NullPointerException clojure println

Question

I'm learning clojure and I found a problem in this very ugly, anti-idiomatic, stupid code that I wrote:

(ns music-tag.core
  (:import
   (java.io.File)
   (com.echonest.api.v4.EchoNestAPI)
   (com.echonest.api.v4.Track)))

(def api-key "JRZSJUMBK8VOOP0L2")

(def music (new java.io.File "/home/simo/Musica/musica_mp3/Rabiosa-Shakira.mp3"))
(def echo-nest (new com.echonest.api.v4.EchoNestAPI api-key))
(def traccia (. echo-nest uploadTrack music true))

(. traccia waitForAnalysis 30)

(do (println (. traccia getArtistName) (. traccia getTitle)))

When I run this file I get the right answer (print artist and title) but it throws a exception:

simo@simo:~/music-tag$ lein run
Shakira Rabiosa (Featuring Pitbull)
Exception in thread "main" java.lang.NullPointerException
    at user$eval39.invoke(NO_SOURCE_FILE:1)
    at clojure.lang.Compiler.eval(Compiler.java:6465)
    at clojure.lang.Compiler.eval(Compiler.java:6455)
    at clojure.lang.Compiler.eval(Compiler.java:6431)
    at clojure.core$eval.invoke(core.clj:2795)
    at clojure.main$eval_opt.invoke(main.clj:296)
    at clojure.main$initialize.invoke(main.clj:315)
    at clojure.main$null_opt.invoke(main.clj:348)
    at clojure.main$main.doInvoke(main.clj:426)
    at clojure.lang.RestFn.invoke(RestFn.java:421)
    at clojure.lang.Var.invoke(Var.java:405)
    at clojure.lang.AFn.applyToHelper(AFn.java:163)
    at clojure.lang.Var.applyTo(Var.java:518)
    at clojure.main.main(main.java:37)

Why? How can I solve it???

Thanks

Answer 1

I'm not sure, but I think it's just because there is no main method defined.

Try changing your code from the last def to:

(defn -main[] 
  (let [traccia (. echo-nest uploadTrack music true)]
    (do
      (. traccia waitForAnalysis 30)
      (println (. traccia getArtistName) (. traccia getTitle)))))

When lein compiles your code, it needs to run the top-level statements, which includes your method calls as they aren't hidden in a function. When it comes to running your code, it barfs with an exception as it has nothing to run.