Clojure Nullpointer exception

Question

I am trying to write a clojure code which generates an Image with each pixel of image having a value of XOR of its x and y co-ordinates but I am getting a NullpointerException. I am not able to figure out what is going wrong in my code

Here is my code:

(ns com.test
  (:import (javax.swing JFrame))
  (:gen-class))


; Each pixel has color value of xor of its x and y co-ordinates
(defn generateXorList [limit]
  (for [x (range limit)
        y (range limit)]
    [x y (bit-xor x y)]
))


(defn makeImage [seq]
  (let [frameSize 250
        frame (JFrame.)
        gfx (.getGraphics frame)]    
    (.setVisible frame true)
    (.setSize frame frameSize frameSize)
    (.setDefaultCloseOperation frame JFrame/EXIT_ON_CLOSE)
    (doseq [[x y color] seq]      
      (.setColor gfx (java.awt.Color. color))
      (.fillRect gfx x y 1 1)
      )))

;; Generate the image
(makeImage (generateXorList 250))

Here is the stacktrace

clojure.lang.Compiler$CompilerException: java.lang.NullPointerException, compiling:(/tmp/t.clj:26:1)
 at clojure.lang.Compiler.load (Compiler.java:7239)
    user$eval1125.invoke (form-init5557027396137860263.clj:1)
    clojure.lang.Compiler.eval (Compiler.java:6782)
    clojure.lang.Compiler.eval (Compiler.java:6745)
    clojure.core$eval.invoke (core.clj:3081)
    clojure.main$repl$read_eval_print__7099$fn__7102.invoke (main.clj:240)
    clojure.main$repl$read_eval_print__7099.invoke (main.clj:240)
    clojure.main$repl$fn__7108.invoke (main.clj:258)
    clojure.main$repl.doInvoke (main.clj:258)
    clojure.lang.RestFn.invoke (RestFn.java:1523)
    clojure.tools.nrepl.middleware.interruptible_eval$evaluate$fn__623.invoke (interruptible_eval.clj:58)
    clojure.lang.AFn.applyToHelper (AFn.java:152)
    clojure.lang.AFn.applyTo (AFn.java:144)
    clojure.core$apply.invoke (core.clj:630)
    clojure.core$with_bindings_STAR_.doInvoke (core.clj:1868)
    clojure.lang.RestFn.invoke (RestFn.java:425)
    clojure.tools.nrepl.middleware.interruptible_eval$evaluate.invoke (interruptible_eval.clj:56)
    clojure.tools.nrepl.middleware.interruptible_eval$interruptible_eval$fn__665$fn__668.invoke (interruptible_eval.clj:191)
    clojure.tools.nrepl.middleware.interruptible_eval$run_next$fn__660.invoke (interruptible_eval.clj:159)
    clojure.lang.AFn.run (AFn.java:22)
    java.util.concurrent.ThreadPoolExecutor.runWorker (ThreadPoolExecutor.java:1142)
    java.util.concurrent.ThreadPoolExecutor$Worker.run (ThreadPoolExecutor.java:617)
    java.lang.Thread.run (Thread.java:745)
Caused by: java.lang.NullPointerException: null
 at com.test$makeImage.invoke (t.clj:22)
    com.test$eval1171.invoke (t.clj:27)
    clojure.lang.Compiler.eval (Compiler.java:6782)
    clojure.lang.Compiler.load (Compiler.java:7227)
    user$eval1125.invoke (form-init5557027396137860263.clj:1)
    clojure.lang.Compiler.eval (Compiler.java:6782)
    clojure.lang.Compiler.eval (Compiler.java:6745)
    clojure.core$eval.invoke (core.clj:3081)
    clojure.main$repl$read_eval_print__7099$fn__7102.invoke (main.clj:240)
    clojure.main$repl$read_eval_print__7099.invoke (main.clj:240)
    clojure.main$repl$fn__7108.invoke (main.clj:258)
    clojure.main$repl.doInvoke (main.clj:258)
    clojure.lang.RestFn.invoke (RestFn.java:1523)
    clojure.tools.nrepl.middleware.interruptible_eval$evaluate$fn__623.invoke (interruptible_eval.clj:58)
    clojure.lang.AFn.applyToHelper (AFn.java:152)
    clojure.lang.AFn.applyTo (AFn.java:144)
    clojure.core$apply.invoke (core.clj:630)
    clojure.core$with_bindings_STAR_.doInvoke (core.clj:1868)
    clojure.lang.RestFn.invoke (RestFn.java:425)
    clojure.tools.nrepl.middleware.interruptible_eval$evaluate.invoke (interruptible_eval.clj:56)
    clojure.tools.nrepl.middleware.interruptible_eval$interruptible_eval$fn__665$fn__668.invoke (interruptible_eval.clj:191)
    clojure.tools.nrepl.middleware.interruptible_eval$run_next$fn__660.invoke (interruptible_eval.clj:159)
    clojure.lang.AFn.run (AFn.java:22)
    java.util.concurrent.ThreadPoolExecutor.runWorker (ThreadPoolExecutor.java:1142)
    java.util.concurrent.ThreadPoolExecutor$Worker.run (ThreadPoolExecutor.java:617)
    java.lang.Thread.run (Thread.java:745)

Please, attach the stacktrace of NullPointerException thrown. — Piotrek Bzdyl
– Piotrek Bzdyl, Commented Jun 1, 2016 at 9:28

Piotrek Bzdyl · Accepted Answer · 2016-06-01 11:40:44Z

6

JFrame.getGraphics will return null until the frame is visible:

Creates a graphics context for this component. This method will return null if this component is currently not displayable.

You have to first call (.setVisible frame true) before calling (.getGraphics frame) and assigning it to a local variable.

(defn makeImage [seq]
  (let [frameSize 250
        frame (JFrame.)]
    (.setVisible frame true)
    (.setSize frame frameSize frameSize)
    (.setDefaultCloseOperation frame JFrame/EXIT_ON_CLOSE)
    (let [gfx (.getGraphics frame)]
      (doseq [[x y color] seq]      
        (.setColor gfx (java.awt.Color. color))
        (.fillRect gfx x y 1 1)))))

or setup your frame using doto:

(defn makeImage [seq]
  (let [frameSize 250
        frame (doto (JFrame.)
                (.setVisible true)
                (.setSize frameSize frameSize)
                (.setDefaultCloseOperation JFrame/EXIT_ON_CLOSE))
        gfx (.getGraphics frame)]    
    (doseq [[x y color] seq]      
      (.setColor gfx (java.awt.Color. color))
      (.fillRect gfx x y 1 1))))

edited Jun 1, 2016 at 11:40

answered Jun 1, 2016 at 9:42

Piotrek Bzdyl

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7 Comments

harrythomas Over a year ago

how do I assign it to a local variable?

Piotrek Bzdyl Over a year ago

Please, look at the enhanced answer.

harrythomas Over a year ago

Thanks for the answer! One question.. which approach should I prefer? 1st or 2nd?

Piotrek Bzdyl Over a year ago

In this particular case I prefer doto as it encapsulates the whole setup of an object before I can use it. I would even consider extracting this part into a separate function so that makeImage responsibility is only "making an image" and not setting up a frame.

Piotrek Bzdyl Over a year ago

As you work with Swing in Clojure you might be interested in seesaw project which provides friendly Clojure API for Swing.

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