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
super.preservingSelection(updateCode);
	applyState(checkedNodes, grayedNodes, getControl());
}
/* (non-Javadoc)
 * Method declared on ICheckable.
 */
public void removeCheckStateListener(ICheckStateListener listener) {
	checkStateListeners.remove(listener);
}
/* (non-Javadoc)
 * Method declared on ICheckable.
 */
public boolean setChecked(Object element, boolean state) {
	Assert.isNotNull(element);
	Widget widget = internalExpand(element, false);
	if (widget instanceof TreeItem) {
		((TreeItem) widget).setChecked(state);
		return true;
	}
	return false;
}
/**
 * Sets the checked state for the children of the given item.
 *
 * @param item the item
 * @param state <code>true</code> if the item should be checked,
 *  and <code>false</code> if it should be unchecked
 */
private void setCheckedChildren(Item item, boolean state) {
	createChildren(item);