Java is an object-oriented programming language that allows developers to create complex and sophisticated applications by using a variety of programming constructs. One such construct is the abstract class, which is a class that cannot be instantiated on its own and is intended to be used as a base class for other classes.

In this article, we will explore the concept of abstract classes in Java, including their definition, syntax, and usage. We will discuss the benefits and drawbacks of using abstract classes in Java, and provide examples of how to create and use abstract classes in real-world scenarios.

What is an Abstract Class?

• In Java, an abstract class is declared using the abstract keyword. An abstract class can contain both abstract and non-abstract methods. A non-abstract method is a method that has an implementation, while an abstract method does not have an implementation.
• Abstract class is not exactly a class, but it is blueprint. By providing a blueprint for other classes to follow, abstract classes can help in achieving code reuse, simplifying the implementation of subclasses, and providing a level of abstraction that can hide the implementation details from the subclasses.
• An abstract method should not be defined inside the parent class. The abstract methods of an abstract class must be defined in its subclass.
• We cannot declare abstract constructors or abstract static methods.
• We can define constructor inside an abstract class because every class has a constructor either default or the user define.
• An abstract class cannot be instantiated, which means that you cannot create an object of an abstract class. Instead, you can only create objects of its subclasses. When a class extends an abstract class, it must implement all the abstract methods defined in the abstract class.
• By making a method “final”, we ensure that the method is not redefine in a subclass, i .e. the method can never be subclassed. Java also provide opposite to it i.e. a method must always redefined in a subclass using keyword “abstract”.

Example of Abstract Class:

public abstract class Shape {
   public abstract double calculateArea();
   public void display(){
      System.out.println("This is a Shape");
   }
}

In this example, Shape is an abstract class that defines an abstract method called calculateArea() and a non-abstract method called display(). The calculateArea() method does not have an implementation, while the display() method has an implementation.

When a subclass of Shape is created, it must implement the calculateArea() method. Here’s an example of a subclass of Shape called Circle:

public class Circle extends Shape {
   private double radius;
   
   public Circle(double radius){
      this.radius = radius;
   }
   
   public double calculateArea(){
      return Math.PI * radius * radius;
   }
}

In this example, Circle extends the Shape abstract class and implements the calculateArea() method. The calculateArea() method calculates the area of a circle using the formula πr².

Benefits of Abstract Classes in Java:

1. Code Reusability: Abstract classes can be used to define a set of methods that can be reused by its subclasses. This can save a lot of development time and effort, as developers can use the same set of methods in different classes.

2. Abstraction: Abstract classes allow developers to define abstract methods that do not have an implementation. This can help in abstraction, where the implementation details can be hidden from the subclasses.

3. Inheritance: Abstract classes can be used as a base class for other classes, which can inherit its properties and methods. This can help in creating a hierarchy of classes that can be used to organize the code and simplify its implementation.

4. Polymorphism: Abstract classes can be used to achieve polymorphism, where different objects can be treated as if they belong to the same class. This can help in creating flexible and extensible code.

Drawbacks of Abstract Classes in Java:

1. Inflexibility: Abstract classes can be inflexible, as once a class is declared as abstract, it cannot be instantiated. This can limit the use of abstract classes in certain situations.

2. Coupling: Abstract classes can create coupling between the base class and its subclasses, which can make the code less flexible and harder to maintain.

3. Complexity: Abstract classes can make the code more complex, as they introduce additional layers of abstraction and inheritance. This can make it harder to understand and debug the code.

4. Overhead: Abstract classes can introduce overhead in the code, as they require additional memory and processing power to maintain the class hierarchy and method definitions.

After reading the above, you may wonder that What is the benefit to declare an abstract class without using an abstract method inside in it?

So, the answer is that In Java, an abstract class is a class that cannot be instantiated on its own and can contain both abstract and non-abstract methods. While it may seem counterintuitive to declare an abstract class without any abstract methods, there are still benefits to doing so.

One of the primary benefits of declaring an abstract class without using an abstract method is that it can serve as a base class for other classes. This allows other classes to inherit the properties and methods of the abstract class, which can help in code reuse and simplify the implementation of the subclasses.

By declaring a class as abstract, you are indicating that it is intended to be used as a base class, and not as a standalone class. This can help in organizing the code and making it more modular, as the abstract class can define a set of common methods and properties that are shared by its subclasses.

Another benefit of declaring an abstract class without using an abstract method is that it can help in future-proofing the code. By defining a class as abstract, you are indicating that it is intended to be extended and modified in the future. This can help in accommodating future changes and requirements, without having to rewrite the entire codebase.

In conclusion, abstract classes are an essential part of object-oriented programming in Java. They provide a powerful mechanism for code reuse and provide a level of abstraction that can hide the implementation details from the subclasses.

We have explored the definition, syntax, and usage of abstract classes in Java. We have discussed the benefits and drawbacks of using abstract classes, and provided examples of how to create and use them in real-world scenarios.

Whether you are a beginner or an experienced Java developer, understanding abstract classes is crucial for building flexible, maintainable, and extensible applications. By using abstract classes, you can create a modular and well-organized codebase that is easy to maintain and extend in the future.