What is Functional Interface in Java

Introduction

Table of Content

Functional Interface

FunctionalInterface.java

Following interface is a functional interface since it has only one abstract method

@FunctionalInterface
interface MyComputeInterface {
  public int compute(int... a);
}

Functional Interface with default method.

MyComputeInterfaceWithDefault.java

Following interface is a functional interface since it has only one abstract method. It can have default methods though.

@FunctionalInterface
interface MyComputeInterfaceWithDefault {

  public int compute(int... a);

  default boolean isEven(int x) {
    return x / 2 == 0;
  }

}

Invalid Functional Interface

ComputeInterface.java

Following interface is invalid since it contains more than one abstract methods. Also as we have the @FunctionalInterface annotation applied, the compiler will complain stating ComputeInterface is not a functional interface.

@FunctionalInterface
interface ComputeInterface {
  public int add(int... a);
  public int average(int... a);
}

Note the @FuncationalInterface annotation is a runtime informative annotation that will make the compiler to complain if the interface contains more than one abstract methods.

Summary

A functional interface has only one abstract method but it can have multiple default methods.
@FunctionalInterface annotation is used to ensure an interface can’t have more than one abstract method. The use of this annotation is optional.
instances of functional interfaces can be created with lambda expressions, method references, or constructor references
Java provides out of the box Functional interfaces that we can use.