import javax.swing.*; 
import javax.swing.plaf.basic.BasicComboBoxEditor; 

public class ComboUI { 
    public static void main(String[] args) throws ClassNotFoundException, UnsupportedLookAndFeelException, InstantiationException, IllegalAccessException { 
        UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName()); 
        JFrame frame = new JFrame(); 
        JPanel panel = new JPanel(); 
        JComboBox comboBox = new JComboBox(new Object[] {"apple", "ball"}); 
        comboBox.setEditable(true); 
        //Reproducible as BasicComboBoxEditor call's setBorder(null) 
        comboBox.setEditor(new BasicComboBoxEditor()); 
        panel.add(comboBox); 
        frame.add(panel); 
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 
        frame.pack(); 
        frame.setVisible(true); 
    } 
}