Abstract Class

An abstract class in Java is a class that is declared using the abstract keyword and may contain abstract methods (methods without a body).

Abstract classes are used when:

You want to provide a common base for multiple classes
You want to enforce certain methods in child classes

What is an Abstract Method?

An abstract method is a method that does not have a body.

java

abstract void m1();

If a method has no body, it must be declared as abstract.

Also, the class containing it must be declared as abstract.

Key Features of Abstract Class

Can contain:
- Variables
- Methods (abstract + normal)
- Constructors
- Static and instance blocks
Cannot be instantiated (no object creation)

java

abstract class A {
    abstract void m1();

    void m2() {
        System.out.println("m2 method");
    }
}

class B extends A {
    void m1() {
        System.out.println("m1 implemented in B");
    }
}

class Soft {
    public static void main(String[] args) {
        B obj = new B();
        obj.m1();
        obj.m2();
    }
}

Why We Cannot Create Object of Abstract Class?

Because an abstract class is incomplete:

It may contain abstract methods
Those methods do not have implementation

So Java does not allow creating its object.

java

abstract class Details {
    int id;
    String name;
    String email;
}

class Student extends Details {}
class Teacher extends Details {}
class Admin extends Details {}

class Soft {
    public static void main(String[] args) {
        Student s = new Student();
        s.id = 101;
        s.name = "Vivek";
        s.email = "vivek@mail.com";
    }
}

Explanation

Details is common for all
We do not create object of Details directly
Instead, we create objects of specific classes like Student

This improves clarity and design.

Rules of Abstract Class

If a class contains an abstract method → class must be abstract
If a class extends an abstract class → it must implement all abstract methods
If not implemented → child class must also be abstract

plaintext

abstract class Animal {
    abstract void eating();
    abstract void sleeping();
}

abstract class Dog extends Animal {
    public void eating() {
        System.out.println("Dog is eating");
    }
}

class MyDog extends Dog {
    public void sleeping() {
        System.out.println("Dog is sleeping");
    }
}

Factory-Based Example

plaintext

abstract class Animal {
    abstract void eating();
    abstract void sleeping();
}

class Dog extends Animal {
    void eating() {
        System.out.println("Dog eating");
    }

    void sleeping() {
        System.out.println("Dog sleeping");
    }
}

class Cat extends Animal {
    void eating() {
        System.out.println("Cat eating");
    }

    void sleeping() {
        System.out.println("Cat sleeping");
    }
}

class AnimalFactory {
    static Animal getAnimal(int type) {
        if (type == 1) return new Dog();
        else return new Cat();
    }
}

class Soft {
    public static void main(String[] args) {
        Animal a = AnimalFactory.getAnimal(1);
        a.eating();
    }
}

Important Points

Abstract class = partially implemented class
Cannot create object directly
Used for common structure
Supports code reuse
Helps in achieving abstraction

Short Interview Answer

Abstract class is a class that cannot be instantiated and may contain abstract methods. It is used to provide a common structure and partial implementation for subclasses.