import java.awt.Color;
import java.awt.Container;
import java.awt.FlowLayout;
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);
}
}