Hola tiegor.
Aquí hay un ejemplo de un JComboBox editable


import java.awt.Dimension;
import java.awt.FlowLayout;
import java.awt.Toolkit;
import java.util.Collections;
import java.util.LinkedList;
import java.util.List;

import javax.swing.JComboBox;
import javax.swing.JFrame;
import javax.swing.JLabel;
import static java.util.logging.Level.*;
import static javax.swing.UIManager.getInstalledLookAndFeels;
import static javax.swing.UIManager.setLookAndFeel;
import static java.util.logging.Logger.*;
import static javax.swing.SwingUtilities.invokeLater;
import static javax.swing.JFrame.*;
import javax.swing.UnsupportedLookAndFeelException;
import javax.swing.UIManager.LookAndFeelInfo;
import static javax.swing.JOptionPane.*;

/**
*
*/

/**
* @author ?
*
*/
public class Sencillo extends JFrame {

public Sencillo() {
setLayout(new FlowLayout());
screenSize = Toolkit.getDefaultToolkit().getScreenSize();
X = (screenSize.width / 4);
Y = (screenSize.height / 4);
muestra = new JLabel("Selecciona");
listaDesplegable = new JComboBox<>();
listaDesplegable.setMaximumRowCount(4);
valores = new LinkedList<>();
valores.add("Rojo");
valores.add("Verde");
valores.add("Azul");
valores.add("Morado");
valores.add("Rosa");
valores.add("Blanco");
valores.add("Amarillo");
valores.add("Negro");
valores.add("Naranja");
valores.add("Guinda");
Collections.sort(valores);
valores.forEach((String v) -> {
listaDesplegable.addItem(v);
});
listaDesplegable.addActionListener(e -> {
coloca = listaDesplegable.getSelectedIndex();
switch (coloca) {
case 0:
showMessageDialog(null, listaDesplegable.getSelectedItem(), "Lista no editable", INFORMATION_MESSAGE);
listaDesplegable.setEditable(false);
break;
case 1:
showMessageDialog(null, listaDesplegable.getSelectedItem(), "Lista no editable", INFORMATION_MESSAGE);
listaDesplegable.setEditable(false);
break;
case 2:
showMessageDialog(null, listaDesplegable.getSelectedItem(), "Lista no editable", INFORMATION_MESSAGE);
listaDesplegable.setEditable(false);
break;
case 3:
showMessageDialog(null, listaDesplegable.getSelectedItem(), "Lista no editable", INFORMATION_MESSAGE);
listaDesplegable.setEditable(false);
break;
case 4:
showMessageDialog(null, listaDesplegable.getSelectedItem(), "Lista no editable", INFORMATION_MESSAGE);
listaDesplegable.setEditable(false);
break;
case 5:
showMessageDialog(null, listaDesplegable.getSelectedItem(), "Lista editable", INFORMATION_MESSAGE);
listaDesplegable.setEditable(true);
break;
case 6:
showMessageDialog(null, listaDesplegable.getSelectedItem(), "Lista no editable", INFORMATION_MESSAGE);
listaDesplegable.setEditable(false);
break;
case 7:
showMessageDialog(null, listaDesplegable.getSelectedItem(), "Lista no editable", INFORMATION_MESSAGE);
listaDesplegable.setEditable(false);
break;
case 8:
showMessageDialog(null, listaDesplegable.getSelectedItem(), "Lista no editable", INFORMATION_MESSAGE);
listaDesplegable.setEditable(false);
break;
case 9:
showMessageDialog(null, listaDesplegable.getSelectedItem(), "Lista editable", INFORMATION_MESSAGE);
listaDesplegable.setEditable(true);
break;
default:
listaDesplegable.setEditable(false);
break;
}
});
add(muestra);
add(listaDesplegable);
setSize(new Dimension(X, Y));
setDefaultCloseOperation(EXIT_ON_CLOSE);
setLocationRelativeTo(null);
}

/**
* @param args
*/
public static void main(String... args) {
// TODO Auto-generated method stub
invokeLater(() -> {
try {
for (LookAndFeelInfo info : getInstalledLookAndFeels()) {
if (NIMBUS.equals(info.getName())) {
setLookAndFeel(info.getClassName());
new Sencillo().setVisible(true);
break;
}
}
} catch (ClassNotFoundException | InstantiationException | IllegalAccessException
| UnsupportedLookAndFeelException ex) {
getLogger(Sencillo.class.getName()).log(SEVERE, null, ex);
}
});
}

/**
*
*/
private static final long serialVersionUID = -9204252413706937916L;
private static final String NIMBUS = "Nimbus";
private List<String> valores;
private JLabel muestra;
private JComboBox<String> listaDesplegable;
private Dimension screenSize;
private int X = 0, Y = 0;
private int coloca;

}

Saludos!!!!