Java Tutorial



Java Tutorial

Functional Interface in Java


In the world of Java programming, functional interfaces have emerged as powerful tools for implementing efficient programming concepts. They enable the use of lambda expressions and method references, making code concise and expressive. In this article, we will dive deep into predefined functional interfaces in Java, understand their syntax, explore various types of functional interfaces, and learn how to implement them effectively in your code. Whether you're a beginner or an experienced Java developer, this comprehensive guide will equip you with the knowledge to leverage the power of functional interfaces in Java 


Functional interfaces are essential for accepting functional programming ideas in the field of Java programming. Using lambda expressions and method references, they enable developers to construct code that is more succinct and expressive. This in-depth manual will take you on a trip to discover functional interfaces in Java. You will learn what functional interfaces in Java 8 are and how to define them in Java. Comprehend their syntax with a straightforward example and become familiar with the different applications of functional interfaces within a class. We will also go in-depth on the list of functional interfaces in Java, highlighting their functions and providing examples. You will also discover more functional interface examples to help you understand designing functional interfaces using lambda expressions and method references. We'll talk about circumstances where functional interfaces can extend non-functional interfaces and situations where they can have several default and static methods. Finally, we'll look at Java's built-in functional interfaces before wrapping up with a recap of the main ideas discussed.

What is a Functional Interface in Java?

A functional interface in Java is an interface that has only one abstract method. It serves as a blueprint for implementing lambda expressions or method references. By restricting the interface to a single abstract method, Java ensures that it qualifies as a functional interface, allowing one to use it in functional programming paradigms. The `@FunctionalInterface` annotation is used to indicate that an interface is a functional interface.

To understand this concept better, let's consider a simple example:

interface MathOperation {
    int operate(int a, int b);

Here, the `MathOperation` interface defines a single abstract method, `operate`, which takes two integer parameters and returns an integer result. With this interface, we can define various mathematical operations using lambda expressions or method references.

Syntax of Functional Interfaces: A simple functional interface in Java example

We will use a straightforward example to explain the syntax of functional interfaces. Here we elaborate on how lambda expressions may be used to construct functional interfaces and demonstrate their versatility and succinctness. You'll discover how to make a functional interface instance and use it to carry out particular tasks. Let's consider the following example to illustrate the syntax:

interface Greeting {
    void greet(String message);
public class FunctionalInterfaceExample {
    public static void main(String[] args) {
        Greeting greeting = message -> System.out.println("Hello, " + message);

In this example, we define a functional interface `Greeting` with a single abstract method `greet,` which takes a `String` parameter. We then create an instance of this interface using a lambda expression, where we define the behavior of the `greet` method. Finally, we call the `greet` method with the argument "World" to output the greeting message.

Ways to Use Functional Interfaces in a Class

Functional interfaces can be used in various ways within a class. Some of them include:

- As method parameters: Functional interfaces can be passed as arguments to methods, allowing dynamic behavior based on different implementations.

- As return types: Methods can return functional interfaces, providing flexibility in selecting the appropriate behavior based on specific conditions.

- As local variables: Functional interfaces can be assigned to local variables, enabling the execution of different behaviors based on the assigned implementation.

By leveraging these techniques, you can create flexible and reusable code that adapts to different scenarios.

Types of Functional Interfaces

There are numerous varieties of functional interfaces, each serving a specific function. In this section, we will discuss some from the java.util.function module that is frequently used. We will describe each type's function and provide examples of its application.

Let's explore some of the commonly used functional interfaces along with their purposes and examples:

- Consumer<T>: Represents an operation that accepts a single input argument of type T and performs some action on it without returning any result.

Consumer<String> printMessage = message -> System.out.println(message);
printMessage.accept("Hello, Java!");

- Supplier<T>: Represents a supplier of results, producing a result of type T without taking any input.

Supplier<Double> getRandomNumber = () -> Math.random();
System.out.println("Random number: " + getRandomNumber.get());

- Function<T, R>: Represents a function that accepts an argument of type T and produces a result of type R.

Function<Integer, String> intToString = number -> "The number is: " + number;

- Predicate<T>: Represents a predicate (boolean-valued function) of one argument of type T.

Predicate<Integer> isEven = number -> number % 2 == 0;
System.out.println("Is 10 even? " + isEven.test(10));

These are just a few examples of functional interfaces available in Java. Each one caters to a specific use case, enabling you to write concise and expressive code.

Implementing the Interface

Working with functional programming concepts requires the implementation of a functional interface. In this section, we'll look at how to use lambda expressions and method references to create functional interfaces. You will see how to specify the behavior of the abstract method and execute it using the implemented interface through a real example.

The following functional interface in Java example covers both approaches:

interface Calculator {
    int calculate(int a, int b);
public class CalculatorExample {
    public static void main(String[] args) {
        // Using a lambda expression
        Calculator addition = (a, b) -> a + b;
        System.out.println("Result of addition: " + addition.calculate(4, 5));
        // Using method references
        Calculator subtraction = CalculatorExample::subtract;
        System.out.println("Result of subtraction: " + subtraction.calculate(10, 3));
 private static int subtract(int a, int b) {
        return a - b;


In the `main` method of the `CalculatorExample` class, we create two instances of the `Calculator` functional interface: `addition` and `subtraction.` 

The `addition` instance uses a lambda expression `(a, b) -> a + b` to define the behavior of the `calculate` method, which performs addition. So when we call `addition.calculate(4, 5)`, it will return the sum of `4` and `5`, which is `9`.

The `subtraction` instance, on the other hand, uses a method reference. `CalculatorExample::subtract` to define the behavior of the `calculate` method, which performs subtraction. The `subtract` method is a static method within the `CalculatorExample` class that subtracts the second parameter from the first. When we call `subtraction.calculate(10, 3)`, it will return the result of `10 - 3`, which is `7`.

Finally, we print the results using `System.out.println`; the output is as shown above.

In this example, we define a functional interface `Calculator` with a single abstract method `calculate.` We implement the interface using a lambda expression to perform addition and method references to perform subtraction. The output of the calculations is displayed accordingly.

More Examples

To further enhance your understanding of functional interfaces, we will provide additional examples. These will cover various scenarios and use cases, such as working with functional interfaces that have multiple parameters and returning results. By examining these examples, you will gain a deeper appreciation for the versatility and power of functional interfaces.

- BiFunction<T, U, R>: Represents a function that accepts two arguments of types T and U and produces a result of type R.

BiFunction<Integer, Integer, String> sumToString = (a, b) -> "Sum: " + (a + b);

System.out.println(sumToString.apply(5, 7));

- UnaryOperator<T>: Represents an operation on a single operand of type T, producing a result of the same type.

UnaryOperator<Integer> square = number -> number * number;

System.out.println("Square of 8: " + square.apply(8));

These examples demonstrate how functional interfaces can be used to perform various operations conveniently.

Functional Interface Having Multiple Default and Static Methods

A functional interface can have multiple default and static methods in addition to the single abstract method. However, only the abstract method qualifies it as a functional interface.

interface Vehicle {
    void drive();

    default void honk() {
    static void stop() {

If you run the following code that utilizes the `Vehicle` functional interface:

public class VehicleExample {
    public static void main(String[] args) {
        Vehicle vehicle = () -> System.out.println("Driving...");;

The output will be:

In the `main` method of the `VehicleExample` class, we create an instance of the `Vehicle` functional interface using a lambda expression `() -> System.out.println("Driving...")`. This lambda expression defines the behavior of the `drive` method, which prints "Driving..." when invoked. 

When we call ``, it executes the lambda expression and prints "Driving...". 

The `honk` method is a default method defined within the `Vehicle` interface, and it prints "Honking...". When we call `vehicle.honk()`, it executes the default method and prints "Honking...".

The `stop` method is a static method defined within the `Vehicle` interface, and it prints "Stopping...". When we call `Vehicle.stop()`, it directly invokes the static method and prints "Stopping...".

Hence, the output of the code will be as shown above.

In this example, the `Vehicle` interface has an abstract method, `drive`, a default method `honk` and a static method, `stop`. The default and static methods provide additional functionality but don't affect the functional nature of the interface.

Functional Interface Extending to a Non-Functional Interface

A functional interface can extend a non-functional interface as long as it still adheres to the single abstract method rule. However, it's important to note that the extended non-functional interface methods won't be considered part of the functional interface.

interface Drawable {
    void draw();
interface ColoredDrawable extends Drawable {
    void setColor(String color);

In this example, the `ColoredDrawable` functional interface extends the `Drawable` non-functional interface. It adds a new abstract method, `setColor`, but the `draw` method from `Drawable` doesn't contribute to the functional interface.

Built-in Java Functional Interfaces

Java provides a set of built-in functional interfaces in the `java.util.function` package. These interfaces cover a wide range of common use cases, reducing the need to define custom functional interfaces. Some notable built-in functional interfaces include:

- `BiConsumer<T, U>`

- `BiFunction<T, U, R>`

- `BinaryOperator<T>`

- `Predicate<T>`

- `Supplier<T>`

- `UnaryOperator<T>`

By leveraging these, you can write concise and readable code, enhancing your productivity as a Java developer.


Java's functional interfaces give programmers the freedom to adopt functional programming paradigms, allowing for the creation of clear and expressive code. They offer versatility by enabling the implementation of behaviors using lambda expressions and method references. You can make the most of functional programming in Java by comprehending its syntax, types, implementation strategies, and built-in functional interfaces. Gaining proficiency with functional interfaces will improve your Java programming abilities and make your code more effective and maintainable, whether working on little projects or complicated apps. 


1. How many abstract methods do a functional interface contain?

  There can only be one abstract method on a functional interface. Although it may contain a number of default and static methods, only one abstract method counts as a functional interface.

2. What are some of the functional interfaces in Java 8?

Examples of functional interfaces in Java 8 include Consumer, Supplier, Function, and Predicate.

3. How do functional interfaces improve code readability?

Since functional interfaces support the usage of lambda expressions and method references, programmers can create shorter and more expressive code. Functional interfaces make code more clear and maintainable by removing the need for lengthy anonymous classes.

4. How to design your own unique functional interfaces? 

You can design your own unique functional interfaces. Simply declare an interface with a single abstract method and annotate it with '@FunctionalInterface' to show that it is functional. To guarantee that it meets the requirements for a functional interface, keep in mind the one abstract method rule.

5. Can streams and collections be utilized with functional interfaces?

In Java, streams and collections may both be used successfully with functional interfaces. In order to effectively filter, convert, and process components in collections and streams, they offer potent structures like "Predicate," "Function," and "Consumer."

Leave a Reply

Your email address will not be published. Required fields are marked *