JComboBox does not play well with GroupLayout

Question

I ran into a strange kind of behavior when putting JComboBox components inside a GroupLayout. I've reduced the code to the below minimal example, featuring exactly one JComboBox layed out by GroupLayout.

The observed behavior is as follows:

When the frame is smaller than the expanded combo box list (with dummy entries a, b, c), clicking on the little down arrow on the combo box opens it correctly.
When I resize the frame to be bigger than the expanded combo box list would be, the expanded combo box list is not opened!

What I've found out already:

When the parent window of a JComboBox is too small, javax.swing.PopupFactory creates a HEAVY_WEIGHT_POPUP component.
When the parent window is big enough, a LIGHT_WEIGHT_POPUP is being created.
The behavior is definitely related to GroupLayout or some side effects of it, since any other layout managers I tried work just fine.
When using addGap() in GroupLayout the problem changes a bit, i.e. the window sizes where the JComboBox does not show up change.

This is the sample code - comments are welcome:

import java.awt.BorderLayout;

import javax.swing.JComboBox;
import javax.swing.JFrame;
import javax.swing.JRootPane;

public class DummyUI_cbdiagnosis extends javax.swing.JPanel {
    private javax.swing.JComboBox cbCategory;

    public DummyUI_cbdiagnosis() {
        initComponents();
    }

    private void initComponents() {
        cbCategory = new JComboBox();
        cbCategory.setModel(new javax.swing.DefaultComboBoxModel(new String[] {
                "a", "b", "c" }));

        javax.swing.GroupLayout layout = new javax.swing.GroupLayout(this);
        this.setLayout(layout);
        layout.setHorizontalGroup(layout.createParallelGroup(
                javax.swing.GroupLayout.Alignment.LEADING).addGroup(
                layout.createSequentialGroup().addComponent(cbCategory,
                        javax.swing.GroupLayout.PREFERRED_SIZE,
                        javax.swing.GroupLayout.PREFERRED_SIZE,
                        javax.swing.GroupLayout.PREFERRED_SIZE)));
        layout.setVerticalGroup(layout.createParallelGroup(
                javax.swing.GroupLayout.Alignment.LEADING).addGroup(
                layout.createSequentialGroup().addComponent(cbCategory)
        ));
    }

    public static void main(String[] args) {
        JFrame frame = new JFrame();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        JRootPane rootPane = frame.getRootPane();
        rootPane.setLayout(new BorderLayout());

        DummyUI_cbdiagnosis panel = new DummyUI_cbdiagnosis();
        rootPane.add(panel, BorderLayout.NORTH);

        frame.pack();
        frame.setVisible(true);
    }
}

You don't have to call package names explicitly, if you import them... — Mordechai, Nov 16 '12 at 17:05

Mordechai · Accepted Answer · 2012-11-20T09:41:50.447

6

Never ever add components to the RootPane itself, rather add them to the contentPane.

frame.add(panel);

or

frame.setContentPane(panel);

A RootPane has control of where to put the:

Menu bar.
Content.
Glass pane.
And most important, lightweight popups (inc. JComboBox), dialogs, drag and drops, etc.

RootPane uses a special layout manager called RootLayout, and shouldn't be changed to BorderLayout.

edited Nov 20 '12 at 09:41

answered Nov 16 '12 at 17:06

Mordechai

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Would you explain why I cannot add to the root pane? – Roman C Nov 16 '12 at 17:11
Not your down-voter, but `frame.add()` _forwards_ to the content pane, making `getContentPane()` superfluous; `frame.setContentPane()` _replaces_ the content pane, perhaps unintentionally. More [here](http://stackoverflow.com/a/11769153/230513). – trashgod Nov 16 '12 at 20:22
@trashgod seems I was backdated, `JFrame#add(...)` used to throw an `Error` on older versions of JDK. – Mordechai Nov 16 '12 at 20:32
Thanks, I did not realize that there is getRootPane() *and* getContentPane() - I mistook getRootPane() for the method that gives you the frame's top level pane (did remember though that frame.add() wasn't working in old JDKs; funny thing). – user1830252 Nov 18 '12 at 21:22

JComboBox does not play well with GroupLayout

1 Answers1