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
Control tree = getControl();
        for (Enumeration e = parentItems.keys(); e.hasMoreElements();) {
            Item parentItem = (Item) e.nextElement();
            if(parentItem.isDisposed()) {
				continue;
			}
            if (!getExpanded(parentItem) && getItemCount(parentItem) == 0) {
                
                if (isExpandable(parentItem, null, parentItem.getData())) {
                    newItem(parentItem, SWT.NULL, -1);
                } else {
                    
                    tree.redraw();
                }
            }
        }
	}
    
    /**
     * Removes the given elements from this viewer, whenever those elements appear as children
     * of the given parent. 
     * 
     * @param elements
     *           the elements to remove
     * @since 3.1
     */
    protected void internalRemove(Object parent, Object[] elements) {
    	
    	CustomHashtable toRemove = new CustomHashtable(getComparer());
    	for (int i = 0; i < elements.length; i++) {