Bug 138608 [Viewers] JFace AbstractTreeViewer: itemExists doesn't handle equal tree entries
I use a TreeViewer with - setUseHashLookup(true) - without any sorter or filter - with a comparer When I try to add an object that is equal to an existing entry in the tree, it will not be added. This is because the method private boolean itemExists(Item[] items, Object element) { if(usingElementMap()) { return findItem(element) != null; } for (int i = 0; i < items.length; i++) { if(items[i].getData().equals(element)) { return true; } } return false; } takes a shortcut if there is an element map, but the shortcut doesn't handle the 'multiple equal elements' case. There is a workaround, however: Setting a dummy sorter will avoid the call to itemExists and multiple equal elements can be added to the tree.
bundles/org.eclipse.jface/src/org/eclipse/jface/viewers/AbstractTreeViewer.java
if (getComparator() != null || hasFilters()) {
			add(parentElementOrTreePath, new Object[] { element });
			return;
		}
		Widget[] items = internalFindItems(parentElementOrTreePath);
		for (int i = 0; i < items.length; i++) {
			Widget widget = items[i];
			if (widget instanceof Item) {
				Item item = (Item) widget;
				Item[] childItems = getChildren(item);
				if (getExpanded(item)
						|| (childItems.length > 0 && childItems[0].getData() != null)) {
					
					int insertionPosition = position;
					if (insertionPosition == -1) {
						insertionPosition = getItemCount(item);
					}
					createTreeItem(item, element, position);
				}
			} else {
				int insertionPosition = position;
				if (insertionPosition == -1) {
					insertionPosition = getItemCount((Control)widget);
				}
				createTreeItem(widget, element, position);
			}
		}
	}
}