Clojure: call java static method/field via instance variable or string (not class name symbol)

Question

I can do the following and it works...

=> (. java.awt.event.KeyEvent getKeyText 10)
"Enter"

But, I have an instance of java.awt.event.KeyEvent called ev. For example,

=> (class ev)
java.awt.event.KeyEvent

I want to call the method like this instead (but, this produces an error):

=> (. (class ev) getKeyText 10)
No matching method getKeyText found taking 1 args for class java.lang.Class

Is it possible to call a static method from an instance?

I have read the docs and searched stack overflow. The question here is not the same.

I may have found a duplicate question but i'll have to read the answers there... stackoverflow.com/questions/2852133/… — Joel M
– Joel M, Commented May 12, 2020 at 17:48

amalloy · Accepted Answer · 2020-05-12 18:14:43Z

1

Just like in Java, you can only call static methods directly if you know at compile time what method of what class you want to call. Otherwise you need to use reflection on the Class object to find method handles.

answered May 12, 2020 at 18:14

amalloy

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4 Comments

Joel M Over a year ago

Thanks... I figured that might be the case (I don't write Java). I guess i'll have to settle with... (condp = (class ev) FirstClass (FirstClass/someMethod) SecondClass (SecondClass/someMethod). Not exactly a solution if you don't know the full set of possible class names of an instance.

Joel M Over a year ago

I'll mark this as the accepted answer for now, as it's a good enough reason for me to re-factor my code away from this pattern. But if someone has a macro/reflection hack, feel free to share it.

amalloy Over a year ago

What are the other classes you're looking at that have a .getKeyText static method?

Joel M Over a year ago

In this case the specific class is just used as an example. There wouldn't be other classes as far as I know that define the getKeyText method.

Rulle · Accepted Answer · 2020-05-12 19:03:20Z

0

Here is an attempt using the MethodHandle API.

(ns playground.static-method-handle
  (:import [java.lang.invoke MethodType MethodHandles]))

;; Get a MethodType instance that encodes the shape of the method (return type and arguments)
(def getKeyText-method-type (MethodType/methodType String Integer/TYPE))

(defn call-getKeyText-on-class [cl key-code]
  (let [lu (MethodHandles/lookup)
        h (.findStatic lu cl "getKeyText" getKeyText-method-type)]
    (.invokeWithArguments h [(int key-code)])))

and we use it like this:

(call-getKeyText-on-class KeyEvent 10)
;; => "Enter"

answered May 12, 2020 at 19:03

Rulle

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