Abstract vs concrete classes in Java - Hands-On Comparison

Abstract vs Concrete Classes in Java

📖 Scenario: Imagine you are creating a simple program for a zoo. Different animals have some common behaviors, but some behaviors are specific to each animal.

🎯 Goal: You will create an abstract class Animal with an abstract method and a concrete method. Then, you will create a concrete subclass Dog that implements the abstract method. Finally, you will create an object of Dog and call its methods to see the output.

📋 What You'll Learn

Create an abstract class called Animal

Add an abstract method makeSound() in Animal

Add a concrete method sleep() in Animal that prints "The animal is sleeping"

Create a concrete class called Dog that extends Animal

Implement the makeSound() method in Dog to print "Woof!"

Create an object of Dog and call both makeSound() and sleep() methods

💡 Why This Matters

🌍 Real World

Abstract classes help programmers define common features for a group of related objects, like animals in a zoo, while allowing specific details to be filled in by subclasses.

💼 Career

Understanding abstract vs concrete classes is important for designing clean, reusable code in many software development jobs, especially in object-oriented programming.

1

Create the abstract class Animal

Create an abstract class called Animal with an abstract method makeSound() and a concrete method sleep() that prints "The animal is sleeping".

Java

abstract class Animal {
    // Your code here
}

Hint

Use the abstract keyword before the class and the method makeSound(). The method sleep() should have a body that prints the message.

2

Create the concrete class Dog

Create a concrete class called Dog that extends Animal and implement the abstract method makeSound() to print "Woof!".

Java

abstract class Animal {
    abstract void makeSound();

    void sleep() {
        System.out.println("The animal is sleeping");
    }
}

class Dog extends Animal {
    // Your code here
}

Hint

Remember to use extends Animal and provide the body for makeSound() that prints "Woof!".

3

Create a Dog object and call methods

Create an object of class Dog named myDog and call its makeSound() and sleep() methods.

Java

abstract class Animal {
    abstract void makeSound();

    void sleep() {
        System.out.println("The animal is sleeping");
    }
}

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

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

Hint

Create the object with new Dog() and call the methods using the object name myDog.

4

Run the program and see the output

Run the program and observe the output. It should print "Woof!" followed by "The animal is sleeping".

Java

abstract class Animal {
    abstract void makeSound();

    void sleep() {
        System.out.println("The animal is sleeping");
    }
}

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

public class Main {
    public static void main(String[] args) {
        Dog myDog = new Dog();
        myDog.makeSound();
        myDog.sleep();
    }
}

// Run the program to see the output

Hint

Make sure you run the Main class. The output should show the dog's sound and the sleeping message.

Practice

1. Which statement best describes an abstract class in Java?

easy

A. It is the same as an interface and cannot have any methods with code.

B. It must have all methods fully implemented and can be instantiated.

C. It can have methods without implementation and cannot be instantiated directly.

D. It is a class that can only contain static methods.

Solution

1. Step 1: Understand abstract class definition

   An abstract class can have methods without implementation (abstract methods) and cannot create objects directly.

2. Step 2: Compare with other options

   Concrete classes have full method implementations and can be instantiated. Interfaces differ from abstract classes. Static-only classes are unrelated.

3. Final Answer:

   It can have methods without implementation and cannot be instantiated directly. -> Option C

4. Quick Check:

   Abstract class = no direct objects [OK]

Hint: Abstract classes can't create objects directly [OK]

Common Mistakes:

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

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

easy

A. abstract class Vehicle {}

B. class abstract Vehicle {}

C. Vehicle abstract class {}

D. class Vehicle abstract {}

Solution

1. Step 1: Recall Java syntax for abstract classes

   The keyword 'abstract' comes before 'class' followed by the class name.

2. Step 2: Check each option

   Only 'abstract class Vehicle {}' matches correct syntax. The other options have incorrect keyword order.

3. Final Answer:

   abstract class Vehicle {} -> Option A

4. Quick Check:

   abstract keyword before class name [OK]

Hint: Put 'abstract' 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 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. Runtime error

B. Animal sound

C. Compilation error

D. Bark

Solution

1. Step 1: Understand class hierarchy and method overriding

   Animal is abstract with abstract method sound(). Dog extends Animal and implements sound() printing "Bark".

2. Step 2: Analyze main method execution

   Animal reference points to Dog object. Calling a.sound() runs Dog's sound(), printing "Bark".

3. Final Answer:

   Bark -> Option D

4. Quick Check:

   Abstract method overridden = Dog's output [OK]

Hint: Abstract method calls run subclass implementation [OK]

Common Mistakes:

• Expecting abstract class method to run
• Thinking abstract class can be instantiated
• Confusing compile and runtime errors

4. Identify the error in the following code snippet:

abstract class Shape {
    abstract void draw();
}

class Circle extends Shape {
    // No draw() method implemented
}

public class Test {
    public static void main(String[] args) {
        Circle c = new Circle();
        c.draw();
    }
}

medium

A. Circle must implement the abstract method draw() or be declared abstract.

B. Cannot create object of class Circle.

C. Abstract class Shape cannot have abstract methods.

D. No error, code runs fine.

Solution

1. Step 1: Check subclass implementation of abstract methods

   Circle extends Shape but does not implement abstract method draw().

2. Step 2: Understand Java rules for abstract methods

   A concrete class must implement all abstract methods or be declared abstract itself. Circle is concrete but missing draw().

3. Final Answer:

   Circle must implement the abstract method draw() or be declared abstract. -> Option A

4. Quick Check:

   Concrete subclass must implement all abstract methods [OK]

Hint: Concrete class must implement all abstract methods [OK]

Common Mistakes:

• Thinking abstract methods can be skipped
• Assuming abstract class can't have abstract methods
• Believing object creation is the error

5. You want to design a system where different types of employees calculate their salary differently. Which approach best uses abstract and concrete classes?

hard

A. Create only concrete classes for each employee type without any abstract class.

B. Create an abstract class Employee with an abstract method calculateSalary(), then create concrete subclasses like Manager and Developer implementing it.

C. Use an interface with no methods and concrete classes implementing it.

D. Create a concrete Employee class with a fixed calculateSalary() method used by all employees.

Solution

1. Step 1: Identify need for shared rules with different implementations

   Employee types share concept of salary calculation but differ in details.

2. Step 2: Use abstract class with abstract method

   Abstract class Employee defines calculateSalary() abstractly. Subclasses implement specific logic.

3. Step 3: Evaluate other options

   Create only concrete classes for each employee type without any abstract class. lacks shared abstraction. Use an interface with no methods and concrete classes implementing it. uses interface with no methods, so no contract. Create a concrete Employee class with a fixed calculateSalary() method used by all employees. fixes salary calculation, no variation.

4. Final Answer:

   Create an abstract class Employee with an abstract method calculateSalary(), then create concrete subclasses like Manager and Developer implementing it. -> Option B

5. Quick Check:

   Abstract class sets rules, subclasses do work [OK]

Hint: Abstract class for rules, concrete classes for details [OK]

Common Mistakes:

• Not using abstraction for shared behavior
• Using concrete class with fixed method only
• Interfaces without methods don't enforce contracts