Difference between Abstract class vs Interface in Java

When to use interface and abstract class is one of the most popular object oriented design questions and almost always asked in Java, C#, and C++ interviews. In this article, we will mostly talk in the context of Java programming language, but it equally applies to other languages as well.
The question usually starts with a difference between abstract class and interface in Java, which is rather easy to answer, especially if you are familiar with the syntax of Java interface and abstract class. Things start getting difficult when interviewer ask about when to use abstract class and interface in Java, which is mostly based upon a solid understanding of popular OOPS concepts like Polymorphism, Encapsulation, Abstraction, Inheritance, and Composition. Many programmers fumbles here, which is natural because most of them haven't gone through real system design process and haven’t seen the impact of choosing one over other.

The repercussion of design decisions are best known during the maintenance phase, a good design allows seamless evolution while maintaining a fragile design is a nightmare.

While deciding when to use interface and abstract class, it’s important to know difference between abstract class and interface in Java. In my opinion, following two differences between them drives decision about when to use abstract class or interface in Java.

1) Interface in Java can only contains declaration. You can not declare any concrete methods inside interface. On the other hand abstract class may contain both abstract and concrete methods, which makes abstract class an ideal place to provide common or default functionality. I suggest reading my post 10 things to know about interface in Java to know more about interfaces, particularly in Java programming language.

2) Java interface can extend multiple interface also Java class can implement multiple interfaces, Which means interface can provide more Polymorphism support than abstract class . By extending abstract class, a class can only participate in one Type hierarchy but by using interface it can be part of multiple type hierarchies. E.g. a class can be Runnable and Displayable at same time. One example I can remember of this is writing GUI application in J2ME, where class extends Canvas and implements CommandListener to provide both graphic and event-handling functionality..

3) In order to implement interface in Java, until your class is abstract, you need to provide implementation of all methods, which is very painful. On the other hand abstract class may help you in this case by providing default implementation. Because of this reason, I prefer to have minimum methods in interface, starting from just one, I don't like idea of marker interface, once annotation is introduced in Java 5. If you look JDK or any framework like Spring, which I does to understand OOPS and design patter better, you will find that most of interface contains only one or two methods e.g. Runnable, Callable, Actio
Listener etc.

When to use interface and abstract class in Java

As I said earlier, it's easy to answer questions like difference between abstract class and interface in Java, but difficult to answer follow-ups. Though most of Java Interview starts with former one, later it goes to see if you have really used abstract class and interface or not. In order to answer this question, you need to have good understanding of OOPS concepts like Polymorphism, Encapsulation, Abstraction and Inheritance. Also familiarity with coupling and cohesion is important. You at least should know that effort of designing should lead to reduce coupling and increased cohesion, ease of maintenance etc. In this part, we will see some scenarios, guidelines, rules which can help you to decide when to use abstract class and interface in Java.

1) In Java particularly, decision between choosing Abstract class and interface may influence by the fact that multiple inheritance is not supported in Java. One class can only extend another class in Java. If you choose abstract class over interface than you lost your chance to extend another class, while at the same time you can implement multiple interfaces to show that you have multiple capability. One of the common example, in favor of interface over abstract class is Thread vs Runnable case. If you want to execute a task and need run() method it's better to implement Runnable interface than extending Thread class.
2) Let's see another case where an abstract class suits better than interface. Since abstract class can include concrete methods, it’s great for maintenance point of view, particularly when your base class is evolving and keep changing. If you need a functionality across all your implementation e.g. a common method, than, you need to change every single implementation to include that change if you have chosen interface to describe your base class. Abstract class comes handy in this case because you can just define new functionality in abstract super class and every sub class will automatically gets it. In short, abstract class are great in terms of evolving functionality. If you are using interface, you need to exercise extra care while defining contracts because its not easy to change them once published.

3) Interface in Java is great for defining Types. Programming for interfaces than implementation is also one of the useful Object oriented design principle which suggests benefit of using interface as argument to function, return type etc.