Functional Interface in Java8

An Interface that contains exactly one abstract method is known as functional interface. It can have any number of default, static methods but can contain only one abstract method. It can also declare methods of object class.

Functional Interface is also known as Single Abstract Method Interfaces or SAM Interfaces. It is a new feature in Java, which helps to achieve functional programming approach.

@FunctionalInterface annotation is added so that we can mark an interface as functional interface.
It is not mandatory to use it, but it’s best practice to use it with functional interfaces to avoid addition of extra methods accidentally.

The major benefit of java 8 functional interfaces is that we can use lambda expressions to instantiate them and avoid using bulky anonymous class implementation.