LESSON 11

3.1 Lesson Outcomes

After completing this lesson, learners will be able to:

• Define abstract classes in Java.
• Create abstract classes and abstract methods.
• Explain the purpose of abstraction in Java.
• Differentiate between abstract and regular classes.
• Implement abstract classes in Java applications.

3.2 Overview

Object-oriented programming allows developers to create generalized structures that can be shared by multiple related classes. Abstract classes are used when a class should provide common functionality but should not be instantiated directly.

This lesson introduces learners to abstract classes and explains how abstraction helps simplify software design and improve code organization. Learners will also explore abstract methods and how subclasses implement inherited abstract behaviour.

Abstract classes are commonly used in:

• banking systems,
• enterprise software,
• frameworks,
• management systems,
• and application architectures.

Understanding abstract classes is essential because abstraction is one of the key principles of object-oriented programming.

KT1101 — Introduction to Abstract Classes

An abstract class is a class declared using the abstract keyword.

Abstract classes:

• cannot be instantiated directly,
• may contain abstract methods,
• and may also contain normal methods.

Syntax of an Abstract Class

abstract class Animal {

}

Explanation

Importance of Abstract Classes

Abstract classes help developers:

• create generalized templates,
• enforce method implementation,
• improve code reuse,
• and support abstraction.

KT1102 — Abstract Methods

An abstract method is a method declared without a body.

Abstract methods must be implemented by subclasses.

Syntax of an Abstract Method

abstract void sound();

Example

abstract class Animal {

    abstract void sound();
}

Explanation

Importance of Abstract Methods

Abstract methods:

• define required behaviour,
• enforce consistency,
• and support standardized application design.

KT1103 — Implementing Abstract Classes

Subclasses inherit abstract classes and implement abstract methods.

Example

abstract class Animal {

    abstract void sound();
}

Subclass Example

class Dog extends Animal {

    void sound() {

        System.out.println("Dog barks");
    }
}

Main Class Example

public class Main {

    public static void main(String[] args) {

        Dog d = new Dog();

        d.sound();
    }
}

Output

Dog barks

Importance of Implementation

Implementation allows subclasses to:

• define specific behaviour,
• customize functionality,
• and inherit shared structures.

KT1104 — Abstract Classes with Normal Methods

Abstract classes may also contain fully implemented methods.

Example

abstract class Animal {

    void sleep() {

        System.out.println("Animal is sleeping");
    }
}

Explanation

Abstract classes may contain:

• abstract methods,
• normal methods,
• variables,
• and constructors.

Importance of Normal Methods

Normal methods:

• provide reusable functionality,
• reduce duplicated code,
• and simplify software maintenance.

KT1105 — Difference Between Abstract and Regular Classes

Example of a Regular Class

class Student {

}

Objects can be created directly from regular classes.

KT1106 — Real-World Use of Abstract Classes

Abstract classes are commonly used in:

• banking applications,
• employee management systems,
• game development,
• frameworks,
• and enterprise software.

Examples:

• Vehicle class
• Employee class
• Account class
• Shape class

Abstract classes define shared structures while allowing subclasses to provide specific implementations.

3.5 Key Notes / Summary

• Abstract classes use the abstract keyword.
• Abstract classes cannot be instantiated directly.
• Abstract methods do not contain method bodies.
• Subclasses implement abstract methods.
• Abstract classes may contain normal methods.
• Abstraction improves code reuse and application structure.
• Abstract classes support object-oriented programming design.

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