Bug 32254 [Viewers] AbstractTreeViewer uses HashSet to remember expansion state
I20030216 The custom hash table should be used instead. Otherwise expansion state may be lost due to a different equals implementation of the element and the IElementComparer.
bundles/org.eclipse.jface/src/org/eclipse/jface/viewers/AbstractTreeViewer.java
public void expandToLevel(Object element, int level) {
		Widget w = internalExpand(element, true);
		if (w != null)
			internalExpandToLevel(w, level);
	}
	/**
	 * Fires a tree collapsed event. Only listeners registered at the time this
	 * method is called are notified.
	 * 
	 * @param event
	 *           the tree expansion event
	 * @see ITreeViewerListener#treeCollapsed
	 */
	protected void fireTreeCollapsed(final TreeExpansionEvent event) {
		Object[] listeners = treeListeners.getListeners();
		for (int i = 0; i < listeners.length; ++i) {
			final ITreeViewerListener l = (ITreeViewerListener) listeners[i];
			Platform.run(new SafeRunnable() {
				public void run() {
					l.treeCollapsed(event);
				}
			});
		}
	}
	/**
	 * Fires a tree expanded event. Only listeners registered at the time this
	 * method is called are notified.
	 * 
	 * @param event
	 *           the tree expansion event