Java Functional Interface has become available for us since the introduction of new features in Java 8. Needless to say, how important functional Interfaces are in Java. If we start learning the most popular feature of Java 8 : 'The Lambda Expression', we should at least know the basics of Functional Interfaces. However, you will get more than basics from this article. Moreover, they also promote functional programming in Java. Without functional Interfaces, you can't imagine to write a code of Lambda Expression and Method References. In fact, introduction of default & static methods inside an interface in Java 8 itself has completely changed the definition of Interfaces in Java. Let's discuss our topic 'Java Functional interface' and other related concepts. How