A functional interface in Java is an interface that contains only one abstract method. Functional interfaces can have multiple default or static methods, but only one abstract method.

From Java 8 onwards, lambda expressions and method references can be used to represent the instance of a functional interface.

Example: Using a Functional Interface with Lambda Expression

public class Geeks {
  
    public static void main(String[] args) {
      
        // Using lambda expression to implement Runnable
        new Thread(() -> System.out.println("New thread created")).start();
    }
}

public class Geeks {

    public static void main(String[] args) {

        // Using lambda expression to implement Runnable

        new Thread(() -> System.out.println("New thread created")).start();

    }

}

New thread created

Explanation:

• Above program demonstrates use of lambda expression with the Runnable functional interface.
• Runnable has one abstract method run(), so it qualifies as a functional interface.
• Lambda ()-> System.out.println("New thread created") defines the run() method.
• new Thread().start() starts a new thread that executes the lambda body

Note: A functional interface can also extend another functional interface.

@FunctionalInterface Annotation

@FunctionalInterface annotation is used to ensure that the functional interface cannot have more than one abstract method. In case more than one abstract methods are present, the compiler flags an "Unexpected @FunctionalInterface annotation" message. However, it is not mandatory to use this annotation.

Note: @FunctionalInterface annotation is optional but it is a good practice to use. It helps catching the error in early stage by making sure that the interface has only one abstract method.

Example: Defining a Functional Interface with @FunctionalInterface Annotation

@FunctionalInterface

interface Square {
    int calculate(int x);
}

class Geeks {
    public static void main(String args[]) {
        int a = 5;

        // lambda expression to define the calculate method
        Square s = (int x) -> x * x;

        // parameter passed and return type must be same as defined in the prototype
        int ans = s.calculate(a);
        System.out.println(ans);
    }
}

@FunctionalInterface

​

interface Square {

    int calculate(int x);

}

​

class Geeks {

    public static void main(String args[]) {

        int a = 5;

​

        // lambda expression to define the calculate method

        Square s = (int x) -> x * x;

​

        // parameter passed and return type must be same as defined in the prototype

        int ans = s.calculate(a);

        System.out.println(ans);

    }

}

25

Explanation:

• Square is a functional interface with a single method calculate(int x).
• A lambda expression (int x) -> x * x is used to implement the calculate method.
• Lambda takes x as input and returns x * x.

Java Functional Interfaces Before Java 8

Before Java 8, we had to create anonymous inner class objects or implement these interfaces. Below is an example of how the Runnable interface was implemented prior to the introduction of lambda expressions.

Example: Java program to demonstrate functional interface

class Geeks {
    public static void main(String args[]) {
      
        // create anonymous inner class object
        new Thread(new Runnable() {
            @Override public void run()
            {
                System.out.println("New thread created");
            }
        }).start();
    }
}

class Geeks {

    public static void main(String args[]) {

        // create anonymous inner class object

        new Thread(new Runnable() {

            @Override public void run()

            {

                System.out.println("New thread created");

            }

        }).start();

    }

}

New thread created

Built-In Java Functional Interfaces

Since Java SE 1.8 onwards, there are many interfaces that are converted into functional interfaces. All these interfaces are annotated with @FunctionalInterface. These interfaces are as follows:

• Runnable: This interface only contains the run() method.
• Comparable: This interface only contains the compareTo() method.
• ActionListener: This interface only contains the actionPerformed() method.
• Callable: This interface only contains the call() method.

Types of Functional Interfaces in Java

Java SE 8 included four main kinds of functional interfaces which can be applied in multiple situations as mentioned below:

1. Consumer
2. Predicate
3. Function 
4. Supplier

1. Consumer 

The consumer interface of the functional interface is the one that accepts only one argument or a gentrified argument. The consumer interface has no return value. It returns nothing. There are also functional variants of the Consumer DoubleConsumer, IntConsumer and LongConsumer. These variants accept primitive values as arguments. 

Other than these variants, there is also one more variant of the Consumer interface known as Bi-Consumer. 

Syntax / Prototype of Consumer Functional Interface:

Consumer<Integer> consumer = (value) -> System.out.println(value);

This implementation of the Java Consumer functional interface prints the value passed as a parameter to the print statement. This implementation uses the Lambda function of Java.

2. Predicate 

The Predicate interface represents a boolean-valued function of one argument. It is commonly used for filtering operations in streams.

Just like the Consumer functional interface, Predicate functional interface also has some extensions. These are IntPredicate, DoublePredicate and LongPredicate. These types of predicate functional interfaces accept only primitive data types or values as arguments.  

Syntax: 

public interface Predicate<T> {
   boolean test(T t);
}

The Java predicate functional interface can also be implemented using Lambda expressions.

Predicate predicate = (value) -> value != null;

3. Function

A function is a type of functional interface in Java that receives only a single argument and returns a value after the required processing. Many different versions of the function interfaces are instrumental and are commonly used in primitive types like double, int, long.

Syntax:

Function<Integer, Integer> function = (value) -> value * value;

• Bi-Function: The Bi-Function is substantially related to a Function. Besides, it takes two arguments, whereas Function accepts one argument. 
• Unary Operator and Binary Operator: There are also two other functional interfaces which are named as Unary Operator and Binary Operator. They both extend the Function and Bi-Function respectively, where both the input type and the output type are same.

4. Supplier

The Supplier functional interface is also a type of functional interface that does not take any input or argument and yet returns a single output.

The different extensions of the Supplier functional interface hold many other suppliers functions like BooleanSupplier, DoubleSupplier, LongSupplier and IntSupplier. The return type of all these further specializations is their corresponding primitives only. 

Syntax:

Supplier<String> supplier = () -> "Hello, World!";

Example: Using Predicate Interface to Filter Strings

import java.util.*;
import java.util.function.Predicate;

class Geeks {
    public static void main(String args[]) {
      
        // create a list of strings
        List<String> n = Arrays.asList(
            "Geek", "GeeksQuiz", "g1", "QA", "Geek2");

        // declare the predicate type as string and use lambda expression to create object
        Predicate<String> p = (s) -> s.startsWith("G");

        // Iterate through the list
        for (String st : n) {
          
            // call the test method
            if (p.test(st))
                System.out.println(st);
        }
    }
}

import java.util.*;

import java.util.function.Predicate;

​

class Geeks {

    public static void main(String args[]) {

        // create a list of strings

        List<String> n = Arrays.asList(

            "Geek", "GeeksQuiz", "g1", "QA", "Geek2");

​

        // declare the predicate type as string and use lambda expression to create object

        Predicate<String> p = (s) -> s.startsWith("G");

​

        // Iterate through the list

        for (String st : n) {

            // call the test method

            if (p.test(st))

                System.out.println(st);

        }

    }

}

Geek
GeeksQuiz
Geek2

Functional Interfaces Table