Functional Interface in Java

     Functional interface is a special type of interface which can have only one abstract method and any number of default methods.

To declare an interface a functional interface use @FunctionalInterface annotation.

Example:

         @FunctionalInterface

         interface Calculator{

                int calculate(int a, int b);

         }

          class Add implements Calculator{

                 @Override

                 public int calculate(int a, int b){

                     return a+b;

                 }

           }

Note: If we add another abstract method inside the Calculator interface, it will result into a compilation error.

       - Best use of functional interface is with lambda expressions. 

In Java there are the following important and widely used inbuilt functional interfaces are defined:

• Function<T, R>: represents a function that accepts one argument of type T and produces a result of type R

• Predicate<T>:  represents a Boolean valued function which accepts one argument of type T and returns a Boolean

• Supplier<T>: represents a supplier which accepts nothing but returns a result of type T.

• Consumer<T>: represents an operation that accepts a single input argument and returns no result.

Continue Learning: Access Modifiers in Java