How can I change the width of a JComboBox dropdown list?

Question

I have an editable JComboBox which contains a list of single letter values. Because of that the combobox is very small.

Every letter has a special meaning which sometimes isn't clear to the user in case of rarely used letters. Because of that I've created a custom ListCellRenderer which shows the meaning of each letter in the dropdown list.

Unfortunately this explanation doesn't fit into the dropdown because it is to small, because it has the same width as the combobox.

Is there any way to make the dropdown list wider than the combobox?

This is what I want to achieve:

 ---------------------
| Small JCombobox | V |
 --------------------------------------------
| "Long item 1"                              |
 --------------------------------------------
| "Long item 2"                              |
 --------------------------------------------
| "Long item 3"                              |
 --------------------------------------------

I cannot change the width of the combobox because the application is a recreation of an old legacy application where some things have to be exactly as they were before. (In this case the combobox has to keep it's small size at all costs)

Answer 1

I believe the only way to do this with the public API is to write a custom UI (there are two bugs dealing with this).

If you just want something quick-and-dirty, I found this way to use implementation details to do it (here):

public void popupMenuWillBecomeVisible(PopupMenuEvent e) {
    JComboBox box = (JComboBox) e.getSource();
    Object comp = box.getUI().getAccessibleChild(box, 0);
    if (!(comp instanceof JPopupMenu)) return;
    JComponent scrollPane = (JComponent) ((JPopupMenu) comp).getComponent(0);
    Dimension size = new Dimension();
    size.width = box.getPreferredSize().width;
    size.height = scrollPane.getPreferredSize().height;
    scrollPane.setPreferredSize(size);
    //  following line for Tiger
    // scrollPane.setMaximumSize(size);
}

Put this in a PopupMenuListener and it might work for you.

Or you could use the code from the first linked bug:

class StyledComboBoxUI extends BasicComboBoxUI {
  protected ComboPopup createPopup() {
    BasicComboPopup popup = new BasicComboPopup(comboBox) {
      @Override
      protected Rectangle computePopupBounds(int px,int py,int pw,int ph) {
        return super.computePopupBounds(
            px,py,Math.max(comboBox.getPreferredSize().width,pw),ph
        );
      }
    };
    popup.getAccessibleContext().setAccessibleParent(comboBox);
    return popup;
  }
}

class StyledComboBox extends JComboBox {
  public StyledComboBox() {
    setUI(new StyledComboBoxUI());
  }
}