Top Most Difference between Abstract class vs Interface in Java and When to use them

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 context of Java programming language, but it equally applies to other languages as well. Question usually starts with difference between abstract class and interface in Java, which is rather easy to answer, especially if you are familiar with 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 solid understanding of popular OOPS concept like Polymorphism, Encapsulation, Abstraction, Inheritance and Composition. Many programmer 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. Repercussion of design decisions are best known during maintenance phase, a good design allows seamless evolution while maintaining a fragile design is nightmare. As I have said previously, some time object oriented design interview questions also helps to understand a topic better, but only if you are willing to do some research and not just mugging the answer. Questions like when to use abstract class and interface falls under same category. In order to best understand this topic, you need to work out some scenarios, examples etc. It's best to get this kind of knowledge as part of your work but even if you don't get there, you can supplement them by reading some good books like Head First design pattern and doing some object-oriented software design exercises. In this article, we will learn difference between abstract class and interface in Java programming language and based upon our understanding of those differences, we will try to find out some tips and guidelines to decide when its better to use abstract class over interface or vice-versa.

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