Java调整组合框下拉列表

Question

我尝试调整combobox的下拉列表，并且代码在第三次点击后起作用:(。 第一次点击为正常combobox，dropdown具有相同的宽度。 然后第二次，下拉列表更大但滚动窗格不会改变。

我在popupmenulistener变得可见之前实施了dropdown，

public void popupMenuWillBecomeVisible(PopupMenuEvent pme)
  {
    JComboBox comboBox = (JComboBox) pme.getSource();
    if (comboBox.getItemCount() == 0)
    {
      return;
    }
    final Object child = comboBox.getAccessibleContext().getAccessibleChild(0);
    if (child instanceof BasicComboPopup)
    {
      JList list = ((BasicComboPopup) child).getList();
      list.ensureIndexIsVisible(3);
      JScrollPane scrollPane = (JScrollPane) SwingUtilities.getAncestorOfClass(JScrollPane.class, list);
      // int scrollBarWidth = 0;
      // if (comboBox.getItemCount() > comboBox.getMaximumRowCount()) {
      // JScrollBar vertical = scrollPane.getVerticalScrollBar();
      // scrollBarWidth = vertical.getPreferredSize().width;
      // }

      int popupWidth = 500;
      Dimension scrollPaneSize = scrollPane.getPreferredSize();
      scrollPane.getViewport().setSize(scrollPaneSize);
      // popupWidth = Math.max(popupWidth, scrollPaneSize.width);
      scrollPaneSize.width = popupWidth;
      scrollPane.setMinimumSize(scrollPaneSize);
      scrollPane.setPreferredSize(scrollPaneSize);
      scrollPane.setMaximumSize(scrollPaneSize);
      // scrollPane.repaint();

      // ((BasicComboPopup) child).setVisible(false);
      ((BasicComboPopup) child).setSize(scrollPaneSize);
      // ((BasicComboPopup) child).repaint();

      // comboBox.setPrototypeDisplayValue("");
      // comboBox.setPrototypeDisplayValue(comboBox.getSelectedItem());

      // ((BasicComboPopup) child).revalidate();
      ((BasicComboPopup) child).setVisible(true);

    }
  }