JComboBox - padding / spacing between arrow and label

Question

I created JComboBox and changed it background and foreground color. But I see not expected white-gray vertical line there and I don't know how to fix it. This verticl line should be painted too, but it's not.

import javax.swing.*;
import java.awt.*;
import java.util.Arrays;

public class TestComboBox {

    private static final String[] ANIMALS = new String[]{"Cat", "Mouse", "Dog", "Elephant", "Bird", "Goat", "Bear"};
    private static final Color COMBO_COLOR = new Color(71, 81, 93);

    public static class MessageComboBox extends JComboBox<String> {

        public MessageComboBox(DefaultComboBoxModel model) {
            super(model);
            setFont(new Font("Arial", Font.PLAIN, 30));
            setPreferredSize(new Dimension(350, 50));
            setRenderer(new MyRenderer());
        }
    }

    private static class MyRenderer extends DefaultListCellRenderer {

        @Override
        public Component getListCellRendererComponent(JList list, Object value,
                                                      int index, boolean isSelected, boolean cellHasFocus) {

            JComponent comp = (JComponent) super.getListCellRendererComponent(list,
                    value, index, isSelected, cellHasFocus);

            list.setBackground(COMBO_COLOR);
            list.setForeground(Color.WHITE);
            list.setOpaque(false);

            return comp;
        }
    }


    public static void main(String[] args) throws Exception {
        String nimbus = Arrays.asList(UIManager.getInstalledLookAndFeels())
                .stream()
                .filter(i -> i.getName().equals("Nimbus"))
                .findFirst()
                .get()
                .getClassName();

        UIManager.setLookAndFeel(nimbus);
        UIManager.put("ComboBox.forceOpaque", false);

        JFrame jf = new JFrame();
        jf.setSize(800, 400);
        jf.setVisible(true);
        jf.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        jf.setLocationRelativeTo(null);

        MessageComboBox comboBox = new MessageComboBox(new DefaultComboBoxModel(ANIMALS));
        JPanel panel = new JPanel();
        panel.add(comboBox);
        jf.add(panel, BorderLayout.NORTH);
    }
}

Maybe someone has ideas on how to fix this?

Answer 1

The Nimbus defaults say, the default ComboBox.padding is Insets(3,3,3,3) (See Insets doc). Therefore the unexpected grey line is due to this padding. Since you want to remove the right padding, you can fix your problem by adding this line somewhere to your first UIManager-call:

UIManager.put("ComboBox.padding", new Insets(3, 3, 3, 0));

Result: