Abstract Class and Interface

Differences

Java 8 (and later)

Feature	Abstract Class	Interface
Instance Methods	Can have fully implemented instance methods	Can also have implemented instance methods, but they must bear the keyword default
Static Methods	Can have fully implemented static methods	Can have fully implemented static methods
Variables	Can have instance variables of any kind	Can have only public static final variables
Visibility	Can have any visibility (public, protected, package-level, private)	Only public

Java 7 (and earlier)

Feature	Abstract Class	Interface
Instance Methods	Can have fully implemented instance methods	Can not have
Static Methods	Can have fully implemented static methods	Can not have
Instance Variables	Can have instance variables of any kind	Can have only public static final variables
Visibility	Can have any visibility (public, protected, package-level, private)	Only public

Usage

Abstract Class

• Defining common behavior: Establish a foundation for a family of related classes, abstract classes provide a clear structure and common methods.

• Providing default implementations: Define concrete methods within abstract classes, which can be inherited by subclasses. This simplifies development and promotes code reuse.

• Creating hierarchies: Subclasses can specialize and extend the behavior of the base class.

• Enforcing a template pattern: Ensure that subclasses follow a certain structure and implement required methods.

Interface

• Defining contracts: Define contracts that classes must adhere to, ensuring consistency and interoperability.

• Achieving polymorphism: Objects of different classes can be treated as the same type if they implement the same interface.

• Supporting multiple inheritance: Java doesn’t allow direct multiple inheritance of classes, but interfaces can be implemented by a class to achieve a similar effect.

• Decoupling components: Promotes loose coupling between classes, makes code more modular and easier to maintain.

Additional Notes

It is possible to put abstract methods into an interface. In fact, that’s one of the primary purposes of interfaces.

We don’t need to use the abstract keyword when declaring methods in an interface, although we can if we want to be explicit.

For example:

public interface MyInterface {
    // Abstract method (no implementation)
    void abstractMethod();
    
    // This is equivalent to the above
    abstract void explicitAbstractMethod();
}

Some IDEs will show the warning message when we add the abstract keyword explicitly:

Modifier 'abstract' is redundant for interface methods