Java to Clojure

April 16, 2024

Making the move to a functional paradigm

In recent years, functional programming has surged in popularity, thanks to its emphasis on simplicity, scalability, and robustness. As developers seek more efficient ways to build software, languages like Clojure have emerged as powerful tools for embracing the functional paradigm.


The rise of functional programming

Functional programming, with its focus on immutability, pure functions, and higher-order functions, has gained traction due to its ability to reduce complexity and improve code maintainability. This approach offers benefits such as enhanced code readability, easier debugging, and better support for concurrency.


Introducing Clojure

Among the array of functional programming languages, Clojure stands out as a dynamic and expressive language built on the Java Virtual Machine (JVM). Developed by Rich Hickey, Clojure combines the elegance of Lisp with modern programming concepts to provide a pragmatic approach to functional programming. Its seamless integration with Java libraries and ecosystem makes it an attractive choice for Java developers looking to explore the functional paradigm.


Purpose of the blog post

In this blog post, we delve into the transition from Java to Clojure, exploring the advantages of embracing a functional paradigm along the way. We’ll discuss the key differences between Java and Clojure, highlight Clojure’s unique features, and provide insights into making a successful migration. Whether you’re a seasoned Java developer or new to functional programming, this post aims to guide you through the exciting journey of making the move to Clojure.


Understanding Java and Clojure

Overview of Java

Java stands as one of the most widely-used programming languages globally (high ranking on TIOBE index for 20 years), renowned for its robustness, platform independence, and extensive ecosystem. Embracing an object-oriented paradigm, Java revolves around the concepts of classes, objects, and inheritance. With its static typing system and emphasis on explicit declaration, Java fosters a disciplined approach to software development, making it a popular choice for building enterprise-level applications, web services, and Android mobile applications.


Introduction to Clojure

In contrast, Clojure presents itself as a modern Lisp dialect, designed to leverage the power of functional programming on the Java Virtual Machine (JVM). Developed by Rich Hickey, Clojure inherits Lisp’s legacy of simplicity, expressiveness, and homoiconicity, while infusing it with modern features such as immutable data structures, higher-order functions, and lazy sequences. By embracing a functional paradigm, Clojure advocates for stateless computation, emphasizing pure functions and immutable data, which leads to more predictable, scalable, and maintainable code.


Highlighting differences

When going from Java to Clojure, developers encounter notable differences in syntax, paradigm, and ecosystem. While Java adheres to an object-oriented paradigm with its imperative style of programming, Clojure embraces a functional paradigm, encouraging declarative and immutable approaches to solving problems. Moreover, Java boasts a vast ecosystem of libraries, frameworks, and tools, while Clojure leverages the extensive Java ecosystem while offering its own set of libraries and tools tailored for functional programming.


Benefits of functional programming

Functional programming offers a paradigm shift from the traditional object-oriented approach, introducing several advantages that resonate with modern software development practices. Let’s explore some of the key benefits of functional programming over object-oriented programming.

Immutability: In functional programming, immutability is a fundamental concept where data structures, once created, cannot be modified. Instead of mutating existing data, functions operate on immutable data structures, creating new data as needed. This approach eliminates side effects and makes programs more predictable and easier to reason about. By embracing immutability, functional programming promotes safer concurrency, thread safety, and easier debugging.

Pure functions: Pure functions are functions that produce the same output for a given input, without modifying any external state or relying on external resources. Pure functions are devoid of side effects, making them deterministic and easier to test and reason about. By emphasizing pure functions, functional programming encourages modular, composable, and reusable code, leading to improved code maintainability and reliability.

Higher-order functions: Functional programming languages treat functions as first-class citizens, allowing functions to be passed as arguments to other functions, returned as results, or stored in data structures. This enables the use of higher-order functions, such as map, filter, and reduce, which operate on collections of data and abstract away common patterns of computation. Higher-order functions promote code abstraction, conciseness, and expressiveness, enabling developers to write more declarative and elegant code.

Functional programming promotes code clarity, maintainability, and scalability by emphasizing immutable data, pure functions, and higher-order functions. By embracing these principles, developers can write code that is easier to understand, test, and maintain, leading to more robust and reliable software systems.


Going from Java to Clojure

As Java developers embark on the journey of going to Clojure, they may encounter common challenges and misconceptions that can hinder their progress. However, with the right guidance and mindset, this transition can be smooth and rewarding. Let’s address some of the key considerations for making the move from Java to Clojure.


Addressing common challenges

Syntax Differences: Clojure’s Lisp-inspired syntax may initially feel unfamiliar to Java developers accustomed to curly braces and semicolons. However, with practice and exposure, developers can quickly adapt to Clojure’s syntax and appreciate its simplicity and expressiveness.

Functional mindset: Shifting from an object-oriented paradigm to a functional paradigm requires a change in mindset. Developers may struggle initially with concepts such as immutability, pure functions, and higher-order functions. However, understanding the benefits of functional programming and practicing these concepts gradually will facilitate the transition.

Tooling and ecosystem: Java developers may be accustomed to a rich ecosystem of IDEs, build tools, and libraries. While Clojure’s ecosystem may appear less mature at first glance, it leverages the vast Java ecosystem and offers its own set of powerful tools and libraries tailored for functional programming.


Guidance on embracing Clojure’s functional paradigm

Embrace immutable data: Clojure encourages the use of immutable data structures, which promote safer concurrency and easier reasoning about code. Embrace immutability by favoring functions that return new data instead of mutating existing data.

Think functionally: Shift your mindset from imperative to declarative programming. Focus on writing pure functions that operate on immutable data, avoiding side effects and mutable state whenever possible. Embrace higher-order functions and functional constructs like map, filter, and reduce to solve problems in a more concise and expressive manner.

Learn by doing: The best way to learn Clojure is by writing Clojure code. Start with small, manageable projects and gradually increase the complexity as you become more comfortable with the language. Leverage resources such as online tutorials, documentation, and community forums to accelerate your learning journey.


Tips and best practices

Practice REPL-driven development: Clojure’s REPL (Read-Eval-Print Loop) enables interactive development, allowing you to experiment with code and iterate quickly. Embrace REPL-driven development as a core part of your workflow, leveraging it to explore language features, test code snippets, and debug issues in real-time.

Embrace functional idioms: Study idiomatic Clojure code and learn from experienced Clojure developers. Embrace functional programming idioms and best practices, such as thread-first and thread-last macros, destructuring, and recursion, to write more elegant and idiomatic Clojure code.

Build a supportive community: Join Clojure communities, attend meetups (Func Prog Sweden), and engage with fellow developers to exchange ideas, ask questions, and seek guidance. Building a supportive community of peers and mentors can greatly accelerate your learning and provide invaluable insights and encouragement along the way.


Key features of Clojure

As Java developers learn Clojure, understanding its key features is essential for harnessing the full power of this dynamic and expressive language. Let’s explore some of the core features of Clojure that distinguish it from Java and highlight its unique strengths.


Immutable data structures

Clojure provides a rich set of immutable data structures, including lists, vectors, sets, and maps. These data structures are persistent and immutable by default, meaning that once created, they cannot be modified. Instead of mutating existing data, Clojure encourages creating new data structures through functional transformations. This emphasis on immutability promotes safer concurrency, easier reasoning about code, and more predictable program behavior.


Higher-order functions

Clojure treats functions as first-class citizens, allowing them to be passed as arguments to other functions, returned as results, or stored in data structures. This enables the use of higher-order functions, such as map, filter, and reduce, which operate on collections of data and abstract away common patterns of computation. Higher-order functions enable developers to write more concise, expressive, and composable code, facilitating code reuse and abstraction.


Lazy sequences

Clojure introduces the concept of lazy sequences, which are sequences that are computed on-demand and only when needed. This lazy evaluation strategy allows for efficient handling of potentially infinite sequences and enables developers to work with large datasets without the need to eagerly compute the entire sequence upfront. Lazy sequences provide a powerful abstraction for dealing with data transformation and manipulation, while also promoting memory efficiency and performance optimization.


Simplicity, expressiveness, and conciseness

Clojure prides itself on its simplicity, expressiveness, and conciseness. With its minimalist syntax and focus on functional programming idioms, Clojure enables developers to express complex ideas in a clear and concise manner. The language’s emphasis on simplicity and expressiveness promotes readability, maintainability, and collaboration, making it an excellent choice for teams working on complex software projects.


Interoperability with Java

One of Clojure’s unique strengths is its interoperability with Java. Clojure code can leverage existing Java libraries and frameworks directly, allowing developers to tap into the vast Java ecosystem without sacrificing the benefits of functional programming. This interoperability enables gradual adoption of Clojure in existing Java projects, facilitates integration with legacy systems, and opens up new possibilities for building hybrid applications that combine the strengths of both languages.


Tools and Development Environment

Transitioning from Java to Clojure involves familiarizing yourself with new tools and development environments tailored for Clojure development. Let’s explore some of the essential tools and setups to kickstart your journey into Clojure programming:



Leiningen is a popular build automation tool for Clojure projects, similar to Apache Maven for Java projects. It simplifies the management of dependencies, project configuration, and building of Clojure projects. With Leiningen, you can easily create new Clojure projects, manage dependencies from Clojars (Clojure’s package repository), run tests, and package your application for deployment.


Clojure CLI

Clojure CLI is another build tool for Clojure projects, offering a more lightweight and flexible alternative to Leiningen. It provides command-line tools for project management, dependency resolution, and REPL-driven development. Clojure CLI embraces the simplicity and composability of Unix-style command-line tools, making it a popular choice for developers seeking a more minimalist approach to Clojure development.


Text editors and IDEs

Clojure development can be done using a variety of text editors and integrated development environments (IDEs). Popular choices include Emacs with CIDER, IntelliJ IDEA with the Cursive plugin, Atom with the Chlorine plugin, and VS Code with the Calva extension. These tools provide features such as syntax highlighting, code completion, REPL integration, and debugging support, enhancing the development experience for Clojure developers.


Setting up your Clojure development environment

  1. Install Java Development Kit (JDK): Clojure runs on the Java Virtual Machine (JVM), so you’ll need to install JDK 8 or later on your system.
  2. Choose a build tool: Decide whether you want to use Leiningen or Clojure CLI for managing your Clojure projects. Both tools have their strengths and can be used interchangeably based on your preferences and project requirements.
  3. Install Clojure: Clojure can be installed via Leiningen or Clojure CLI, or downloaded as a standalone JAR file from the official Clojure website. Follow the installation instructions provided by the chosen build tool to set up Clojure on your system.
  4. Set up your editor or IDE: Install the necessary plugins or extensions for your preferred text editor or IDE to support Clojure development. Configure your editor to use the Clojure REPL for interactive development and enable features such as syntax highlighting and code completion.
  5. Create your first Clojure project: Use Leiningen or Clojure CLI to create a new Clojure project, and open it in your chosen editor or IDE. Start writing Clojure code, experimenting with the language features, and exploring the Clojure ecosystem.


By setting up a development environment and familiarizing yourself with essential tools for Clojure development, you’ll be well-equipped to start your journey into the world of functional programming with Clojure. Whether you prefer a feature-rich IDE or a lightweight text editor, there are plenty of options available to suit your preferences and workflow.


Building your first Clojure application

Now that you’ve set up your Clojure development environment, it’s time to dive into building your first Clojure application. In this section, we’ll walk you through the process of creating a simple Clojure application from scratch, covering project structure, module definition, and basic functionality implementation.


Project structure

Begin by creating a new directory for your Clojure project. Inside this directory, create a file named project.clj if you’re using Leiningen or deps.edn if you’re using Clojure CLI. These files contain project metadata and dependencies.


Define modules

Clojure applications are organized into modules, each containing functions and data related to a specific aspect of the application. Create a new Clojure namespace (a file with the .clj extension) for your main application logic. For example, you could create a file named core.clj and define your application’s main namespace as follows:


(ns my-app.core
  (:require [clojure.string :as str]))


Implement basic functionality

Now, let’s implement some basic functionality in your Clojure application. For example, let’s create a function that greets the user

(ns my-app.core
  (:require [clojure.string :as str]))

(defn greet [name]
  (str "Hello, " name "!"))


Run your application

You can run your Clojure application using the Clojure REPL (Read-Eval-Print Loop) or by executing the main function defined in your namespace. If you’re using Leiningen, you can run your application using the lein run command. If you’re using Clojure CLI, you can run your application using the clojure command followed by the namespace containing your main function. For example:

$ lein run




$ clojure -m my-app.core


Test your application

As you build your Clojure application, it’s important to write tests to ensure its correctness and reliability. Clojure provides built-in testing facilities such as the clojure.test library. Write test cases for your functions and run them using the lein test command if you’re using Leiningen or clojure -A:test if you’re using Clojure CLI.

(ns my-app.core-test
  (:require [clojure.test :refer :all]
            [my-app.core :refer :all]))

(deftest test-greet
  (is (= "Hello, World!" (greet "World"))))



Congratulations! You’ve successfully built your first Clojure application. This simple example demonstrates the basics of creating a Clojure project, defining namespaces, implementing functions, and running tests. As you continue your journey into Clojure development, explore more advanced topics, libraries, and tools to unlock the full potential of functional programming with Clojure.



As we conclude our exploration of transitioning from Java to Clojure and embracing the functional paradigm, it’s essential to reflect on the key insights and takeaways from this journey. Let’s summarize the key points discussed in this blog post and emphasize the transformative potential of making the move to Clojure.


Going to Clojure

In this blog post, we’ve delved into the transition from Java to Clojure, highlighting the benefits of embracing a functional paradigm and exploring the unique features of Clojure as a dynamic and expressive language. We’ve addressed common challenges and considerations faced by Java developers during the shift and provided strategies for overcoming obstacles and maximizing productivity in Clojure development.


Embracing Clojure’s functional paradigm

Clojure offers a pragmatic approach to functional programming, combining the elegance of Lisp with modern programming concepts to provide a powerful tool for building elegant and scalable software solutions. By embracing Clojure’s functional paradigm, developers can leverage immutability, pure functions, and higher-order functions to write concise, expressive, and maintainable code.


Exploring real-world applications

Clojure’s functional paradigm extends beyond theoretical concepts to practical applications in real-world projects. From web development to data processing and distributed systems, Clojure’s simplicity, expressiveness, and conciseness empower developers to tackle complex challenges with confidence and efficiency.


Inspiration for developers

As we conclude our journey, we encourage you to embrace the opportunity to learn and master Clojure for building elegant and scalable software solutions. Whether you’re a seasoned Java developer seeking new horizons or a newcomer to functional programming exploring exciting possibilities, Clojure offers a transformative experience that will expand your programming horizons and elevate your skills to new heights.


Final words

Going from Java to Clojure is not just a change in programming languages; it’s a shift in mindset and approach to software development. By embracing Clojure’s functional paradigm, you’ll unlock new avenues for creativity, innovation, and collaboration.


Additional resources

Check out the Ada Beat Functional Programming blog for more topics, including functional programming principles, summaries of MeetUps, language specific articles, and much more. Whether you’re interested in functional programming theory or practical application, we have something for everyone.