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
internalSetExpandedTreePaths(expandedTreePaths, getControl(), new TreePath(new Object[0]));
    }
	/**
     * Sets whether the node corresponding to the given element or tree path is expanded or
     * collapsed.
     * 
     * @param elementOrTreePath
     *           the element
     * @param expanded
     *           <code>true</code> if the node is expanded, and <code>false</code>
     *           if collapsed
     */
    public void setExpandedState(Object elementOrTreePath, boolean expanded) {
        Assert.isNotNull(elementOrTreePath);
        Widget item = internalExpand(elementOrTreePath, false);
        if (item instanceof Item) {
            if (expanded) {
                createChildren(item);
            }
            setExpanded((Item) item, expanded);
        }
    }
    /**
     * Sets the selection to the given list of items.
     * 
     * @param items
     *           list of items (element type: <code>org.eclipse.swt.widgets.Item</code>)