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/CheckboxTreeViewer.java
/**
	 * List of check state listeners (element type: <code>ICheckStateListener</code>).
	 */
	private ListenerList checkStateListeners = new ListenerList(3);
	/**
	 * Last item clicked on, or <code>null</code> if none.
	 */
	private TreeItem lastClickedItem = null;	
/**
 * Creates a tree viewer on a newly-created tree control under the given parent.
 * The tree control is created using the SWT style bits: <code>CHECK</code> and <code>BORDER</code>.
 * The viewer has no input, no content provider, a default label provider, 
 * no sorter, and no filters.
 *
 * @param parent the parent control
 */
public CheckboxTreeViewer(Composite parent) {
	this(parent, SWT.BORDER);
}
/**
 * Creates a tree viewer on a newly-created tree control under the given parent.
 * The tree control is created using the given SWT style bits, plus the <code>CHECK</code> style bit.
 * The viewer has no input, no content provider, a default label provider, 
 * no sorter, and no filters.