Abstract Class vs Interface in Java: Key Differences Explained

Introduction to Abstract Classes and Interfaces in Java

In object-oriented programming, Java provides two key constructs for defining the structure and behaviour of classes: abstract classes and interfaces. Both are essential for creating a flexible and modular codebase, but they serve different purposes and have unique characteristics. Understanding the differences between abstract classes and interfaces is crucial for Java developers to design and implement robust software systems.

What is an Abstract Class?

In Java, an abstract class is a class that cannot be instantiated on its own and is meant to be subclassed. It can contain both abstract methods (without a body) and concrete methods (with an implementation). Abstract classes are used to define a common template for a group of subclasses.

Let's illustrate this with an example where we have a superclass Animal with an abstract method makeSound(). Different subclasses such as Dog and Cat will provide their own implementations of the makeSound() method.

Code Example:

In the example, makeSound() is an abstract method (a method without a body) declared in the abstract class Animal. Since Animal contains this abstract method, it is also an abstract class. The Dog and Cat classes extend the Animal class and provide their own implementations of the makeSound() method. Each subclass must implement the abstract method to provide specific behaviour.

What is an Interface in Java?

Like a class, an interface can have methods and variables, but the methods declared in an interface are by default abstract (only method signature, without body).  

Interfaces specify what a class must do and not how. It is the blueprint of the class.

An Interface is about capabilities like a Player may be an interface and any class implementing Player must be able to (or must implement) move(). So it specifies a set of methods that the class has to implement.

If a class implements an interface and does not provide method bodies for all functions specified in the interface, the class must be declared abstract.

A Java library example is the Comparator Interface. If a class implements this interface, then it can be used to sort a collection.

Here is an example

In this example, the Animal interface declares the makeSound() method, which must be implemented by any class that claims to be an Animal. The Dog and Cat classes implement the Animal interface and provide their own versions of the makeSound() method.

Following are the differences between abstract class and interface in java

Conclusion

In summary, abstract classes and interfaces in Java are foundational constructs for building well-structured and reusable code. Abstract classes are best suited for cases where there is a need to share common behaviour and state across subclasses. In contrast, interfaces are ideal for defining a common contract that multiple classes can implement, regardless of their location in the class hierarchy. Understanding when and how to use these constructs is crucial for effective Java programming, allowing developers to create flexible, maintainable, and scalable applications.