Set Maximum Popup Displayed Items

jComboBox1.setMaximumRowCount(3);

Reset:
    jComboBox1.setMaximumRowCount(8);

Show Popup

jComboBox1.showPopup();

Reset:
    jComboBox1.hidePopup();

Enable LightWeight Popup

jComboBox1.setLightWeightPopupEnabled(true); //default

Reset:
    jComboBox1.setLightWeightPopupEnabled(true); //default

Note:
    Determines if the Popup is composed of light or heavy weight components

Set Width by Rendered String

jComboBox1.setPrototypeDisplayValue("Use this string to determine the width of the JComboBox");

Reset:
   jComboBox1.setPrototypeDisplayValue(null);

Set Action

jComboBox1.setAction(new AbstractAction("New Action") {
    @Override
    public void actionPerformed(ActionEvent e) {
        System.out.println("The new action has fired");
    }
});

Reset:
    jComboBox1.setAction(null);

Set Action Command Text

jComboBox1.setActionCommand("The New Action Command");

Reset:
    jComboBox1.setActionCommand("comboBoxChanged");

Set Enabled

jComboBox1.setEnabled(true); //default

Reset:
    jComboBox1.setEnabled(true); //default

Set New Item Type

jComboBox1.setModel(new DefaultComboBoxModel(new Color[]{Color.WHITE,Color.BLUE,Color.MAGENTA}));

Reset:
    jComboBox1.setModel(new DefaultComboBoxModel());

Note:
    Will have to add a new ListDataListener to the new Model (if needed)