Abstract classes in Java - Mini Project: Build & Apply

Using Abstract Classes in Java

📖 Scenario: You are creating a simple program to represent different types of vehicles. Each vehicle has a method to display its sound. You want to use an abstract class to define the common structure for all vehicles.

🎯 Goal: Build a Java program that uses an abstract class called Vehicle with an abstract method makeSound(). Then create two subclasses Car and Bike that provide their own implementation of makeSound(). Finally, create objects of these subclasses and call their makeSound() methods.

📋 What You'll Learn

Create an abstract class called Vehicle with an abstract method makeSound().

Create a class Car that extends Vehicle and implements makeSound() to print "Car goes vroom".

Create a class Bike that extends Vehicle and implements makeSound() to print "Bike goes ring ring".

In the Main class, create one Car object and one Bike object.

Call the makeSound() method on both objects and print the results.

💡 Why This Matters

🌍 Real World

Abstract classes help organize code when many objects share common features but have different specific behaviors, like different types of vehicles.

💼 Career

Understanding abstract classes is important for designing flexible and reusable code in many software development jobs.

1

Create the abstract class Vehicle

Create an abstract class called Vehicle with an abstract method makeSound() that returns void.

Java

abstract class Vehicle {
    // Your code here
}

Hint

Use the keyword abstract before the class and method. The method has no body.

2

Create the Car and Bike classes

Create a class called Car that extends Vehicle and implements the makeSound() method to print "Car goes vroom". Also create a class called Bike that extends Vehicle and implements the makeSound() method to print "Bike goes ring ring".

Java

abstract class Vehicle {
    abstract void makeSound();
}

// Your code here

Hint

Remember to use extends Vehicle and override the makeSound() method with void makeSound().

3

Create objects of Car and Bike

Create a class called Main with a main method. Inside the main method, create one object of Car called myCar and one object of Bike called myBike.

Java

abstract class Vehicle {
    abstract void makeSound();
}

class Car extends Vehicle {
    void makeSound() {
        System.out.println("Car goes vroom");
    }
}

class Bike extends Vehicle {
    void makeSound() {
        System.out.println("Bike goes ring ring");
    }
}

class Main {
    public static void main(String[] args) {
        // Your code here
    }
}

Hint

Use new Car() and new Bike() to create the objects inside the main method.

4

Call makeSound() on the objects and print output

In the main method of the Main class, call the makeSound() method on the myCar and myBike objects to print their sounds.

Java

abstract class Vehicle {
    abstract void makeSound();
}

class Car extends Vehicle {
    void makeSound() {
        System.out.println("Car goes vroom");
    }
}

class Bike extends Vehicle {
    void makeSound() {
        System.out.println("Bike goes ring ring");
    }
}

class Main {
    public static void main(String[] args) {
        Car myCar = new Car();
        Bike myBike = new Bike();
        // Your code here
    }
}

Hint

Call myCar.makeSound() and myBike.makeSound() to print the sounds.

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