What Is an Abstract Class in Java and How to Define It ?

Java is an object-oriented programming language that allows you to create classes to represent objects in your programs. In some cases, you may want to create a class that cannot be instantiated but instead serves as a base or template for other classes to extend from. This is where abstract classes come into play.

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 directly. Instead, abstract classes are used as a base or template for other classes to extend from. An abstract class may contain both abstract and non-abstract methods, but it must contain at least one abstract method.

Why use an abstract class?

There are several reasons why you might want to use an abstract class in your Java program:

1. To provide a common template for multiple classes that share similar characteristics or behaviors.

2. To enforce a set of rules or requirements that subclasses must follow.

3. To encapsulate common functionality that can be reused by multiple subclasses.

How to define an abstract class

Defining an abstract class in Java involves the following steps:

Declaring an abstract class

To declare an abstract class, you must use the abstract keyword in the class declaration:

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public abstract class MyAbstractClass { // class members }

Abstract methods

An abstract method is a method that is declared but not implemented in the abstract class. Abstract methods must be implemented in any subclass that extends the abstract class. To declare an abstract method, use the abstract keyword in the method declaration:

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public abstract void myAbstractMethod();

Concrete methods

A concrete method is a method that is implemented in the abstract class. Concrete methods can be called from any subclass that extends the abstract class. To define a concrete method, simply implement it in the abstract class:

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public void myConcreteMethod() { // method implementation }

Example of an abstract class

Here's an example of an abstract class that defines an abstract method and a concrete method:

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public abstract class Shape { public abstract double area(); public void printArea() { System.out.println("The area is: " + area()); } }

In this example, the Shape class defines an abstract method called area() that returns a double. The class also defines a concrete method called printArea() that prints the area to the console. Note that the area() method is abstract, so any subclass that extends the Shape class must provide an implementation for it.

Abstract class vs. Interface (cont.)

1. An abstract class can have instance variables and constructors, while an interface cannot.

2. A class can only extend one abstract class, but it can implement multiple interfaces.

So, when should you use an abstract class instead of an interface? If you need to define a set of default implementations for some methods and require that the subclass provides the implementation for others, then use an abstract class. If you need to define only the method signatures and require that the subclass implements them, then use an interface.

Conclusion

Here, we've discussed what an abstract class is in Java, how to define it, and how it differs from an interface. An abstract class is a class that cannot be instantiated and serves as a template for other classes to extend from. It can contain both abstract and non-abstract methods, but it must contain at least one abstract method. An abstract class is useful when you want to define a common template for multiple classes or enforce a set of rules or requirements that subclasses must follow.

Frequently Asked Questions (FAQs)

Q. Can an abstract class be instantiated?

No, an abstract class cannot be instantiated. You must create a subclass that extends the abstract class and provide implementations for its abstract methods.

Q. Can an abstract class have a constructor?

Yes, an abstract class can have a constructor. It can be used to initialize the instance variables of the abstract class.

Q. Can a class extend multiple abstract classes?

No, a class can only extend one abstract class. However, it can implement multiple interfaces.

Q. Can a subclass of an abstract class be abstract?

Yes, a subclass of an abstract class can be abstract if it does not implement all the abstract methods of its superclass.

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