Java Swing How to - Change JComboBox appearance

Question

We would like to know how to change JComboBox appearance.
Answer

import java.awt.Color;
import java.awt.Container;
import java.awt.FlowLayout;
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import javax.swing.JComboBox;
import javax.swing.JFrame;
import javax.swing.UIManager;
import javax.swing.plaf.ColorUIResource;

public class Main {
  public static void main(String[] args) throws Exception {
    String[] items = new String[] { "", "Apple", "Banana", "Carrot" };
    Color bgColor = UIManager.getColor("TextField.background");

    UIManager.put("ComboBox.selectionBackground", new ColorUIResource(bgColor));

    JComboBox<String> combo1 = new JComboBox<>(items);
    combo1.setPrototypeDisplayValue("XXXXXXXXXXXXXXX");
    combo1.setEditable(true);
    combo1.setSelectedIndex(-1);

    JComboBox<String> combo2 = new JComboBox<>(items);
    combo2.setPrototypeDisplayValue("XXXXXXXXXXXXXXX");
    combo2.setEditable(false);
    combo2.setSelectedIndex(-1);
    combo2.setBackground(bgColor);

    JFrame frame = new JFrame();
    Container c = frame.getContentPane();
    c.setLayout(new FlowLayout());
    c.add(combo1);
    c.add(combo2);

    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setSize(200, 100);
    frame.setLocationRelativeTo(null);
    frame.setVisible(true);
  }
}