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Abstract Class in Java

Definition:
A class definition prefixed with the abstract keyword is called an abstract class.

An abstract class may have both abstract methods and concrete methods.

Level of Abstraction

Type	Level of Abstraction
Regular Class	0% abstraction
Abstract Class	0% to 100% abstraction
Interface	100% abstraction

Why Use an Abstract Class?

An abstract class is useful to create a class with partial implementation.

Some methods can contain common implementation, while other methods can be left for subclasses to implement.

Example 1: Abstract Class

Java Code

abstract class Animal
{
    abstract void eat();

    void sleep()
    {
        System.out.println("I can sleep");
    }
}

Explanation:

Any animal can sleep and it is common to all animals. But each animal may eat differently. Therefore, sleep() is a concrete method, while eat() is an abstract method.

Example 2: Abstract and Concrete Methods

Java Code

abstract class Sample
{
    int x;

    abstract void test();   // abstract method

    void show()              // concrete method
    {

    }
}

Remember:

An abstract method does not have a method body. A concrete method has a method body and contains implementation.

Constructor in an Abstract Class

An abstract class can have a constructor, but it cannot be instantiated directly.

Important:

The constructor of an abstract class is executed when an object of its subclass is created.

Example: Constructor in Abstract Class

Java Code

abstract class Sample
{
    int x;

    Sample()
    {
        x = 15;
    }

    abstract void test();

    void show()
    {
        System.out.println("x=" + x);
    }
}

Creating Object of Abstract Class

Java Code

public class AbstractEg
{
    public static void main(String[] args)
    {
        Sample obj = new Sample();   // Error
    }
}

Error:

An abstract class cannot be instantiated directly. Therefore:

Sample obj = new Sample();

is not allowed.

Extending an Abstract Class

If you are extending an abstract class that has an abstract method, you must either:

Provide the implementation of the abstract method.
Make the new class abstract.

Java Code

abstract class Animal
{
    abstract void eat();

    void sleep()
    {
        System.out.println("I can sleep");
    }
}

class Dog extends Animal
{
    void eat()
    {
        System.out.println("Dog eats bones");
    }
}

Here, the Dog class provides the implementation of the abstract eat() method.

Interface vs Abstract Class vs Regular Class

Type	When to Use
Interface	When you want to define a capability or role.
Abstract Class	When you want to share code and enforce some method implementation.
Regular Class	When you don't need abstraction.

Easy Real-Life Analogy

Interface = Job Description
It tells the worker what he/she should be able to do.

Abstract Class = Partially Trained Worker
The worker knows some tasks but must learn other tasks.

Regular Class = Fully Trained Worker
The worker is completely ready to work.

Quick Revision

Abstract class is declared using the abstract keyword.
It may contain abstract as well as concrete methods.
An abstract class cannot be instantiated directly.
An abstract class can have a constructor.
The constructor executes when a subclass object is created.
A subclass must implement inherited abstract methods or itself be declared abstract.
Abstract classes are useful for partial implementation.

Practice Questions

1. Shape, Polygon and Rectangle

Write a Java program with an interface Shape having method area().

Create an abstract class Polygon that implements Shape and has a concrete method display().

Create a class Rectangle that extends Polygon and implements area().

2. Bank Interest Rates

Write a Java program where interface Bank has method getRateOfInterest().

Implement it in classes SBI, ICICI, and AXIS.

In main(), create objects and display interest rates.

3. Vehicle Interface

Write a Java program where interface Vehicle has:

Abstract method start()
Default method stop()
Static method service()

Implement Vehicle in class Car and demonstrate calling all methods.

4. Playable Interface

Write a Java program where interface Playable has method play().

Implement it in classes Guitar and Piano.

In main(), use an interface reference to call play() on both objects.

5. Multiple Interfaces

Write a Java program where interface Flyable has method fly().

Interface Swimmable has method swim().

Class Duck implements both interfaces.

In main(), create a Duck object and call both methods.

Challenge Question

Create an abstract class Employee containing:

A variable name
A constructor to initialize the name
An abstract method calculateSalary()
A concrete method displayName()

Create two subclasses:

FullTimeEmployee
PartTimeEmployee

Implement calculateSalary() differently in both classes and demonstrate runtime polymorphism using an Employee reference.

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