Subclass must implement all abstract methods [OK]

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

Common Mistakes:

• Forgetting to implement abstract methods in subclasses
• Trying to instantiate abstract classes directly
• Misunderstanding abstract method requirements

5. You want to design a Java program where different vehicle types must provide their own startEngine() behavior. Which approach using abstract methods is best to ensure this?

hard

A. Create an abstract class Vehicle with an abstract method startEngine(), then subclass it for each vehicle type implementing startEngine().

B. Create a concrete class Vehicle with a startEngine() method that prints a generic message, and override it in subclasses.

C. Create an interface Vehicle with a default startEngine() method, and override it in subclasses if needed.

D. Create a class Vehicle with a private startEngine() method and call it from subclasses.

Solution

1. Step 1: Understand requirement for mandatory implementation

   The program requires each vehicle type to provide its own startEngine() behavior, so subclasses must be forced to implement this method.

2. Step 2: Evaluate options for enforcing implementation

   Create an abstract class Vehicle with an abstract method startEngine(), then subclass it for each vehicle type implementing startEngine(). uses an abstract class with an abstract method, forcing subclasses to implement startEngine(). Create a concrete class Vehicle with a startEngine() method that prints a generic message, and override it in subclasses. provides a generic method that can be overridden but does not force implementation. Create an interface Vehicle with a default startEngine() method, and override it in subclasses if needed. uses an interface with a default method, which subclasses may skip overriding. Create a class Vehicle with a private startEngine() method and call it from subclasses. uses a private method, which is not accessible to subclasses.

3. Final Answer:

   Create an abstract class Vehicle with an abstract method startEngine(), then subclass it for each vehicle type implementing startEngine(). -> Option A

4. Quick Check:

   Abstract method forces subclass implementation [OK]

Hint: Use abstract method in abstract class to force subclass implementation [OK]

Common Mistakes:

• Using concrete methods without forcing override
• Using private methods inaccessible to subclasses
• Relying on default interface methods without enforcement