use a newspaper-style JList that can scroll and display multiple items as a ComboBoxEditor

• download example.jar
• download src.zip
• repository

Code

ListModel<ListItem> model = makeModel();
JList<ListItem> list = new NewspaperStyleList<>(model);
JScrollPane scroll = new JScrollPane(list) {
  @Override public Dimension getPreferredSize() {
    Dimension d = super.getPreferredSize();
    d.width = 62 * 3 + 10;
    d.height = 32;
    return d;
  }
};
scroll.setVerticalScrollBarPolicy(ScrollPaneConstants.VERTICAL_SCROLLBAR_NEVER);
scroll.setBorder(BorderFactory.createEmptyBorder());
scroll.setViewportBorder(BorderFactory.createEmptyBorder());

JList<ListItem> list2 = new NewspaperStyleList<>(model);
JScrollPane scroll2 = new JScrollPane(list2) {
  @Override public Dimension getPreferredSize() {
    Dimension d = super.getPreferredSize();
    d.width = 62 * 4 + 10;
    return d;
  }
};
scroll2.setBorder(BorderFactory.createEmptyBorder());
scroll2.setViewportBorder(BorderFactory.createEmptyBorder());

JPopupMenu popup = new JPopupMenu();
popup.setLayout(new BorderLayout());
list2.addMouseListener(new MouseAdapter() {
  @Override public void mouseClicked(MouseEvent e) {
    if (popup.isVisible() && e.getClickCount() >= 2) {
      popup.setVisible(false);
    }
  }
});
popup.add(scroll2);
popup.setBorder(BorderFactory.createLineBorder(Color.GRAY));

JToggleButton dropDown = new JToggleButton(new DropDownArrowIcon());
popup.addPopupMenuListener(new PopupMenuListener() {
  @Override public void popupMenuWillBecomeVisible(PopupMenuEvent e) {
    list2.setSelectedIndex(list.getSelectedIndex());
    EventQueue.invokeLater(popup::pack);
  }

  @Override public void popupMenuWillBecomeInvisible(PopupMenuEvent e) {
    dropDown.setSelected(false);
    list.requestFocusInWindow();
    int i = list2.getSelectedIndex();
    if (i >= 0) {
      list.setSelectedIndex(i);
      list.scrollRectToVisible(list.getCellBounds(i, i));
    }
  }

  @Override public void popupMenuCanceled(PopupMenuEvent e) {
    popupMenuWillBecomeInvisible(e);
  }
});

dropDown.setBorder(BorderFactory.createEmptyBorder());
dropDown.setContentAreaFilled(false);
dropDown.setFocusPainted(false);
dropDown.setFocusable(false);
dropDown.addItemListener(e -> {
  AbstractButton b = (AbstractButton) e.getItemSelectable();
  if (e.getStateChange() == ItemEvent.SELECTED) {
    popup.show(b, -scroll.getWidth(), b.getHeight());
  }
});

JScrollBar verticalScrollBar = scroll.getVerticalScrollBar();
JPanel verticalBox = new JPanel(new BorderLayout()) {
  @Override public Dimension getPreferredSize() {
    Dimension d = super.getPreferredSize();
    d.height = 32 + 5 + 5;
    return d;
  }
};
verticalBox.setOpaque(false);
verticalBox.add(verticalScrollBar);
verticalBox.add(dropDown, BorderLayout.SOUTH);

JPanel panel = new JPanel(new BorderLayout(0, 0));
panel.add(scroll);
panel.add(verticalBox, BorderLayout.EAST);
panel.setBorder(BorderFactory.createLineBorder(Color.DARK_GRAY));

References

• use a newspaper-style JList that can scroll and display multiple items as a ComboBoxEditor(Japanese)