Difference between an Abstract class and an Interface?

Let’s discuss some differences between an abstract class and an interface in Java. Interfaces provide the flexibility for unrelated classes to implement  multiple interfaces.

In Java, both abstract classes and interfaces are used to achieve abstraction — a core concept in object-oriented programming that allows you to hide complex implementation details and show only the necessary features of an object. However, many beginners get confused between the two because they seem similar but are used in different scenarios and have different rules. This tutorial will help you understand what abstract classes and interfaces are, and how they differ from each other.

What is an Abstract Class?

An abstract class in Java is a class that cannot be instantiated on its own and is meant to be extended by other classes. It can have both abstract methods (without implementation) and concrete methods (with implementation). Abstract classes are used when some common behavior should be inherited by multiple subclasses, but each subclass can have its own specific implementation for certain methods.

• Defined using the abstract keyword.
• Can contain constructors and member variables.
• Can have both abstract and non-abstract methods.
• Used for providing partial abstraction.

What is an Interface?

An interface in Java is a completely abstract class that is used to specify a set of methods that a class must implement. It is like a contract that ensures certain behaviors are implemented by the class that chooses to implement the interface.

• Defined using the interface keyword.
• Cannot contain constructors or instance variables (except static final constants).
• All methods are implicitly abstract and public (before Java 8).
• Used for achieving full abstraction and multiple inheritance.

Abstract class vs Interface

Some of the differences are as follows:

Feature	Abstract Class	Interface
Keyword Used	abstract Abstract Class is declared as : public abstract class ClassName { … }	interface Interface is declared as : public interface InterfaceName { … }
Method Implementation	Can have both abstract and concrete methods. Abstract Class can have abstract methods and may also have implemented methods. Abstract classes can define some default implementation and leave abstract method implementations to sub classes.	Only abstract methods (default and static methods allowed from Java 8) Interface methods are by default abstract. There is no need to declare with keyword abstract. Interfaces only contain method signatures.
Instantiation	Cannot be instantiated	Cannot be instantiated
Inheritance	Supports single inheritance Derived class can only extend on Abstract class in Java. Java doesn’t support multiple class inheritance. Derived classes of an abstract class directly come under the Abstract class in the class hierarchy.	Supports multiple inheritance Java classes can implement multiple interfaces and from any where in the class hierarchy.
Constructors	Can have constructors	Cannot have constructors
Variables	Can have instance variables	Can have only public static final constants
Access Modifiers	Can have any access modifier	All methods are public by default
Use Case	Used when classes share common behavior	Used when classes must follow a contract