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Table Header Icons

Did you know that icon images can be displayed in a table component's column headers? Figure 8 shows an example.

Figure 8

Assigning icons to a JTable component's header columns makes them more colorful and informative.

To display an icon in a column header, first establish a renderer (a component that draws graphics) for that column header. This renderer is a class that implements the TableCellRenderer interface by providing a getTableCellRendererComponent method. In turn, this method returns a component capable of rendering the column header.

What component can be used to perform this rendering? How about a component based on the JLabel class? After all, JLabel is capable of rendering an icon image. The following code fragment creates a JLabel object that represents the rendering component:

JLabel l = new JLabel (new ImageIcon ("icon.gif"));

The above code fragment creates an ImageIcon object that loads an icon image from a file called icon.gif. This object is subsequently passed to a JLabel constructor during the creation of a JLabel object. After making a few other changes, this JLabel object can be returned from getTableCellRendererComponent.

To specify an icon for a particular column, call JTable's getColumn method to retrieve a reference to a TableColumn object that describes that column. Then call TableColumn's setHeaderRenderer method with an object created from the renderer class.

Listing 5 presents source code to a TableHeaderIcons application. This application was used to generate the GUI in Figure 8 and shows how easy it is to render icons in a table's header.

Listing 5  The TableHeaderIcons application source code

// TableHeaderIcons.java

import java.awt.*;
import java.awt.event.*;

import javax.swing.*;
import javax.swing.table.*;

class TableHeaderIcons extends JFrame
   TableHeaderIcons (String title)
      // Pass the title argument to the JFrame superclass so that the
      // contents of title appear in the JFrame's title bar.

      super (title);

      // Exit the application by calling System.exit when the user
      // either selects Close from the System menu or clicks the X
      // button.

      setDefaultCloseOperation (EXIT_ON_CLOSE);

      // Define the data model for the JTable.

      TableModel dataModel = new AbstractTableModel ()
         final static int NROWS = 20;
         final static int NCOLS = 5;

         Object [][] rowData = new Object [NROWS] [];

         // This instance initializer creates the data when an object
         // is created from this subclass of the AbstractTableModel
         // class.

            for (int i = 0; i < NROWS; i++)
                 rowData [i] = new Object [NCOLS];

            int x = 0;
            for (int row = 0; row < NROWS; row++)
                 for (int col = 0; col < NCOLS; col++)
                      rowData [row] [col] = new Integer (2 * x++);

         public int getColumnCount ()
            return NCOLS;

         public int getRowCount ()
            return NROWS;

         public Object getValueAt (int row, int col)
            return rowData [row] [col];

         public boolean isCellEditable (int row, int col)
            return false;

         public void setValueAt (Object value, int row, int col)
            rowData [row] [col] = value;
            fireTableCellUpdated (row, col);

      // Create the JTable and assign it the predefined data model.

      JTable table = new JTable (dataModel);

      // Assign a custom renderer to column A, so that an icon can
      // be displayed in column A's header.

      table.getColumn ("A").setHeaderRenderer (new MyRenderer ());

      // Create a JScrollPane and add the JTable to this container.

      JScrollPane sp = new JScrollPane (table);

      // Set the JTable's preferred "viewport" size for viewing
      // JTable data.

      table.setPreferredScrollableViewportSize (new Dimension (250,

      // Add the JScrollPane to the JFrame's content pane.

      getContentPane ().add (sp);

      // Set the size of the JFrame window.

      setSize (300, 250);

      // Show the JFrame window.

      setVisible (true);

   public static void main (String [] args)
      new TableHeaderIcons ("Table Header Icons");

class MyRenderer implements TableCellRenderer
   // Create a JLabel for use as a renderer and pre-load this label
   // with an icon image.

   private JLabel l = new JLabel (new ImageIcon ("icon.gif"));

   public Component getTableCellRendererComponent (JTable table,
                                                   Object value,
                                                   boolean isSelected,
                                                   boolean hasFocus,
                                                   int row,
                                                   int column)
      // Extract the original header renderer for this column.

      TableCellRenderer tcr = table.getTableHeader ()
                                   .getDefaultRenderer ();

      // Extract the component used to render the column header.

      Component c = tcr.getTableCellRendererComponent (table, value,
                                                       row, column);

      // Establish the font, foreground color, and border for the
      // JLabel so that the rendered header will look the same as the
      // other rendered headers.

      l.setFont (c.getFont ());
      l.setForeground (c.getForeground ());
      l.setBorder (((JComponent) c).getBorder ());

      // Establish the column name.

      l.setText ((String) value);

      // Return the cached JLabel a the renderer for this column
      // header.

      return l;

TableHeaderIcons' getTableCellRendererComponent method doesn't create a separate copy of the JLabel object each time this method is called. Instead, it always returns the same cached JLabel object. Some renderers always return a new rendering component object when getTableCellRendererComponent is called. This object is thrown away once JTable finishes rendering. If a separate object is always created and then thrown away, this can have an impact on performance—especially for a renderer that's responsible for rendering table cells. Therefore, consider caching a rendering component object (which is what TableHeaderIcons does) and only returning the cached object.

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