Understanding Functional Interfaces in Java 8

Photo by Joan Gamell on Unsplash

Understanding Functional Interfaces in Java 8


Java 8 introduced a powerful new concept: functional interfaces. These interfaces are a key part of Java's support for functional programming concepts, and they work hand-in-hand with lambda expressions and method references. But what exactly are functional interfaces, and how do they work? Let's dive in.

What is a Functional Interface?

A functional interface in Java is an interface that contains exactly one abstract method. This abstract method is often referred to as the "functional method." While a functional interface can contain any number of default or static methods, it can only have one abstract method.

Here's an example of a functional interface:

public interface MyFunctionalInterface {
    void execute();

The @FunctionalInterface annotation is optional but recommended, as it causes the compiler to generate an error if the annotated interface does not satisfy the conditions of a functional interface.

Working with Lambda Expressions

One of the main reasons functional interfaces were introduced in Java 8 was to facilitate the use of lambda expressions. A lambda expression is a short block of code that can be passed around and executed later. It can be used to provide the implementation of the abstract method in a functional interface.

Here’s an example of a lambda expression that matches the MyFunctionalInterface from earlier:

MyFunctionalInterface myLambda = () -> System.out.println("Executing...");

In this example, the lambda expression () -> System.out.println("Executing...") provides the implementation for the execute method in the MyFunctionalInterface interface.

Leveraging Method References

Method references are another feature introduced in Java 8 that work well with functional interfaces. A method reference is a simplified, shorthand syntax for a lambda expression that executes just ONE method.

Here’s an example of a method reference:


This method reference matches a functional interface with a method that takes an Object and returns a String.


Functional interfaces, lambda expressions, and method references are powerful features in Java 8 that support functional programming concepts. Understanding these features can help you write more flexible and concise code.

Remember, the key to mastering these concepts is practice. So don’t hesitate to write some code and experiment with these features yourself. Happy coding!

Did you find this article valuable?

Support Jaydeep Ravat by becoming a sponsor. Any amount is appreciated!