如果JComboBox列表中的内容过长的话，下拉框中不全部显示 

解决方法一：当鼠标放在这个过长的项目中时显示tooltip，提示用户选择这项的全部信息 

setComboBoxUI(combobox); 

private void setComboBoxUI(final JComboBox combobox) {
      combobox.setUI(new WindowsComboBoxUI() {
          protected ComboPopup createPopup() {
              return new BasicComboPopup(combobox) {
                  protected JList createList() {
                      return new JList(comboBox.getModel()) {
                          public void processMouseEvent(MouseEvent e) {
                              if (e.isControlDown()) {
                                  e = new MouseEvent((Component) e.getSource(), e.getID(), e.getWhen(),                                         e.getModifiers() ^ InputEvent.CTRL_MASK, e.getX(), e.getY(),                                         e.getClickCount(), e.isPopupTrigger());                             }                             super.processMouseEvent(e);                         }                         public String getToolTipText(MouseEvent event) {
                              int index = locationToIndex(event.getPoint());                             if (index != -1) {
                                  Object value = getModel().getElementAt(index);                                 ListCellRenderer renderer = getCellRenderer();                                 Component rendererComp =                                         renderer.getListCellRendererComponent(this, value, index, true, false);                                 if (rendererComp.getPreferredSize().width > getVisibleRect().width) {
                                      return value == null ? null : value.toString();                                 } else {
                                      return null;                                 }                             }                             return null;                         }                         public Point getToolTipLocation(MouseEvent event) {
                              int index = locationToIndex(event.getPoint());                             if (index != -1) {
                                  Rectangle cellBounds = getCellBounds(index, index);                                 return new Point(cellBounds.x, cellBounds.y);                             }                             return null;                         }                     };                 }             };         }     }); } 解决方法二：获取最大长项目宽度，并设置为弹出下拉框时的宽度  combobox.setUI(new LargerComboBoxUI()); 

class LargerComboBoxUI extends BasicComboBoxUI {
      protected ComboPopup createPopup() {
          return new LargerComboPopup(comboBox);     }     public class LargerComboPopup extends BasicComboPopup {
          public LargerComboPopup(JComboBox comboBox) {
              super(comboBox);         }         public void show() {
              int selectedIndex = comboBox.getSelectedIndex();             if (selectedIndex == -1) {
                  list.clearSelection();             } else {
                  list.setSelectedIndex(selectedIndex);                 list.ensureIndexIsVisible(selectedIndex);             }             Insets insets = getInsets();             Dimension listDim = list.getPreferredSize();             boolean hasScrollBar = scroller.getViewport().getViewSize().height != listDim.height;             if (hasScrollBar) {
                  JScrollBar scrollBar = scroller.getVerticalScrollBar();                 listDim.width += scrollBar.getPreferredSize().getWidth();             }             int width = Math.max(listDim.width, comboBox.getWidth() - (insets.right + insets.left));             int height = getPopupHeightForRowCount(comboBox.getMaximumRowCount());             Rectangle popupBounds = computePopupBounds(0, comboBox.getHeight(), width, height);             Dimension scrollSize = popupBounds.getSize();             scroller.setMaximumSize(scrollSize);             scroller.setPreferredSize(scrollSize);             scroller.setMinimumSize(scrollSize);             list.revalidate();             show(comboBox, popupBounds.x, popupBounds.y);         }     } } 来源：https://www.cnblogs.com/tiankafei/p/10340541.html