Functional Interface Java Example: Java Explained

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The rise of functional programming in Java has been a major talking point in the programming community for many years since the introduction of lambda expressions in Java 8. Consequently, functional interfaces have become an important part of working with the modern versions of Java. In this article, we’ll look at what functional interfaces are and how they work, look at some of the benefits of functional interfaces, provide an overview of Java 8’s new features, explain how to create a functional interface in Java, and provide examples of functional interfaces. We’ll also look at how to implement lambda expressions in functional interfaces, share best practices for using them, and talk about their advantages.

What is a Functional Interface in Java?

A functional interface, also known as a single abstract method interface or a SAM interface, is an interface in Java that consists of one and only one abstract method. It serves to act as a common denominator for passing value-returning functions as parameters, or using assignments whenever the need arises. To create an interface that qualifies as a functional interface, it must not have any other abstract methods beyond the single one. This helps to make it easier for the developer to focus on the key concept without being overwhelmed by an excessive number of methods.

Functional interfaces are used to create lambda expressions, which are anonymous functions that can be used to pass code as data. This allows developers to write code that is more concise and easier to read. Additionally, functional interfaces can be used to create method references, which are used to refer to existing methods without having to explicitly write out the code. This can be especially useful when dealing with complex code that needs to be reused in multiple places.

Benefits of Functional Interfaces

One of the main benefits of using functional interfaces is that they dramatically shorten the process of writing code. This means that developers can drastically reduce the amount of coding time necessary to accomplish any specific task. Furthermore, they also allow developers to clearly and accurately represent the essential elements of their code within an easily read form. This makes code much easier to understand for both the initial creator and any subsequent developers who may need to work on the same project.

In addition, functional interfaces can also help to reduce the amount of errors that occur in code. By providing a clear and concise representation of the code, developers can more easily identify any potential errors and quickly fix them. This helps to ensure that the code is as efficient and effective as possible, and can help to reduce the amount of time spent debugging and troubleshooting.

Overview of Java 8’s New Features

Introduced in Java 8 were a variety of new features, each designed to enhance the speed and efficiency at which developers could achieve tasks, as well as increase the readability and maintainability of their code. These include lambda expressions and method references as well as a range of support for parallelism and streams. All of these features are enabled by the functional interfaces that come with Java 8.

Lambda expressions are a powerful tool that allow developers to write concise code that is easier to read and maintain. They also enable developers to write code that is more efficient and can be executed in parallel. Method references are another feature that allows developers to refer to existing methods in their code, making it easier to reuse code and reduce the amount of code that needs to be written. Finally, the support for parallelism and streams allows developers to take advantage of multiple cores in their system to execute tasks in parallel, resulting in faster execution times.

How to Create a Functional Interface in Java

To create a functional interface in Java one has to declare an interface that contains one and only one abstract method. Once declared, this interface should be annotated with the @FunctionalInterface annotation. This is an important step because it prevents the accidental introduction of extra abstract methods, which would render the interface non-functional. There is an exception to this annotation requirement for “default” methods, which are abstract methods that are defined within an interface but are not considered to be part of the interface’s single abstract method.

When creating a functional interface, it is important to remember that the interface must contain only one abstract method. Any additional methods, such as static methods, must be defined as default methods. Additionally, the interface must be annotated with the @FunctionalInterface annotation to ensure that the interface remains functional. By following these steps, developers can create a functional interface in Java that can be used to create powerful and efficient applications.

Examples of Functional Interfaces

Functional interfaces can come in various shapes and sizes depending upon the situation. A few examples include Runnable, which is used to define threads; Consumer, which can be used to define functors that have to consume or process data; and Supplier, which provides objects on request. Another example is Predicate, which is used to examine and evaluate data without the need for further instructions.

Functional interfaces can also be used to define callbacks, which are functions that are passed as arguments to other functions. This allows the callback to be executed at a later time, when the other function is finished. This is a powerful tool for asynchronous programming, as it allows for tasks to be executed in parallel, without blocking the main thread.

Implementing Lambda Expressions in Functional Interfaces

Lambda expressions are another key component that was introduced with Java 8 and are considered to be key components in utilizing functional programming in Java. To use a lambda expression with a functional method, the syntax is to match a compatible functional interface definition. For example, if we want to define a Runnable (which requires the “run()” function to be implemented), we would match it with this syntax: “() -> do something”. This will define a runnable object that will do something when the “run()” method is called.

Best Practices for Using Functional Interfaces

When using functional interfaces in Java it is important to follow certain best practices to ensure that everything runs smoothly. One such practice is to only declare one abstract method within a given functional interface. This is important because having more than one abstract method can lead to confusion, bugs, and other issues that may impact the overall performance and readability of code.Another important practice is to ensure that code clarity is maintained whenever possible. This can be accomplished by using Lambda expressions where appropriate and avoiding lengthy blocks of code that can be difficult to parse.

Advantages of Using Functional Interfaces

The advantages of using functional interfaces extend beyond just making it simpler for developers to write code for complex tasks. They also make debugging and maintainability easier, as well as allowing for more versatile and efficient concurrent programming. Additionally, it provides a more natural way for developers to express certain algorithms which allows for improved readability when reading code written by others.


Functional interfaces provide developers with a powerful tool for creating robust, efficient, and readable code in modern versions of Java. They make it easier for developers to work on complex tasks while still maintaining code readability and versatility. By following best practices such as declaring only one abstract method within a given functional interface and using Lambda expressions where appropriate, developers can ensure that their code is written quickly without sacrificing code quality.

Anand Das

Anand Das

Anand is Co-founder and CTO of Bito. He leads technical strategy and engineering, and is our biggest user! Formerly, Anand was CTO of Eyeota, a data company acquired by Dun & Bradstreet. He is co-founder of PubMatic, where he led the building of an ad exchange system that handles over 1 Trillion bids per day.

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