Question: I'm using the following example from [here](http://javadoc.jenkins-ci.org/index.html?hudson/FilePath.html):

```
 void someMethod(FilePath file) {
     // make 'file' a fresh empty directory.
     file.act(new Freshen());
 }
 // if 'file' is on a different node, this FileCallable will
 // be transferred to that node and executed there.
 private static final class Freshen implements FileCallable<Void> {
     private static final long serialVersionUID = 1;
     @Override public Void invoke(File f, VirtualChannel channel) {
         // f and file represent the same thing
         f.deleteContents();
         f.mkdirs();
         return null;
     }
 }

```

The Freshen class will be serialized and sent to a slave for execution. How can i get access and log progress to the logger on the master from inside my Freshen class?

Answer: 
I suspect the problem is your call to `glVertexPointer()`. The last parameter is supposed to be a pointer to the start of the array you're using, but at the moment you're just passing it a null pointer, so it's got nothing to work with.

Try this instead:

```
glVertexPointer(3, GL_FLOAT, 0, vertices);

```

EDIT: By the way, your code won't give you a box. If it works, I think it'll just give you two lines. Try using GL\_QUADS instead of GL\_LINES.
The main thing that pops out at me is the array: 
`//static GLfloat vertices[] = {0.0, 0.0, 0.0, 0.5, 0.0, 0.0, 0.5, 0.5, 0.0, 0.0, 0.5, 0.0, };`

should be

`static GLfloat vertices[] = {0.0, 0.0, 0.0, 0.5, 0.0, 0.0, 0.5, 0.5, 0.0, 0.0, 0.5, 0.0 };`
(note the removed comma)

Next thing is the enabling and disabling of the client state: that could be done in the main loop for example as it is expensive to enable and disable and your draw loop will suffer because of it.