Question

I am following few tutorials on JMonkey2.1

When I run those tutorials I get NullPointerExceptions where new ImageIcons are loaded using java.net.URL.

// generate a terrain texture with 3 textures
ProceduralTextureGenerator pt = new ProceduralTextureGenerator(heightMap);
pt.addTexture(new ImageIcon(Lesson3.class.getClassLoader().getResource("path/to/file")), -128, 0, 128); //line 199
pt.addTexture(new ImageIcon(Lesson3.class.getClassLoader().getResource("dirt.jpg")), 0, 128, 255);
pt.addTexture(new ImageIcon(Lesson3.class.getClassLoader().getResource("highest.jpg")), 128, 255, 384);
pt.createTexture(32);

But when I parse file path as a string instead of reading them as java.net.URL they work fine.

e.g: new ImageIcon("path/to/file")

This is the stack trace

Jun 4, 2011 4:18:22 PM class jme.test.Lesson3 start()
SEVERE: Exception in game loop
java.lang.NullPointerException
at javax.swing.ImageIcon.(ImageIcon.java:167)
at jme.test.Lesson3.buildTerrain(Lesson3.java:199)
at jme.test.Lesson3.initGame(Lesson3.java:151)
at com.jme.app.BaseGame.start(BaseGame.java:74)
at jme.test.Lesson3.main(Lesson3.java:62)
at jme.test.Main.main(Main.java:14)
Jun 4, 2011 4:18:22 PM com.jme.app.BaseGame start
INFO: Application ending.

What might be the reason for this?
Thank you..

Answer 1

In order to use ClassLoader#getResource(), the resource needs to be in the classpath.

When it’s located in the same package as Lesson3 class, then do so

new ImageIcon(Lesson3.class.getResource("image.png"));

When it’s located in a different package, then use an absolute path from the classpath root on, i.e. start with /.

new ImageIcon(Lesson3.class.getResource("/com/example/image.png"));

This expects the file image.png to be in the package com.example.

Using new File() is not recommended for relative paths as it depends on the current working directory which in turn depends on the way how you started the application which is not controllable from inside the application.