How to disable blue focus indicator effect inside JComboBox?

Question

It looks like i am asking for help to disable the blue focus indicator effect inside a JComboBox in Java Swing. i want to remove the visual indication of focus (the blue border) when the JComboBox is selected, i think i can achieve this by using a custom UI. I don't know how i can do that:

JComboBox<Integer> day = new JComboBox<>();
      for (int i = 1; i <= 31; i++) {
          day.addItem(i);
      }
day.setForeground(Color.LIGHT_GRAY);
day.setFont(new Font("SansSerif", Font.PLAIN, 11));
day.setFocusable(false);
day.setComponentOrientation(ComponentOrientation.LEFT_TO_RIGHT);
day.setBorder(new RoundBorder());
day.setBackground(Color.WHITE);
day.setBounds(40, 192, 73, 40);
form.add(day);

public class RoundBorder extends AbstractBorder {

    private static final long serialVersionUID = 1L;

    @Override
    public void paintBorder(Component c, Graphics g, int x, int y, int width, int height) {
        Graphics2D g2 = (Graphics2D) g.create();
        g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);

        RoundRectangle2D round = new RoundRectangle2D.Float(x, y, width - 1, height - 1, 8, 8);

        g2.setColor(new Color(0xe9eaec, false));
        g2.draw(round);
        g2.dispose();
    }
    

    @Override
    public Insets getBorderInsets(Component c) {
        return new Insets(4, 8, 4, 8);
    }

    @Override
    public Insets getBorderInsets(Component c, Insets insets) {
        insets.set(4, 8, 4, 8);
        return insets;
    }

}

Answer 1

Found the answer here. Since that link will probably disappear in future, here is its essence.

The question:

What I really want, is to know how to access the border painted in the box left to the arrow button, and be able to change that compound border (in Metal L&F) to a simple soft line border.

The answer:

Maybe something like this?

cb.setUI(new MetalComboBoxUI() {
    public void paintCurrentValueBackground(Graphics g, Rectangle bounds, boolean hasFocus) {
        g.setColor(MetalLookAndFeel.getControlDarkShadow());
        g.drawRect(bounds.x, bounds.y, bounds.width, bounds.height - 1);
        g.setColor(((BasicComboBoxRenderer) comboBox.getRenderer()).getBackground());
        g.drawRect(bounds.x + 1, bounds.y + 1, bounds.width - 2, bounds.height - 3);
    }
});

Here is a SSCCE adapted from the code in your question that incorporates the above code:

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Component;
import java.awt.ComponentOrientation;
import java.awt.EventQueue;
import java.awt.Font;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Insets;
import java.awt.Rectangle;
import java.awt.RenderingHints;
import java.awt.geom.RoundRectangle2D;

import javax.swing.JButton;
import javax.swing.JComboBox;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.border.AbstractBorder;
import javax.swing.plaf.metal.MetalComboBoxUI;
import javax.swing.plaf.metal.MetalLookAndFeel;

public class ComBordr implements Runnable {

    public void run() {
        showGui();
    }

    private JPanel createButton() {
        JButton button = new JButton("Exit");
        JPanel panel = new JPanel();
        panel.add(button);
        return panel;
    }

    private JComboBox<Object> createCombo() {
        Object[] list = new Object[31];
        for (int i = 0; i < 31; i++) {
            list[i] = i + 1;
        }
        JComboBox<Object> day = new JComboBox<>(list);
        day.setUI(new MetalComboBoxUI() {
            public void paintCurrentValueBackground(Graphics g, Rectangle bounds, boolean hasFocus) {
                g.setColor(MetalLookAndFeel.getControlDarkShadow());
                g.drawRect(bounds.x, bounds.y, bounds.width, bounds.height - 1);
                g.setColor(new Color(0xe9eaec, false));
                g.drawRect(bounds.x + 1, bounds.y + 1, bounds.width - 2, bounds.height - 3);
            }
        });
        day.setForeground(Color.LIGHT_GRAY);
        day.setFont(new Font("SansSerif", Font.PLAIN, 11));
        day.setFocusable(false);
        day.setComponentOrientation(ComponentOrientation.LEFT_TO_RIGHT);
        day.setBorder(new RoundBorder());
        day.setBackground(Color.WHITE);
        return day;
    }

    private void showGui() {
        JFrame frame = new JFrame();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.add(createCombo(), BorderLayout.PAGE_START);
        frame.add(createButton(), BorderLayout.PAGE_END);
        frame.pack();
        frame.setLocationByPlatform(true);
        frame.setVisible(true);
    }

    public static void main(String[] args) {
        EventQueue.invokeLater(new ComBordr());
    }
}

class RoundBorder extends AbstractBorder {

    private static final long serialVersionUID = 1L;

    @Override
    public void paintBorder(Component c, Graphics g, int x, int y, int width, int height) {
        Graphics2D g2 = (Graphics2D) g.create();
        g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);

        RoundRectangle2D round = new RoundRectangle2D.Float(x, y, width - 1, height - 1, 8, 8);

        g2.setColor(new Color(0xe9eaec, false));
        g2.draw(round);
        g2.dispose();
    }
    

    @Override
    public Insets getBorderInsets(Component c) {
        return new Insets(4, 8, 4, 8);
    }

    @Override
    public Insets getBorderInsets(Component c, Insets insets) {
        insets.set(4, 8, 4, 8);
        return insets;
    }
}

Here is a screen capture of the result.