Widgets in JTable cells

Question

Widgets in JTable columns are expected to be not distinguishable from normal ones, right? There seems to be behavioral difference, take Swing documentation example and move mouse over checkboxes in the Vegetarian column... They don't react at all. I understand that those are just widget surrogates, so highlighting has to be done manually, so how would I fix this? I tried widget.requestFocusInWindow(); in mouseMoved() for the surrogate widget event handler without success. Any other workaround?

Answer 1

You can create your own cell renderer that applies a rollover effect. Then, add a mouse listener that tracks the mouse movement and repaints the relevant cells. You need to apply the effect to the current cell under the cursor and clear the previous cell.

Below is a short example that demonstrates this approach on a checkbox renderer. The example extends default BooleanRenderer. The only change is getModel().setRollover(...) in getTableCellRendererComponent().

import java.awt.Component;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import javax.swing.JCheckBox;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JScrollPane;
import javax.swing.JTable;
import javax.swing.SwingUtilities;
import javax.swing.UIManager;
import javax.swing.border.Border;
import javax.swing.border.EmptyBorder;
import javax.swing.plaf.UIResource;
import javax.swing.table.DefaultTableModel;
import javax.swing.table.TableCellRenderer;

public class TableRolloverDemo {
    private static void createAndShowGUI() {
        JFrame frame = new JFrame("TableRolloverDemo");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        final JTable table = new JTable();

        final DefaultTableModel model = new DefaultTableModel(new Object[][] {
                { false }, { false }, { true }, { true } },
                new Object[] { "Column" }) {
            public Class<?> getColumnClass(int columnIndex) {
                return Boolean.class;
            }
        };

        RolloverMouseAdapter rolloverAdapter = new RolloverMouseAdapter(table);
        RolloverBooleanRenderer renderer = new RolloverBooleanRenderer(rolloverAdapter);

        table.addMouseListener(rolloverAdapter);
        table.addMouseMotionListener(rolloverAdapter);

        table.setDefaultRenderer(Boolean.class, renderer);
        table.setModel(model);

        frame.add(new JScrollPane(table));
        frame.pack();
        frame.setLocationByPlatform(true);
        frame.setVisible(true);
    }

    static class RolloverMouseAdapter extends MouseAdapter {
        private int row = -1;
        private int column = -1;
        private JTable table;

        public RolloverMouseAdapter(JTable table) {
            this.table = table;
        }

        public boolean isRolloverCell(int row, int column) {
            return this.row == row && this.column == column;
        }

        @Override
        public void mouseMoved(MouseEvent e) {
            int lastRow = row;
            int lastColumn = column;

            row = table.rowAtPoint(e.getPoint());
            column = table.columnAtPoint(e.getPoint());

            if (row == lastRow && column == lastColumn)
                return;

            if (row >= 0 && column >= 0) {
                table.repaint(table.getCellRect(row, column, false));
            }
            if (lastRow >= 0 && lastColumn >= 0) {
                table.repaint(table.getCellRect(lastRow, lastColumn, false));
            }
        }

        @Override
        public void mouseExited(MouseEvent e) {
            if (row >= 0 && column >= 0) {
                table.repaint(table.getCellRect(row, column, false));
            }
            row = column = -1;
        }

    }

    static class RolloverBooleanRenderer extends JCheckBox implements
            TableCellRenderer, UIResource {
        private static final Border noFocusBorder = new EmptyBorder(1, 1, 1, 1);
        private RolloverMouseAdapter adapter;

        public RolloverBooleanRenderer(RolloverMouseAdapter adapter) {
            super();
            this.adapter = adapter;
            setHorizontalAlignment(JLabel.CENTER);
            setBorderPainted(true);
        }

        public Component getTableCellRendererComponent(JTable table,
                Object value, boolean isSelected, boolean hasFocus, int row,
                int column) {

            getModel().setRollover(adapter.isRolloverCell(row, column));

            if (isSelected) {
                setForeground(table.getSelectionForeground());
                super.setBackground(table.getSelectionBackground());
            } else {
                setForeground(table.getForeground());
                setBackground(table.getBackground());
            }
            setSelected((value != null && ((Boolean) value).booleanValue()));

            if (hasFocus) {
                setBorder(UIManager.getBorder("Table.focusCellHighlightBorder"));
            } else {
                setBorder(noFocusBorder);
            }

            return this;
        }
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                createAndShowGUI();
            }
        });
    }
}

Answer 2

It's rollover (or lack of thereof) which makes widgets in the table to appear dead. If JPanel is genuine container of widgets, why JTable and JTree are not?

Both JTable and JTree use the flyweight pattern for rendering. The default renderer & editor have no intrinsic mouse-over behavior. You have to supply the desired behavior yourself, as @Max shows.