JListのセルに項目選択チェックボックスを追加する

@Override public void setSelectionInterval(int anchor, int lead) {
  if (checkedIndex < 0 && isDragging()) {
    super.setSelectionInterval(anchor, lead);
  } else {
    EventQueue.invokeLater(() -> {
      if (checkedIndex >= 0 && lead == anchor && checkedIndex == anchor) {
        super.addSelectionInterval(checkedIndex, checkedIndex);
      } else {
        super.setSelectionInterval(anchor, lead);
      }
    });
  }
}

protected boolean isDragging() {
  Rectangle r = getRubberBand().getBounds();
  return r.width != 0 || r.height != 0;
}

@Override public void removeSelectionInterval(int index0, int index1) {
  if (checkedIndex < 0) {
    super.removeSelectionInterval(index0, index1);
  } else {
    EventQueue.invokeLater(() -> super.removeSelectionInterval(index0, index1));
  }
}

private static <E> Optional<AbstractButton> getItemCheckBox(
    JList<E> list, MouseEvent e, int index) {
  if (e.isShiftDown() || e.isControlDown() || e.isAltDown()) {
    return Optional.empty();
  }
  E proto = list.getPrototypeCellValue();
  ListCellRenderer<? super E> cr = list.getCellRenderer();
  Component c = cr.getListCellRendererComponent(
      list, proto, index, false, false);
  Rectangle r = list.getCellBounds(index, index);
  c.setBounds(r);
  Point pt = e.getPoint();
  pt.translate(-r.x, -r.y);
  return Optional
    .ofNullable(SwingUtilities.getDeepestComponentAt(c, pt.x, pt.y))
    .filter(AbstractButton.class::isInstance)
    .map(AbstractButton.class::cast);
}

private final class ItemCheckBoxesListener extends MouseAdapter {
  private final Point srcPoint = new Point();

  @Override public void mouseDragged(MouseEvent e) {
    checkedIndex = -1;
    JList<?> l = (JList<?>) e.getComponent();
    l.setFocusable(true);
    Point destPoint = e.getPoint();
    Path2D rb = getRubberBand();
    rb.reset();
    rb.moveTo(srcPoint.x, srcPoint.y);
    rb.lineTo(destPoint.x, srcPoint.y);
    rb.lineTo(destPoint.x, destPoint.y);
    rb.lineTo(srcPoint.x, destPoint.y);
    rb.closePath();

    int[] indices = IntStream.range(0, l.getModel().getSize())
            .filter(i -> rb.intersects(l.getCellBounds(i, i))).toArray();
    l.setSelectedIndices(indices);
    l.repaint();
  }

  @Override public void mouseExited(MouseEvent e) {
    rollOverIndex = -1;
    e.getComponent().repaint();
  }

  @Override public void mouseMoved(MouseEvent e) {
    Point pt = e.getPoint();
    int idx = locationToIndex(pt);
    if (!getCellBounds(idx, idx).contains(pt)) {
      idx = -1;
    }
    Rectangle rect = new Rectangle();
    if (idx > 0) {
      rect.add(getCellBounds(idx, idx));
      if (rollOverIndex >= 0 && idx != rollOverIndex) {
        rect.add(getCellBounds(rollOverIndex, rollOverIndex));
      }
      rollOverIndex = idx;
    } else {
      if (rollOverIndex >= 0) {
        rect.add(getCellBounds(rollOverIndex, rollOverIndex));
      }
      rollOverIndex = -1;
    }
    ((JComponent) e.getComponent()).repaint(rect);
  }

  @Override public void mouseReleased(MouseEvent e) {
    getRubberBand().reset();
    Component c = e.getComponent();
    c.setFocusable(true);
    c.repaint();
  }

  @Override public void mousePressed(MouseEvent e) {
    JList<?> l = (JList<?>) e.getComponent();
    int index = l.locationToIndex(e.getPoint());
    if (l.getCellBounds(index, index).contains(e.getPoint())) {
      l.setFocusable(true);
      cellPressed(l, e, index);
    } else {
      l.setFocusable(false);
      l.clearSelection();
      l.getSelectionModel().setAnchorSelectionIndex(-1);
      l.getSelectionModel().setLeadSelectionIndex(-1);
    }
    srcPoint.setLocation(e.getPoint());
    l.repaint();
  }

  private void cellPressed(JList<?> l, MouseEvent e, int index) {
    if (e.getButton() == MouseEvent.BUTTON1 && e.getClickCount() > 1) {
      ListItem item = getModel().getElementAt(index);
      JOptionPane.showMessageDialog(l.getRootPane(), item.title);
    } else {
      checkedIndex = -1;
      getItemCheckBox(l, e.getPoint(), index).ifPresent(rb -> {
        checkedIndex = index;
        if (l.isSelectedIndex(index)) {
          l.setFocusable(false);
          removeSelectionInterval(index, index);
        } else {
          setSelectionInterval(index, index);
        }
      });
    }
  }
}