Functional Interface in Java

Last Updated: 28 May, 2022

Functional Interface was introduced in Java 8.

An Interface that contains only one abstract method is known as a Functional Interface in Java. It is also known as Single Abstract Method Interfaces or SAM Interfaces.

A functional interface can have any number of default methods or static methods but an abstract method must be only one. A functional interface can have methods of the Object class.

Before Java 8, We had to use anonymous inner class objects to implement functional interfaces.

Java Functional Interface

  
  // An Interface that has only one abstract method
  interface TestFunctionalInterface {
    void showWebURL();
  }
  
  class TestClass implements TestFunctionalInterface {
    @Override
    public void showWebURL() {
      System.out.println("Web URL: https://javabytechie.com");
    }
  }
  
  public class FunctionInterfaceExample {
    public static void main(String args[]) {
      TestClass obj = new TestClass();
      obj.showWebURL();
    }
  }

Output

Web URL: https://javabytechie.com

Define A Funcational Interface with @FunctionalInterface Annotation

Any Interface that contains only one abstract method is treated as a Functional Interface in Java but if we have to ensure that Interface must be a functional interface and contains only one abstract method we should declare that interface with @FunctionalInterface Annotation.

Java Functional Interface with @FunctionalInterface Annotation

  
  @FunctionalInterface
  interface TestFunctionalInterface {
    void showWebURL();
  }
  
  class TestClass implements TestFunctionalInterface {
    @Override
    public void showWebURL() {
      System.out.println("Web URL: https://javabytechie.com");
    }
  }
  
  public class FunctionInterfaceExample {
    public static void main(String args[]) {
      TestClass obj = new TestClass();
      obj.showWebURL();
    }
  }

Output

Web URL: https://javabytechie.com

Java allows to declare any number of default methods and static methods inside functional interface. Let's understand it with the help of given example.

Example 3 : Functional Interface with default methods and static methods

  @FunctionalInterface
  interface MyFunctionalInterface {
    void printYourName();
  
    // Here, I have declared another abstract method which is invalid because I have
    // declared the interface as @FunctionalInterface
    // Error Message -> Invalid '@FunctionalInterface' annotation;
    // MyFunctionalInterface is not a functional interface
    // void printYourNickName();
  
    default void printYourCity() {
      System.out.println("Bangalore");
    }
  
    default void printYourState() {
      System.out.println("Karnataka");
    }
  
    static void printYourPinCode() {
      System.out.println("560021");
    }
  
    static void printYourCountry() {
      System.out.println("INDIA");
    }
  }
  
  class MyDetails implements MyFunctionalInterface {
  
    @Override
    public void printYourName() {
      System.out.println("Sandeep");
    }
  }
  
  public class FunctionalInterfaceExample3 {
    public static void main(String[] args) {
  
      MyDetails detail = new MyDetails();
  
      detail.printYourName();
      detail.printYourCity();
      detail.printYourState();
      
      // MyDetails.printYourPinCode(); -> Compile Time Error
      
      MyFunctionalInterface.printYourPinCode();
      MyFunctionalInterface.printYourCountry();
    }
  }

Output

Sandeep
Bangalore
Karnataka
560021
INDIA

Functional Interface in Java

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