Exception encountered while repainting Polyline

How do I prevent a ConcurrentModificationException while rendering a polyline?

I have an ArrayDeque of 25 positionObjects in a thread that sleeps on a 500ms interval. This is the run method:

public synchronized void run() {
historyLayer.addRenderable(polyline);
boolean layerAdded = true;

Object lock = new Object();
while (!stopped) {

if (!positionObjects.isEmpty()) {
//NOTE: WWJ polyline class is throwing concurrent modification exception
synchronized (lock) {
polyline.setPositions(positionObjects);

}
}
//if not viewing the history, remove it from screen
if (!displaying) {
historyLayer.removeRenderable(polyline);
layerAdded = false;
}
// only add one polyline to the historyLayer
else if(!layerAdded) {
historyLayer.addRenderable(polyline);
layerAdded = true;
}

}

Oct 26, 2010 8:39:12 PM gov.nasa.worldwind.AbstractSceneControll er draw
SEVERE: Exception encountered while repainting
java.util.ConcurrentModificationExceptio n
at java.util.AbstractList$Itr.checkForComod ification(AbstractList.java:372)
at java.util.AbstractList$Itr.next(Abstract List.java:343)
at gov.nasa.worldwind.render.Polyline.drawO rderedRenderable(Unknown Source)
at gov.nasa.worldwind.render.Polyline.draw( Unknown Source)
at gov.nasa.worldwind.render.Polyline.rende r(Unknown Source)
at gov.nasa.worldwind.AbstractSceneControll er.draw(Unknown Source)
at gov.nasa.worldwind.BasicSceneController. doRepaint(Unknown Source)
at gov.nasa.worldwind.AbstractSceneControll er.repaint(Unknown Source)
at gov.nasa.worldwind.WorldWindowGLAutoDraw able.doDisplay(Unknown Source)
at gov.nasa.worldwind.WorldWindowGLAutoDraw able.display(Unknown Source)
at com.sun.opengl.impl.GLDrawableHelper.dis play(GLDrawableHelper.java:78)
at javax.media.opengl.GLCanvas$DisplayActio n.run(GLCanvas.java:435)
at com.sun.opengl.impl.GLDrawableHelper.inv okeGL(GLDrawableHelper.java:194)
at javax.media.opengl.GLCanvas.maybeDoSingl eThreadedWorkaround(GLCanvas.java:412)
at javax.media.opengl.GLCanvas.display(GLCa nvas.java:244)
at javax.media.opengl.GLCanvas.paint(GLCanv as.java:277)
at javax.media.opengl.GLCanvas.update(GLCan vas.java:354)
at sun.awt.RepaintArea.updateComponent(Repa intArea.java:239)
at sun.awt.RepaintArea.paint(RepaintArea.ja va:216)
at sun.awt.windows.WComponentPeer.handleEve nt(WComponentPeer.java:306)
at java.awt.Component.dispatchEventImpl(Com ponent.java:4706)
at java.awt.Component.dispatchEvent(Compone nt.java:4460)
at java.awt.EventQueue.dispatchEvent(EventQ ueue.java:599)
at java.awt.EventDispatchThread.pumpOneEven tForFilters(EventDispatchThread.java:269 )
at java.awt.EventDispatchThread.pumpEventsF orFilter(EventDispatchThread.java:184)
at java.awt.EventDispatchThread.pumpEventsF orHierarchy(EventDispatchThread.java:174 )
at java.awt.EventDispatchThread.pumpEvents( EventDispatchThread.java:169)
at java.awt.EventDispatchThread.pumpEvents( EventDispatchThread.java:161)
at java.awt.EventDispatchThread.run(EventDi spatchThread.java:122)