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
*
 * @param parent the parent control
 * @param style the SWT style bits
 */
public CheckboxTreeViewer(Composite parent, int style) {
	this(new Tree(parent, SWT.CHECK | style));
}
/**
 * Creates a tree viewer on the given tree control.
 * The <code>SWT.CHECK</code> style bit must be set on the given tree control.
 * The viewer has no input, no content provider, a default label provider, 
 * no sorter, and no filters.
 *
 * @param tree the tree control
 */
public CheckboxTreeViewer(Tree tree) {
	super(tree);
}
/* (non-Javadoc)
 * Method declared on ICheckable.
 */
public void addCheckStateListener(ICheckStateListener listener) {
	checkStateListeners.add(listener);
}
/**
 * Applies the checked and grayed states of the given widget and its
 * descendents.
 *
 * @param checked a set of elements (element type: <code>Object</code>) 
 * @param grayed a set of elements (element type: <code>Object</code>)