Display the insert cursor for a new column on the boundary of a JTable column

• download example.jar
• download src.zip
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Code

class ColumnInsertLayerUI extends LayerUI<JScrollPane> {
  private static final Color LINE_COLOR = new Color(0x00_78_D7);
  private static final int LINE_WIDTH = 4;
  private final Rectangle2D line = new Rectangle2D.Double();
  private final Ellipse2D plus = new Ellipse2D.Double(0d, 0d, 10d, 10d);

  @Override public void paint(Graphics g, JComponent c) {
    super.paint(g, c);
    if (c instanceof JLayer && !line.isEmpty()) {
      JScrollPane scroll = (JScrollPane) ((JLayer<?>) c).getView();
      JTableHeader header =
          ((JTable) scroll.getViewport().getView()).getTableHeader();
      Graphics2D g2 = (Graphics2D) g.create();
      g2.setRenderingHint(
          RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
      Point pt0 = line.getBounds().getLocation();
      Point pt1 = SwingUtilities.convertPoint(header, pt0, c);
      g2.translate(pt1.getX() - pt0.getX(), pt1.getY() - pt0.getY());
      // paint Insert Line
      g2.setPaint(LINE_COLOR);
      g2.fill(line);
      // paint Plus Icon
      g2.setPaint(Color.WHITE);
      g2.fill(plus);
      g2.setPaint(LINE_COLOR);
      double cx = plus.getCenterX();
      double cy = plus.getCenterY();
      double w2 = plus.getWidth() / 2d;
      double h2 = plus.getHeight() / 2d;
      g2.draw(new Line2D.Double(cx - w2, cy, cx + w2, cy));
      g2.draw(new Line2D.Double(cx, cy - h2, cx, cy + h2));
      g2.draw(plus);
      g2.dispose();
    }
  }

  private void updateLineLocation(JScrollPane scroll, Point loc) {
    JTable table = (JTable) scroll.getViewport().getView();
    JTableHeader header = table.getTableHeader();
    Rectangle rect = scroll.getVisibleRect();
    JScrollBar bar = scroll.getHorizontalScrollBar();
    int scrollHeight = bar.isVisible() ? bar.getHeight() : 0;
    Dimension d = new Dimension(
        LINE_WIDTH, rect.height - scrollHeight);
    for (int i = 0; i < table.getColumnCount(); i++) {
      if (canInsert(header, loc, i, d)) {
        return;
      }
    }
  }

  private boolean canInsert(
        JTableHeader header, Point loc, int i, Dimension d) {
    Rectangle r = header.getHeaderRect(i);
    Rectangle r1 = getWestRect(r, i);
    Rectangle r2 = getEastRect(r);
    boolean hit = false;
    if (r1.contains(loc)) {
      updateInsertLineLocation(r1, loc, d, header);
      hit = true;
    } else if (r2.contains(loc)) {
      updateInsertLineLocation(r2, loc, d, header);
      hit = true;
    } else if (r.contains(loc)) {
      line.setFrame(0d, 0d, 0d, 0d);
      header.setCursor(Cursor.getDefaultCursor());
      hit = true;
    }
    return hit;
  }

  private Rectangle getWestRect(Rectangle r, int i) {
    Rectangle rect = r.getBounds();
    Rectangle bounds = plus.getBounds();
    if (i != 0) {
      rect.x -= bounds.width / 2;
    }
    rect.setSize(bounds.getSize());
    return rect;
  }

  private Rectangle getEastRect(Rectangle r) {
    Rectangle rect = r.getBounds();
    Rectangle bounds = plus.getBounds();
    rect.x += rect.width - bounds.width / 2;
    rect.setSize(bounds.getSize());
    return rect;
  }

  private void updateInsertLineLocation(
        Rectangle r, Point loc, Dimension d, Component c) {
    if (r.contains(loc)) {
      double cx = r.getCenterX();
      double cy = r.getCenterY();
      line.setFrame(
          cx - d.getWidth() / 2d, r.getY(), d.width, d.height);
      double pw = plus.getWidth() / 2d;
      double ph = plus.getHeight() / 2d;
      plus.setFrameFromCenter(cx, cy, cx - pw, cy - ph);
      c.setCursor(Cursor.getDefaultCursor());
    } else {
      line.setFrame(0d, 0d, 0d, 0d);
      c.setCursor(Cursor.getPredefinedCursor(Cursor.W_RESIZE_CURSOR));
    }
  }

  @Override public void installUI(JComponent c) {
    super.installUI(c);
    if (c instanceof JLayer) {
      ((JLayer<?>) c).setLayerEventMask(
          AWTEvent.MOUSE_EVENT_MASK | AWTEvent.MOUSE_MOTION_EVENT_MASK);
    }
  }

  @Override public void uninstallUI(JComponent c) {
    if (c instanceof JLayer) {
      ((JLayer<?>) c).setLayerEventMask(0);
    }
    super.uninstallUI(c);
  }

  @Override protected void processMouseEvent(
        MouseEvent e, JLayer<? extends JScrollPane> l) {
    super.processMouseEvent(e, l);
    if (e.getID() == MouseEvent.MOUSE_CLICKED) {
      JScrollPane scroll = l.getView();
      Point pt = e.getPoint();
      if (plus.contains(pt) && !line.isEmpty()) {
        JTable table = (JTable) scroll.getViewport().getView();
        TableModel model = table.getModel();
        int columnCount = table.getColumnCount();
        int maxColumn = model.getColumnCount();
        if (columnCount < maxColumn) {
          int idx = table.columnAtPoint(
              line.getBounds().getLocation());
          TableColumn column = new TableColumn(columnCount);
          column.setHeaderValue("Column" + columnCount);
          table.addColumn(column);
          table.moveColumn(columnCount, idx + 1);
          updateLineLocation(scroll, pt);
        }
      }
      l.repaint(scroll.getBounds());
    }
  }

  @Override protected void processMouseMotionEvent(
        MouseEvent e, JLayer<? extends JScrollPane> l) {
    super.processMouseMotionEvent(e, l);
    Component c = e.getComponent();
    int id = e.getID();
    JScrollPane scroll = l.getView();
    if (id == MouseEvent.MOUSE_MOVED && c instanceof JTableHeader) {
      updateLineLocation(scroll, e.getPoint());
    } else {
      line.setFrame(0d, 0d, 0d, 0d);
    }
    l.repaint(scroll.getBounds());
  }
}

References

• Simplified insert options - Modernized Excel Grid | Microsoft Community Hub
• Display the insert cursor for a new column on the boundary of a JTable column(Japanese)