Question

I’m using a second JTable in the Viewport of a JScrollPane to build a RowHeader for a main table.
DragAndDrop on the main table is disabled. On the rowheader table DnD is enabled.

If a Drag on the rowheader is started by the user, I want to extend the painted rowheader dropline (the black line in the image) over the main table (like the green line in the image).

Does anybody have an advice for me?
Here’s the SSCCE:

import java.awt.Component;
import java.awt.EventQueue;
import java.awt.Font;
import java.awt.datatransfer.StringSelection;
import java.awt.datatransfer.Transferable;
import java.beans.PropertyChangeEvent;
import java.beans.PropertyChangeListener;
import javax.swing.DropMode;
import javax.swing.JComponent;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JScrollPane;
import javax.swing.JTable;
import javax.swing.JViewport;
import javax.swing.TransferHandler;
import javax.swing.UIManager;
import javax.swing.event.ChangeEvent;
import javax.swing.event.ChangeListener;
import javax.swing.table.DefaultTableCellRenderer;
import javax.swing.table.JTableHeader;
import javax.swing.table.TableColumn;


public class DNDLinePainterExampleMain extends JFrame {

  public DNDLinePainterExampleMain() {
    JTable mainTable = new JTable(4, 3);
    mainTable.setAutoResizeMode(JTable.AUTO_RESIZE_OFF);

    JTable rowTable = new RowHeaderTable(mainTable);
    rowTable.setAutoscrolls(true);
    rowTable.setDragEnabled(true);
    rowTable.setTransferHandler(new RowHeaderTransferHandler());
    rowTable.setDropMode(DropMode.INSERT_ROWS);

    JScrollPane scrollPane = new JScrollPane(mainTable);
    scrollPane.setRowHeaderView(rowTable);
    scrollPane.setCorner(JScrollPane.UPPER_LEFT_CORNER,
        rowTable.getTableHeader());
    this.add(scrollPane);
  }

  public static void main(String[] args) {
    EventQueue.invokeLater(new Runnable() {
      @Override
      public void run() {
        JFrame f = new DNDLinePainterExampleMain();
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.pack();
        f.setVisible(true);
      }
    });
  }


  /*
   * Use a JTable as a renderer for row numbers of a given main table. This
   * table must be added to the row header of the scrollpane that contains the
   * main table. from:
   * http://tips4java.wordpress.com/2008/11/18/row-number-table/
   */
  public class RowHeaderTable extends JTable implements ChangeListener,
      PropertyChangeListener {

    private final JTable table;

    public RowHeaderTable(JTable table) {
      this.table = table;
      table.addPropertyChangeListener(this);

      setFocusable(false);
      setAutoCreateColumnsFromModel(false);

      updateRowHeight();
      updateModel();
      updateSelectionModel();

      TableColumn column = new TableColumn();
      column.setHeaderValue("");
      addColumn(column);
      column.setCellRenderer(new RowNumberRenderer());

      getColumnModel().getColumn(0).setPreferredWidth(50);
      setPreferredScrollableViewportSize(getPreferredSize());

      getTableHeader().setReorderingAllowed(false);
    }

    @Override
    public void addNotify() {
      super.addNotify();
      Component c = getParent();
      // Keep scrolling of the row table in sync with the main table.
      if (c instanceof JViewport) {
        JViewport viewport = (JViewport) c;
        viewport.addChangeListener(this);
      }
    }

    /*
     * Delegate method to main table
     */
    @Override
    public int getRowCount() {
      return table.getRowCount();
    }

    @Override
    public int getRowHeight(int row) {
      return table.getRowHeight(row);
    }

    /*
     * This table does not use any data from the main TableModel, so just return
     * a value based on the row parameter.
     */
    @Override
    public Object getValueAt(int row, int column) {
      return Integer.toString(row + 1);
    }

    /*
     * Don't edit data in the main TableModel by mistake
     */
    @Override
    public boolean isCellEditable(int row, int column) {
      return false;
    }

    // implements ChangeListener
    @Override
    public void stateChanged(ChangeEvent e) {
      // Keep the scrolling of the row table in sync with main table
      JViewport viewport = (JViewport) e.getSource();
      JScrollPane scrollPane = (JScrollPane) viewport.getParent();
      scrollPane.getVerticalScrollBar().setValue(viewport.getViewPosition().y);
    }

    // implements PropertyChangeListener
    @Override
    public void propertyChange(PropertyChangeEvent e) {
      // Keep the row table in sync with the main table
      if ("rowHeight".equals(e.getPropertyName()))
        updateRowHeight();

      if ("selectionModel".equals(e.getPropertyName()))
        updateSelectionModel();

      if ("model".equals(e.getPropertyName()))
        updateModel();
    }

    private void updateRowHeight() {
      setRowHeight(table.getRowHeight());
    }

    private void updateModel() {
      setModel(table.getModel());
    }

    private void updateSelectionModel() {
      setSelectionModel(table.getSelectionModel());
    }


    /*
     * Borrow the renderer from JDK1.4.2 table header
     */
    private class RowNumberRenderer extends DefaultTableCellRenderer {

      public RowNumberRenderer() {
        setHorizontalAlignment(JLabel.CENTER);
      }

      @Override
      public Component getTableCellRendererComponent(JTable table,
          Object value, boolean isSelected, boolean hasFocus, int row,
          int column) {
        if (table != null) {
          JTableHeader header = table.getTableHeader();

          if (header != null) {
            setForeground(header.getForeground());
            setBackground(header.getBackground());
            setFont(header.getFont());
          }
        }

        if (isSelected) {
          setFont(getFont().deriveFont(Font.BOLD));
        }

        setText((value == null) ? "" : value.toString());
        setBorder(UIManager.getBorder("TableHeader.cellBorder"));

        return this;
      }
    }//class RowNumberRenderer

  }//class RowHeaderTable


  public class RowHeaderTransferHandler extends TransferHandler {

    @Override
    public int getSourceActions(JComponent c) {
      return COPY_OR_MOVE;
    }

    @Override
    protected Transferable createTransferable(JComponent c) {
      return new StringSelection(c.getName());
    }

    @Override
    public boolean canImport(TransferSupport supp) {
      return true;
    }
  }//class RowHeaderTransferHandler


}//class DNDLinePainterExampleMain

Answer 1

I suggest you to do like this (using Swing’s drag & drop functionality):

    RowHeaderTable rowTable = new RowHeaderTable(mainTable);
    rowTable.setAutoscrolls(true);
    rowTable.setDragEnabled(true);
    rowTable.setTransferHandler(new RowHeaderTransferHandler(mainTable));
    rowTable.setDropMode(DropMode.INSERT_ROWS);
    try {
        DropTarget dropTarget = rowTable.getDropTarget();
        dropTarget.addDropTargetListener(rowTable);
    }
    catch(TooManyListenersException e) {
        e.printStackTrace();
    }

Now you have to implement DropTargetListener:

public class RowHeaderTable extends JTable implements ChangeListener, PropertyChangeListener, DropTargetListener {

These are 2 methods that you should be interested in:

    @Override
    public void dragEnter(DropTargetDragEvent dtde) { shouldPaint = true }

    @Override
    public void dragExit(DropTargetEvent dte) { shouldPaint = false }

Now you should manipulate your mainTable (to paint this “green line”) using your TranserHandler:

    @Override
    public boolean canImport(TransferSupport supp) {
        DropLocation location = supp.getDropLocation();
        if(location instanceof javax.swing.JTable.DropLocation && shouldPaint) {
            javax.swing.JTable.DropLocation tableLocation = (javax.swing.JTable.DropLocation) location;
            int rowToInsert = tableLocation.getRow();
            //paint somehow the "green line"
            //remember that canImport is invoked when you move your mouse
        }
        return true;
    }

Basically you should implement your own row renderer or something similar (like in @naugler answer). You should somehow provide boolean shouldPaint to your TransferHandler as well. But this is not a big deal to implement.

Hope this helps.