Thread: Difference between interface and abstract class?

What is the difference between interface and abstract class in Java? is the most frequently asked question in Java Interview and it is one of the toughest question to answer. After the introduction of Java 8 this has become quite tougher because the interfaces for both static and default methods can be included in the default methods, in this new version.

There are some interview panel members who also check for the practical experience of the candidates on when to use interface in Java rather than just focusing on the difference. This is very tough to answer and one must have a very good knowledge about interface and abstract class in Java. Therefore, you need to keep in mind that if you are preparing for Java Programming Interview then you also refer a book of Java concepts named as Java Programming Interview Exposed. In it, there are lots of important questions and topics included Eg: multi threading, OOPS concepts, collections, frameworks similar to Hibernate and Spring, data structure, unit testing and algorithms apart from that recent technology questions like Scala, Android, and other JVM languages.

Read More : What is an Abstract Class in Java?

Now, let us check out some difference between abstract class and interface in Java programming language.

Abstract class vs Interface in Java

1) An Interface is an interface and an abstract class is a class and this is the major difference between abstract class and an interface which implies that by extending the abstract class it is not possible to extend another class as Java does not support multiple inheritances but it is possible to implement multiple inheritance in Java.

2) It is not possible to create a non-abstract method in an interface, is the second difference between interface and abstract class and it is by default abstract. You can develop a non-abstract method in abstract class. With the help of abstract keyword a class which does not contain any abstract method can be made abstract.

Read More : Significance Of Abstract Methods and Classes In Java

3) For type declaration interface is best suited and for reusing the code and evolution perspective abstract class is used. Why there is a need to use interface for type declaration can be explained by the book “Effective Java” very well.

4) Abstract class are faster when compared to interface because before calling any overridden method in Java there is a search carried out by the interface. Although it is of not that importance it works well in time critical application.

Read More : What is Java Interface And how does it Work?

5) If a new method is added in an existing interface then the interface which is present breaks all the implementation and it is necessary to implement all the clients that is not advisable. A default implementation can be provided with the help of an abstract class for a new method without breaking the current clients.

abstract class vs Interface in Java

1) An Interface is an interface and an abstract class is a class and this is the major difference between abstract class and an interface which implies that by extending the abstract class it is not possible to extend another class as Java does not support multiple inheritances but it is possible to implement multiple inheritance in Java.

2) It is not possible to create a non-abstract method in an interface, is the second difference between interface and abstract class and it is by default abstract. You can develop a non-abstract method in abstract class. With the help of abstract keyword a class which does not contain any abstract method can be made abstract.


3) For type declaration interface is best suited and for reusing the code and evolution perspective abstract class is used. Why there is a need to use interface for type declaration can be explained by the book “Effective Java” very well.

4) Abstract class are faster when compared to interface because before calling any overridden method in Java there is a search carried out by the interface. Although it is of not that importance it works well in time critical application.


5) If a new method is added in an existing interface then the interface which is present breaks all the implementation and it is necessary to implement all the clients that is not advisable. A default implementation can be provided with the help of an abstract class for a new method without breaking the current clients.

Abstract class vs Interface

Type of methods: Interface can have only abstract methods. Abstract class can have abstract and non-abstract methods. From Java 8, it can have default and static methods also.
Final Variables: Variables declared in a Java interface are by default final. An abstract class may contain non-final variables.
Type of variables: Abstract class can have final, non-final, static and non-static variables. Interface has only static and final variables.
Implementation: Abstract class can provide the implementation of interface. Interface can’t provide the implementation of abstract class.
Inheritance vs Abstraction: A Java interface can be implemented using keyword “implements” and abstract class can be extended using keyword “extends”.
Multiple implementation: An interface can extend another Java interface only, an abstract class can extend another Java class and implement multiple Java interfaces.
Accessibility of Data Members: Members of a Java interface are public by default. A Java abstract class can have class members like private, protected, etc.
Abstract Class vs Interface

Abstraction: Hiding the internal implementation of the feature and only showing the functionality to the users. i.e. what it works (showing), how it works (hiding). Both abstract class and interface are used for abstraction.

Abstract class vs Interface

Type of methods: Interface can have only abstract methods. Abstract class can have abstract and non-abstract methods. From Java 8, it can have default and static methods also.
Final Variables: Variables declared in a Java interface are by default final. An abstract class may contain non-final variables.
Type of variables: Abstract class can have final, non-final, static and non-static variables. Interface has only static and final variables.
Implementation: Abstract class can provide the implementation of interface. Interface can’t provide the implementation of abstract class.
Inheritance vs Abstraction: A Java interface can be implemented using keyword “implements” and abstract class can be extended using keyword “extends”.
Multiple implementation: An interface can extend another Java interface only, an abstract class can extend another Java class and implement multiple Java interfaces.
Accessibility of Data Members: Members of a Java interface are public by default. A Java abstract class can have class members like private, protected, etc.

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.

Sub-classes 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 sub-classes use implements keyword to implement interfaces and should provide implementation for all the methods declared in the interface.

Abstract classes can have methods with implementation whereas interface provides absolute abstraction and can’t have any method implementations.
Abstract classes can have constructors but interfaces can’t have constructors.

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.

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.

A subclass can extend only one abstract class but it can implement multiple interfaces.

Abstract classes can extend other class and implement interfaces but interface can only extend other interfaces.

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.

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.