It is impossible to specify tree in jtree widgets. Only adding root node works without problem: jtree .tree .tree node add root "Root node" Now when I try to add other nodes command failed .tree node add 0 "First node" Error message: Exception in thread "AWT-EventQueue-0" java.lang.ArrayIndexOutOfBoundsException: node has no children at javax.swing.tree.DefaultMutableTreeNode.getChildAt(DefaultMutableTreeNode.java:228) at com.onemoonscientific.swank.SwkJTreeWidgetCmd$NodeAdd.run(SwkJTreeWidgetCmd.java:210) at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:209) at java.awt.EventQueue.dispatchEvent(EventQueue.java:597) at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:269) at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:184) at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:174) at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:169) at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:161) at java.awt.EventDispatchThread.run(EventDispatchThread.java:122) As I discovered in source there is no possibility to add nodes which follow root node - I temporary modify in SwkJTreeWidgetCmd.java method run of NodeAdd class: "for" loop (over iNodes for calculating refNode) is executed only when iNodes[0]>=0, patch is follow (don sure that I made it in right way because I done it under windows with WinMerge utility) 206a207 > if (iNodes[0]>=0) { 209a211 > } Now these commands work .tree node add -1 "First node" .tree node add -1 "Second node" .tree node add -1 "Third node" .tree node add 0 "Second level node" .tree node add {0 0} "Third level node" My version of SwkJTreeWidgetCmd.java file is included as attachment