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 add(Object parentElement, Object childElement) {
		add(parentElement, new Object[] { childElement });
	}
	/**
	 * Adds the given SWT selection listener to the given SWT control.
	 * 
	 * @param control
	 *           the SWT control
	 * @param listener
	 *           the SWT selection listener
	 * @deprecated
	 */
	protected void addSelectionListener(
		Control control,
		SelectionListener listener) {
	    
	}
	/**
	 * Adds a listener for expand and collapse events in this viewer. Has no
	 * effect if an identical listener is already registered.
	 * 
	 * @param listener
	 *           a tree viewer listener
	 */
	public void addTreeListener(ITreeViewerListener listener) {
		treeListeners.add(listener);
	}
	/**
	 * Adds the given SWT tree listener to the given SWT control.
	 *