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// Error: non-static variable tree cannot be referenced from a static context:  tree = new JTree(treeModel);            public static void main(String args[]) {              TreeTest MyTree = new TreeTest();        MyTree.setVisible(true);                DefaultMutableTreeNode rootNode;        DefaultTreeModel treeModel;                rootNode = new DefaultMutableTreeNode("Root Node");        treeModel = new DefaultTreeModel(rootNode);                        tree = new JTree(treeModel);        tree.setEditable(true);        tree.getSelectionModel().setSelectionMode                (TreeSelectionModel.SINGLE_TREE_SELECTION);        tree.setShowsRootHandles(true);       }