Calling Java from Clojure
Discover how to invoke Java methods, access fields, and instantiate objects directly from Clojure code.
Calling Java from Clojure is a free Clojure Functional Programming & JVM Backend Development lesson on CoddyKit — lesson 1 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Clojure Functional Programming & JVM Backend Development learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.
Clojure & Java: Best Friends!
Clojure runs on the Java Virtual Machine (JVM), which means it can use all the amazing Java libraries directly! This is called Java Interop.
- Access Java classes and methods.
- Leverage the vast Java ecosystem.
- Combine Clojure's power with Java's stability.
This lesson will show you how to call Java code from Clojure.
Creating Java Objects
To use a Java class, you often need to create an instance of it. In Clojure, you can do this using a special syntax.
- Use
(new ClassName args)or the shorthand(ClassName. args). - The
.after the class name is a convenient way to call a constructor.
Example: (java.util.Date.) creates a new Date object.
Demo: New Date Object
Try creating a new java.util.Date object. It will represent the current date and time.
(ns my-app.core
(:gen-class))
(defn -main
"Creates and prints a new Java Date object."
[& args]
(let [current-date (java.util.Date.)]
(println "Current date:" current-date)))Calling Static Java Methods
Static methods belong to the class itself, not to a specific object instance. You can call them directly on the class.
- Syntax:
(ClassName/staticMethodName args) - The
/separates the class name from the static method name.
Example: (java.lang.Math/random) calls the random static method of the Math class.
Demo: Math.random()
Let's get a random number using Java's Math/random static method. It returns a double between 0.0 (inclusive) and 1.0 (exclusive).
(ns my-app.core
(:gen-class))
(defn -main
"Generates and prints a random number using Java's Math/random."
[& args]
(let [rand-num (java.lang.Math/random)]
(println "Random number:" rand-num)))Calling Instance Java Methods
Once you have an object, you can call its instance methods. These methods operate on that specific object's data.
- Syntax:
(.methodName instance args)or(. instance methodName args). - The
.before the method name indicates an instance method call.
Example: (.getTime my-date) calls the getTime method on the my-date object.
Demo: Date.getTime()
Create a Date object and then call its getTime() method. This method returns the number of milliseconds since January 1, 1970, 00:00:00 GMT.
(ns my-app.core
(:gen-class))
(defn -main
"Creates a Date object and calls its getTime() method."
[& args]
(let [current-date (java.util.Date.)
time-in-millis (.getTime current-date)]
(println "Current date:" current-date)
(println "Milliseconds since epoch:" time-in-millis)))Accessing Java Fields
Java objects can have fields (variables) that store data. You can access these fields directly from Clojure.
- Syntax:
(.-fieldName instance)for getting a field's value. - For static fields, use
(.-staticFieldName ClassName).
Example: (.-out System) accesses the out static field of the System class, which is a PrintStream.
Demo: System.out Field
The java.lang.System class has a static field named out, which is a PrintStream object. We can access it to print to the console.
(ns my-app.core
(:gen-class))
(defn -main
"Accesses System.out and prints a message using its println method."
[& args]
(let [out-stream (.-out java.lang.System)]
(.println out-stream "Hello from System.out!")))Quick Check: Java Interop
You've learned how to interact with Java classes. Which Clojure expression uses the ClassName. shorthand to instantiate a java.io.File object for the path "myfile.txt"?
Recap: Java Interop Basics
Great job! You've learned the fundamentals of interacting with Java from Clojure:
- Instantiate objects: Use
(ClassName. args)or(new ClassName args). - Call static methods: Use
(ClassName/staticMethodName args). - Call instance methods: Use
(.methodName instance args)or(. instance methodName args). - Access fields: Use
(.-fieldName instance).
This powerful capability allows you to leverage the entire Java ecosystem within your Clojure projects. Next, we'll explore implementing Java interfaces.
Frequently asked questions
Is the “Calling Java from Clojure” lesson free?
Yes — the full text of “Calling Java from Clojure” is free to read here on the web, and the Clojure Functional Programming & JVM Backend Development course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Clojure Functional Programming & JVM Backend Development course, upgrade to CoddyKit PRO.
What will I learn in “Calling Java from Clojure”?
Discover how to invoke Java methods, access fields, and instantiate objects directly from Clojure code. You practise Clojure Functional Programming & JVM Backend Development with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start Clojure Functional Programming & JVM Backend Development?
No prior experience is required. Clojure Functional Programming & JVM Backend Development on CoddyKit is structured for beginners through advanced learners; this is — lesson 1 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “Calling Java from Clojure” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this Clojure Functional Programming & JVM Backend Development lesson?
Yes. Every Clojure Functional Programming & JVM Backend Development lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.