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/CheckboxTableViewer.java
TableItem item = children[i];
		if (item.getChecked())
			v.add(item.getData());
	}
	return v.toArray();
}
/**
 * Returns the grayed state of the given element.
 *
 * @param element the element
 * @return <code>true</code> if the element is grayed,
 *   and <code>false</code> if not grayed
 */
public boolean getGrayed(Object element) {
	Widget widget = findItem(element);
	if (widget instanceof TableItem) {
		return ((TableItem) widget).getGrayed();
	}
	return false;
}
/**
 * Returns a list of elements corresponding to grayed nodes in this
 * viewer.
 * <p>
 * This method is typically used when preserving the interesting
 * state of a viewer; <code>setGrayedElements</code> is used during the restore.
 * </p>
 *
 * @return the array of grayed elements
 * @see #setGrayedElements