Functional interface in java

Let’s learn functional interface in java.

Functional interface in java

Functional interface is one kind of an interface which has only one abstract method.

Syntax:

@FunctionalInterface
interface interfacename
{
   // one abstract method
}

NOTE: @FunctionalInterface is an annotation. This annotation is optional.

Before Java 8 an interface had abstract method signature but didn’t define the method itself. It’s upto the implementation class to implement the method of interface.

But in Java 8 we can write interfaces which have implementation methods. Functional interface is one kind. This is the type of interface which can be used for declaring or defining lambda expression.

We have a new way of marking a specific interface as functional interface. This gives a indication to other developer that this is meant to be a functional interface.

Here’s functional interface example.

@FunctionalInterface
interface HelloWorld
{
   void print();
}
public class FunctionalInterfaceExample
{
   public static void main(String[] args)
   {
      HelloWorld obj = () -> System.out.println("Hello World Java");
      obj.print();
   }
}

Output:

Hello World Java

Also read – classes and objects in java