how to load Tree inside JComboBox?

How can I show a tree inside a JComboBox popup?

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There is no default way to put a tree in a combo box. There are a couple of options:

If you can give allowing expansion of nodes, you can achieve a similar effect by adding space before some of the options in a standard JComobBox. Or even space and a dash in front of leaf options.

If you need expansion of nodes, then a better option would be to add a popup that appears below a button that listens for selections of items in the tree. Something like this might be a better choice depending on how your GUI is laid out.

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When swinglabs was active there used to be JXComboBox that allows you to have other components in the dropdown like a JTable/JTree. Check here or here you may find the source for it or the documentation.

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You coud write your own renderer and put a treenode picture in front of the returning label for the subnodes.

Something like:

private static class NodeComboBoxRenderer implements ListCellRenderer {

    protected DefaultListCellRenderer defaultRenderer = new DefaultListCellRenderer();

    public Component getListCellRendererComponent(JList list, Object value, int index, boolean isSelected, boolean cellHasFocus) {
        JLabel renderer = (JLabel) defaultRenderer.getListCellRendererComponent(list, value, index, isSelected, cellHasFocus);
        if(value.isSubNode()) { //something to find out
            renderer.setIcon("here comes the resource");
        }
        return renderer;
    }
}

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I use the TreeComboBox of mindgame (link). It just requires the class AbstractComboBoxUI of the same project.