Difference between Abstract class and Interface in Java 8? Answer

Ever since JDK 8 has allowed concrete (non-abstract) methods on the interface like default and static methods, many of my readers have asked me how should they answer the classical abstract class vs interface questions. Earlier, an interface cannot have any concrete methods and that was the main difference between abstract class and interface but now that is not the case. In this post, I'll revisit this hugely popular Java interview question in light of Java 8 changes. This is also a popular Java interview question and knowing the difference will help you to answer this question in a real interview.

As I said, before JDK 8, the level of abstraction was the clear-cut difference between abstract class and interface like interface was the purest form of Abstraction which only defines what interface is supposed to do without specifying how they should be implemented, in other words, it only declares API methods and leaves implementation to its subclasses.

But, the main difference between an abstract class and an interface in Java 8 is the fact that an abstract class is a class and an interface is an interface. A class can have a state which can be modified by non-abstract methods but an interface cannot have the state because they can't have instance variables.

The second difference is that an interface cannot have a constructor even in Java 8 but you may remember that abstract class can also have a constructor in Java. All methods of an interface were abstract but since Java 8 you can define non-abstract methods in the form of default and static methods inside the interface in Java.

This brings many questions to come to mind, which we'll discuss in the next section, btw, if you are new to the Java world and just started learning Java, I suggest you use a course that is up-to-date like The Complete Java MasterClass on Udemy because Java is changing very fast. 

In just a couple of years we have moved from Java 9 to Java 21. Thankfully this course is up-to-date and the author tries to update it with every Java release.

Difference between Abstract class vs Interface  in Java 8

Prima facia, in Java 8, an interface looks like an abstract class and one can reason about, can we use an interface with default methods in place of an abstract class in Java?

Well, I believe they are for two different purposes and we will learn more once we start using Java 8 regularly, but following the semantics difference between abstract class and interface with default method will guide you further :

1) Abstract classes are classes, so they are not restricted to other restrictions of the interface in Java, like abstract class can have the state, but you cannot have the state on the interface in Java.

2) Another semantic difference between an interface with default methods and an abstract class is that you can define constructors inside an abstract class, but you cannot define constructors inside an interface in Java.

In reality, default or defender methods are introduced to maintain backward compatibility and the same time making Collection API more suitable to be used inside key Java 8 features like lambda expressions.

Without adding default methods, it wasn't possible to declare any new method on the existing interface in Java without breaking all classes which implement it, but because of the default method, you can now better evolve your API.

They defend your code against implementing new methods hence they are also called defender methods. If you want to know more about default methods or new changes in Java 8 in general, I suggest you check out these Java 8 to Java 21 courses from sites like Udemy and Pluralsight. 

Difference between Abstract class and Interface  in Java 8

That's all about the difference between an Abstract class and an Interface in Java 8. Though I certainly agree that the difference between abstract class and the interface has reduced with the introduction of default methods, as well as allowing static methods inside the interface and their usage will evolve once Java 8 becomes a mainstream Java development version, but you must remember that an abstract class is a class and an interface is an interface.

The key difference is that an abstract class can maintain a state but the interface cannot, and an abstract class can also have a constructor that is not allowed inside the interface even in Java 8. If you are designing API then use interface and if you are abstracting both state and behavior then use an abstract class.

Other Java 8 Tutorials you may like
If you are interested in learning more about the new features of Java 8, here are my earlier articles covering some of the important concepts of Java 8:
  • 5 Books to Learn Java 8 from Scratch (books)
  • How to join String in Java 8 (example)
  • Top 5 Courses to learn Full Stack Java development (courses)
  • How to use filter() method in Java 8 (tutorial)
  • 10 Advanced Core Java courses for Programmers (courses)
  • How to format/parse the date with LocalDateTime in Java 8? (tutorial)
  • My favorite free courses to learn Java in-depth (courses)
  • How to use Stream class in Java 8 (tutorial)
  • Top 5 Courses to learn Lambdas and Stream in Java (courses)
  • How to use forEach() method in Java 8 (example)
  • How to convert List to Map in Java 8 (solution)
  • 20 Examples of Date and Time in Java 8 (tutorial)
  • How to use peek() method in Java 8 (example)
  • How to sort the map by keys in Java 8? (example)
  • How to sort the may by values in Java 8? (example)
  • 10 examples of Options in Java 8? (example)
  • How to convert lambda expression to method reference in Java 8 (tutorial)
  • 5 Free Java 8 and  Java 9 courses for Programmers (courses)
Thanks for reading this article so far. If you like this article then please share it with your friends and colleagues. If you have any questions or feedback then please drop a comment.

P. S. - If your goal is to learn new features introduced in Java 8 then you can also check out these Java Functional Programming and Stream API courses which only focuses on the most important new features (lambda expressions and stream) and nothing else.

Now, one question for you? When do you use Abstract class over interface in Java? This question is easy to answer if you have read Effective Java but you can still try, its not that tough. 


  1. What do you mean by "abstract class can have the state"?

    1. Hello Eugene, I mean abstract class can instance variable which can hold values.

  2. They should've left interfaces for pure abstraction and added something like delegates for lambdas like in C#. The bridge between abstract class and interface is very very thin now. It's just that lambdas can only be used with f interfaces otherwise abstract class could do the same job.

  3. Also, you can extend only one abstract class, but you can implement as much interfaces as you would need right?

  4. If you ask me I think java8 comes with interface patch works just to introduce lambda expressions on collection framework, else java was facing so much competition before scripting programming

  5. How does 100 percent abstraction works in java 8? Read a lot of articles but still not clear. Your writing is superb so hoping you will publish the answer❣

    1. Hello Bikash, I Am first time hearing about 100% abstraction? can you provide bit more context? Where did you heard this term etc? Because there is nothing like 100% abstraction the close we get is interface but there is no percentage associate it with.

  6. Why we have required Abstract class if interface is there already.

    1. I hope, when we required to use the value of an instance variable of an abstract class in the abstract method implemented by the child class, we can use the abstract class, because that will provide the abstraction.


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