Clojure’s deftype allows you to define new types that implement Java interfaces or extend Java classes. This can be particularly useful when you want to integrate functionality from a Java library in your Clojure code. If you have a Java JAR file containing interfaces or abstract classes, you can create Clojure implementations using deftype!

Why use deftype?

  • Performance: Types created using deftype compile down to efficient Java bytecode, providing the speed of direct Java implementations.
  • Interop: Clojure’s seamless interop with Java makes it easy to extend Java classes and implement interfaces using familiar Clojure syntax.

Implementing a Java Interface:

Let’s assume you have a Java Jar file with an interface Greeting:

public interface Greeting {
    String sayHello(String name);
}

To implement this interface in Clojure using deftype, you could write:

(ns myapp.greeter
  (:import [com.example Greeting]))

(deftype Greeter []
  Greeting
  (sayHello [this name]
    (str "Hello, " name "!")))

Here, deftype creates a new type Greeter that implements the Greeting interface. The sayHello method is overridden with Clojure’s concise syntax.

Once you’ve created the implementation, you can use it as if it were a java object:

(let [greeter (Greeter.)]
  (.sayHello greeter "John Connor"))
;; => "Hello, John Connor!"
  • deftype works with Java interfaces and abstract classes.
  • It’s ideal for cases where performance is critical, as it compiles down to optimized bytecode.
  • This approach allows for seamless integration between Clojure and Java libraries.

By leveraging deftype, you can extend existing Java libraries in a more functional, expressive way using Clojure, without sacrificing performance.