Question

In Clojure, how to use a java Class that is stored in a variable?

How should I fix the following code?

(def a java.lang.String)
(new a "1"); CompilerException java.lang.IllegalArgumentException: Unable to resolve classname: a

And why this one works fine?

(def a str)
(a "1")

Answer 1

The most elegant solution is to write construct that does the same as new but is able to receive a class dynamically:

 (defn construct [klass & args]
    (clojure.lang.Reflector/invokeConstructor klass (into-array Object args)))
 (def a HashSet)
 (construct a '(1 2 3)); It works!!!

This solution overcomes the limitation of @mikera‘s answer (see comments).

Special Thanks to @Michał Marczyk that made me aware of invokeConstructor answering another question of mine: Clojure: how to create a record inside a function?.

Another option is to store the call to the constructor as an anonymous function. In our case:

(def a #(String. %1))
(a "111"); "111"