Abstract Class in Java

The abstract keyword can only be used on classes and methods in java. An abstract class cannot be instantiated and an abstract method can have no implementation. Lets dig further.
When a class is declared with abstract keyword then that particular class cannot be instantiated.It can only be extended and the all the methods of the abstract class needs to be implemented by the class which extends our abstract class

Example of abstract class

abstract class A{}

An abstract class can have both abstract or non-abstract methods as well

Abstract Method in Java

When a method has abstract keyword then the class should definitely be a abstract class and when a method is declared abstract then it cannot have implementation.

Example of abstract method

abstract void disp();

Abstract classes and Abstract methods are like skeletons. It defines a structure, without any implementation. lets put it all together

abstract class A
{
    abstract void disp();
    public void show()
    {
        System.out.println("Show method"):
    }
}

Example of Abstract Class and Abstract Method

In this example, we have abstract class Car and an abstract method move(), the implementation to the Car class is provided by Bmw Class and Audi Class.

abstract class Car 
{
    public abstract void move();
}
class Bmw extends Car
{
    @Override
    public void move() 
    {
        System.out.println("Move method of BMW");
    }

}
class Audi extends Car
{
    @Override
    public void move() 
    {
        System.out.println("Move method of Audi");
    }

}
public class Logic 
{
    public static void main(String args[])
    {
        Car c = new Bmw();
        c.move();
    }
}

Output

Move method of BMW

In the real world scenario we will not know the implementation class and object of the implementation will be provided by Factory Method (public method something like getCarObject(),so that the user will not know about the implementation).But in this example we have created instance for Bmw and Audi classes directly.

Can Abstract Class have constructor?

An abstract class can have constructor,abstract method, non abstract method , data member and even main method as well.

abstract class Shape
{
    Shape()
    {
        System.out.println("Shape constructor called");
    }
    abstract void color();
    void size()
    {
        System.out.println("Size method called");
    }
    
}
class Rectangle extends Shape
{
    void color()
    {
        System.out.println("Color of Rectange is Blue");
    }
}
public class Logic 
{
    public static void main(String args[])
    {
        Rectangle rect = new Rectangle();
        rect.color();
        rect.size();
    }
}

Output

Shape constructor called
Color of Rectange is Blue
Size method called

Important points to remember

• If we have declared a method as abstract then you must declare the class as abstract, abstract method cannot be present in a non-abstract class.
• Abstract Class can have concrete methods(non-abstract) as well.
• Abstract method should not have implementation(no body).
• Abstract method declaration should end with a semicolon (;)
• The class extending the abstract class should implement all the abstract methods.