jWeaver jWeaver - 4 months ago 15
Java Question

What is the purpose of the default keyword in Java?


An interface in Java is similar to a class, but the body of an
interface can include only abstract methods and
final
fields
(constants).


Recently, I saw a question, which looks like this

interface AnInterface {
public default void myMethod() {
System.out.println("D");
}
}


According to the interface definition, only abstract methods are allowed. Why does it allow me to compile the above code? What is the
default
keyword?

On the other hand, when I was trying to write below code, then it says
modifier default not allowed here


default class MyClass{

}


instead of

class MyClass {

}


Can anyone tell me the purpose of the
default
keyword? Is it only allowed inside an interface? How does it differ from
default
(no access modifier)?

Answer

It's a new feature in Java 8 which allows an interface to provide an implementation. Described in Java 8 JLS-13.5.6. Interface Method Declarations which reads (in part)

Adding a default method, or changing a method from abstract to default, does not break compatibility with pre-existing binaries, but may cause an IncompatibleClassChangeError if a pre-existing binary attempts to invoke the method. This error occurs if the qualifying type, T, is a subtype of two interfaces, I and J, where both I and J declare a default method with the same signature and result, and neither I nor J is a subinterface of the other.

What's New in JDK 8 says (in part)

Default methods enable new functionality to be added to the interfaces of libraries and ensure binary compatibility with code written for older versions of those interfaces.