Difference between Abstract Class and Interface in Java

Filed Under: Java
Abstract Class Vs Interface

Difference between Abstract Class and Interface is one of the popular interview questions. Abstract Class and Interface are a core part of the Java programming language. Whether to choose an interface or abstract class is a design decision that every architect faces.

In my last articles, I have provided as much as possible details about java interface and abstract class.

In this post, we will learn about the difference between abstract class and Interface and when should we use interface over the abstract class and vice versa.

Difference between Abstract Class and Interface

  1. abstract keyword is used to create an abstract class and it can be used with methods also whereas interface keyword is used to create interface and it can’t be used with methods.
  2. Subclasses use extends keyword to extend an abstract class and they need to provide implementation of all the declared methods in the abstract class unless the subclass is also an abstract class whereas subclasses use implements keyword to implement interfaces and should provide implementation for all the methods declared in the interface.
  3. Abstract classes can have methods with implementation whereas interface provides absolute abstraction and can’t have any method implementations. Note that from Java 8 onwards, we can create default and static methods in interface that contains the method implementations.
  4. Abstract classes can have constructors but interfaces can’t have constructors.
  5. Abstract class have all the features of a normal java class except that we can’t instantiate it. We can use abstract keyword to make a class abstract but interfaces are a completely different type and can have only public static final constants and method declarations.
  6. Abstract classes methods can have access modifiers as public, private, protected, static but interface methods are implicitly public and abstract, we can’t use any other access modifiers with interface methods.
  7. A subclass can extend only one abstract class but it can implement multiple interfaces.
  8. Abstract classes can extend other class and implement interfaces but interface can only extend other interfaces.
  9. We can run an abstract class if it has main() method but we can’t run an interface because they can’t have main method implementation.
  10. Interfaces are used to define contract for the subclasses whereas abstract class also define contract but it can provide other methods implementations for subclasses to use.

That’s all for the difference between an interface and abstract classes, now we can move on to know when should we use Interface over Abstract class and vice versa.

Interface or Abstract Class

Whether to choose between Interface or abstract class for providing a contract for subclasses is a design decision and depends on many factors. Let’s see when Interfaces are the best choice and when can we use abstract classes.

  1. Java doesn’t support multiple class level inheritance, so every class can extend only one superclass. But a class can implement multiple interfaces. So most of the times Interfaces are a good choice for providing the base for class hierarchy and contract. Also coding in terms of interfaces is one of the best practices for coding in java.
  2. If there are a lot of methods in the contract, then abstract class is more useful because we can provide a default implementation for some of the methods that are common for all the subclasses. Also if subclasses don’t need to implement a particular method, they can avoid providing the implementation but in case of interface, the subclass will have to provide the implementation for all the methods even though it’s of no use and implementation is just empty block.
  3. If our base contract keeps on changing then interfaces can cause issues because we can’t declare additional methods to the interface without changing all the implementation classes, with the abstract class we can provide the default implementation and only change the implementation classes that are actually going to use the new methods.

Use Abstract classes and Interface both

Using interfaces and abstract classes together is the best approach to design a system. For example, in JDK java.util.List is an interface that contains a lot of methods, so there is an abstract class java.util.AbstractList that provides a skeletal implementation for all the methods of List interface so that any subclass can extend this class and implement only required methods.

We should always start with an interface as the base and define methods that every subclass should implement and then if there are some methods that only certain subclass should implement, we can extend the base interface and create a new interface with those methods.

The subclasses will have the option to chose between the base interface or the child interface to implement according to its requirements.

If the number of methods grows a lot, it’s not a bad idea to provide a skeletal abstract class implementing the child interface and providing flexibility to the subclasses to chose between interface and an abstract class.

Java 8 interface changes

From Java 8 onwards, we can have method implementations in the interfaces. We can create default as well as static methods in the interfaces and provide an implementation for them.

This has bridged the gap between abstract classes and interfaces and now interfaces are the way to go because we can extend it further by providing default implementations for new methods.

For more details, check out Java 8 interface default static methods.

Comments

  1. Hari says:

    Hi Pankaj,

    Nowadays many sites for explanations but your blog quite impressive because few points added more and those are very helpful for interview point of view. Thanks a lot

  2. annonymous says:

    Hi ,

    first of all your every post is useful .

    I have one request , can you change the difference between abstract class and interface according to java 8 changes

  3. Kenneth says:

    Just a friendly reminder, I’ve noticed that you mistakenly used “chose” in many articles where “choose” should be used in fact. Nonetheless, this is a great set of tutorials on Core Java 🙂

  4. Vivek Dubey says:

    Dear Pankaj,
    Please tell me where we use abstract class and where we use interface in java project.

  5. GST Billing Software says:

    Nice and very straight forward post. Thanks for posting.

  6. Chetan says:

    We can run interface from Java8 as it can have static method and we can execute so the difference of not running interface is invalid after java8, either u can update or add.

  7. sanjay pradhan says:

    Nice explanation.

  8. Anurag Singh says:

    Thanks for sharing this post.
    keep it up.

  9. Great Explanation,
    Your article provides quality content on difference between Abstract class and Interface. It is a very common question usually asked in interviews. I agree with your all points. We can achieve Abstraction 0-100% through abstract class and 100% abstraction from Interfaces because Interface can contain only abstract methods but Abstract class may have abstract method as well as methods with implementation. I am also running a blog (java2blog) which is tutorial on Java. It also provides useful interview questions and answers on Java.

  10. Stan says:

    Hi Pankaj,

    I think “Subclasses use extends keyword to extend an abstract class and they need to provide implementation of all the declared methods in the abstract class” is not correct. It should be “Subclasses use extends keyword to extend an abstract class and they need to provide implementation of all the declared abstract methods in the abstract class “

    1. Abdul Latheef says:

      Correct, but if the subclass doesn’t have an implementation for the abstract method then the subclass should be declare as abstract.that is the method should declare as abstract in subclass.but it should have the implementation in any of its child non abstract class

  11. Vikas Palakurthi says:

    Can abstract methods in an abstract class be private? No right?

    1. Pankaj says:

      No it can’t, if you ever face these kind of questions, then best way to just write quick code in Eclipse and see if you get errors. 🙂

  12. Oshin Talreja says:

    You mention that abstract class can be run throw main method while interface can’t However abstract class cant be instantiated so how JVM is going to run the abstract class.

    You can call any static method with Abstract class implementation, No matter it is main method or not.

  13. KALADHAR KHAMITHKAR says:

    Hi Pankaj,
    Good Explanation on Abstract and Interfaces.
    Can you Post Tutorials on MAVEN, LOG4J and JUnit.

    Thanks

  14. Aliasger Motiwala says:

    You have written
    “The subclasses will have option to chose between the base interface or the child interface to implement according to its requirements”
    Hows this possible ?

    1. Pankaj says:

      Let’s say we have interface A with abstract method foo() and child interface B with method bar().

      Now my subclass can either implement A or B, based on whether it just need to provide foo() method or bar() method too.

    2. Jayavardhana says:

      HI Pankaj,

      Good explanation about abstract and interface, but i have one doubt about abstract class,In future added abstaract method ,it should be override all subclasses. but you said not required to implement.I am confusing this point .please clarify

  15. Shwetha says:

    Hi. I have a question.
    Assume that there is an abstract class with method String Name() and an Interface with the same method String Name(). The signatures of both methods in abstract class and interface is same.

    Now Class A extends the abstract class and implements the interface.

    Which String Name() method is implemented in Class A???

    Example :

    //AbstractClass
    public abstract AbstractClass {

    public abstract String Name();
    }

    //Interface
    Public interface SampleInterface{

    public String Name();
    }

    // which method is implemented here???
    public class A extends AbstractClass implements SampleInterface{

    public String Name(){
    //Implementation
    // Which method is implemented here. The abstract class method or interface method???
    }

    I tried executing a small sample program. The program runs fine but i am not sure as to which method is implemented here.

    1. Pankaj says:

      Since both super methods are abstract, it’s working fine. It doesn’t matter which method is implemented.

    2. Shiva says:

      very good question Shwetha :-). your thinking is awesome

  16. Waseem Ahmed Kalhoro says:

    What is the simple definition of abstract??
    Give me an example..

  17. abhishek chanda says:

    “… Also if subclasses don’t need to implement particular method, they can avoid providing the implementation… ” – does that mean we can choose NOT to implement any method of the abstract class?

  18. suresh says:

    very good collective information on abstract and interfcae

  19. Swati Aher says:

    thank You …for giving very good explanation…

  20. prabhu says:

    best explanation,Thank you.

  21. rishi says:

    the best explanation i came across. going to copy this page forever.

  22. sha says:

    sir
    8. interface can only extend other interfaces.
    but interface only implements other interface
    correct

    1. Pankaj says:

      Abstract class can implement interfaces because it’s like normal class with some abstract methods.

      Interfaces don’t have implementation, so they can ONLY extend other interfaces.

  23. lakshmi says:

    thank u sir

  24. lakshmi says:

    sir can u give me reply for the question i have asked
    on last point

  25. lakshmi says:

    you mentioned in last point”Interfaces are used to define contract for the subclasses” like what is ‘ contract here’ klindly explain that point

    1. Pankaj says:

      Contract are the behavior methods that are declared in the interface that all the implementation classes must define.

      1. Tuntun kumar says:

        very good collective information on abstract and interfcae

  26. Madiraju Krishna Chaitanya says:

    Hi Pankaj Sir,
    Thank You for sharing this wonderful site with us.
    Would be Thankful to you,if you can shed light on ANT,JUnit,TestNG topics too.
    I am an Automation tester in Selenium and am looking forward for the topics mentioned above,since,your experience and understanding helps us understand the topics better.
    Thanks Again.

    ~Chaitanya

    1. Pankaj says:

      ANT is an outdated technology now when it comes to build and deployment tools, I am planning to post some tutorials for Maven.
      JUnit and TestNG tutorials will also arrive in coming months.

      Thanks for your kind words, it pushes me to keep going.

      1. Lucas says:

        You have given
        8.Abstract classes can extend other class and implement interfaces but interface can only extend other interfaces.

        I have a doubt can abstract class “extend” another abstract class or “implement” another abstract class
        reply to my email please……

        1. Pankaj says:

          Yes, an abstract class can extend other classes or other abstract classes and implement interfaces. You can check this easily with sample code in Eclipse.

          public abstract class Ab1 {

          public abstract void ss();
          }

          public interface It1 {

          public void xx();
          }

          public abstract class Ab2 extends Ab1 implements It1{

          public abstract void tt();
          }

          Ab2 is extending abstract class Ab1 and implementing interface It1.

  27. sunil patil says:

    Hello Pankaj,
    I like this post much. I got clear idea about when to use interfaces and when to use abstract class.

    But i didnt understand last paragraph i.e extending interface.

    could u elobarate more. So that it will understandable.

    I will be clear if u provide some bit of code. Please
    Thanks.

    1. Pankaj says:

      It feels great that you liked the post, as I mentioned that once you have created an interface, its hard to change its behaviors because that would result in compilation error for all the implementation classes, so we can use interfaces extend functionality to create a new interface rather than changing the base interface.

      To learn more about this, read https://www.journaldev.com/1601/what-is-interface-in-java-example-tutorial

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