Functional interfaces contain one—and only one—abstract method. An abstract method is one which should have a body in the implementation class if the implementation class is not abstract. It can have any number of regular methods (methods which have a body in the implementation classes), but the prerequisite of a functional interface is that the number of abstract methods must be only one. These interfaces are used hand-in-hand with Lambda expressions.

In the following code block, the demo method is inside an interface Greeting. Therefore, this interface should only have one abstract method, which is the demo method. In order to instruct other users that this is a functional interface, we annotate this interface with the @FunctionalInterface annotation.

The type of blockofCodeA will be of this functional interface type. This annotation is optional:

@FunctionalInterface
public interface Greeting {
        public void demo();
}