Strange tab-order with editable JComboBoxes

Question

I've got a `JFrame` that looks like this: It's got two `JTextField`s on it, one `JComboBox` between them and a `JPanel` at the bottom (that you can't see). One of the features of the `JComboBox` is that it can be given a custom editor. These implement the `ComboBoxEditor` interface. In each of the following three cases, the GUI looks exactly the same, and I would have expected them all to behave exactly the same: - I do not specify a custom editor, and use the default one. - I create a custom editor whose editor component is a `JTextField`. - I create a custom editor whose editor component is a `JPanel` with a `JTextField` on it (using a `BorderLayout`). When the editor for the editable combo box is set to the default, pressing Tab moves the focus from the top `JTextField` into the editing area on the `JComboBox` and then into the other `JTextField`. If I create a custom editor whose editor component is a `JTextField` and otherwise does what you would expect, the same thing happens. B…

Accepted Answer

The basic problem is that the combo's ui delegate can't handle compound editor components. There are several places where it assumes that the editor component is the target of whatever configuration it needs to do. The concrete mis-behaviour here is that it explicitly sets the editor's focusability to that of the combo itself

```
// in BasicComboBoxUI
protected void configureEditor() {
    ....
    editor.setFocusable(comboBox.isFocusable());
    ....
]
```

The implications

- by default, the panel's focusable is true because the combo's is true

- forcing the panel's focusable to false in its constructor has no effect (the ui resets it later on and whenever the LAF is switched)

- disabling combo's focusable disables the panel's as well

To fix on the level of the editor, you can implement its isFocusable to return false unconditionally:

```
private class PanelEditor extends JPanel implements ComboBoxEditor

public boolean isFocusable() {
        return false;
    }
    ...
}
```

An aside: for code hygiene, better not extend a view to implement its role as ComboBoxEditor (even though here you need a subclassed JPanel to avoid the problem, so it's arguably borderline :-) - instead implement the editor and let it use the tweaked panel.

Also beware that you might stumble into more problems with the compound editor (check the code of BasicComboUI for more places where it assumes a plain childless component), so you might consider not doing it at all but think of a different way to achieve your requirement.

Strange tab-order with editable JComboBoxes

Solution

Problem