Java Abstract Classes – Essential Understanding

Introduction

Object-oriented programming (OOP) is a key concept in the world of Java programming, and abstract classes are one of the important tools that facilitate Object-oriented programming. In this article, we’ll walk you through abstract classes in Java, providing a clear understanding of what they are, why they’re used, and how to use them effectively with the help of straightforward examples.

What Is An Abstract Class?

An abstract class in Java is a class that cannot be instantiated, meaning you cannot create an object of an abstract class. It typically contains one or more abstract methods (methods declared without an implementation). An abstract class is declared using the keyword abstract.

Here’s an example of an abstract class:

public abstract class Animal {
    public abstract void sound();
}

In the above code, Animal is an abstract class and sound() is an abstract method.

Why Do We Need Abstract Classes?

Abstract classes are primarily used to achieve abstraction, a core principle of OOP. Abstraction hides the complex details and shows only the necessary information.

With abstract classes, we can declare methods without implementing them, just like interfaces. However, unlike interfaces, abstract classes can have fields that are not static and final, and they can have implemented methods.

Abstract classes offer a way to create methods that are common to all subclasses while leaving some methods abstract to be implemented by each subclass, enforcing a certain level of common behavior across subclasses.

Creating an Abstract Class

An abstract class in Java is created using the abstract keyword. Here’s an example:

public abstract class Animal {
    // Abstract method
    public abstract void sound();
   
    // Concrete method
    public void breathe() {
        System.out.println("Breathing...");
    }
}

In the above code, Animal is an abstract class with an abstract method sound() and a concrete method breathe().

Extending Abstract Classes

An abstract class in itself is incomplete, and its sole purpose is to be extended (inherited). To use an abstract class, you have to extend it with another class.

Here’s how to extend an abstract class:

public class Dog extends Animal {
    public void sound() {
        System.out.println("The dog barks");
    }
}

In the above code, Dog is a concrete class that extends the abstract class Animal and provides implementation for the sound() method.

Instantiating Abstract Classes

Remember, we cannot instantiate an abstract class. This means the following is invalid:

Animal myAnimal = new Animal(); // This will cause an error

Instead, we can create an instance of the Dog class, which inherits from Animal:

Animal myDog = new Dog(); // This is valid

Conclusion

Abstract classes in Java are a powerful tool for abstraction. They allow us to define a template for what methods a group of subclasses should have, but they also allow us to determine how some of those methods should work. Understanding abstract classes is fundamental for any Java developer. By practicing the use of abstract classes in your own code, you’ll become more proficient in using this important feature of object-oriented programming.