// This class renders a JProgressBar in a table cell.
class ProgressRenderer extends JProgressBar
implements TableCellRenderer
// Constructor for ProgressRenderer.
public ProgressRenderer(int min, int max) {
super(min, max);
/* Returns this JProgressBar as the renderer
for the given table cell. */
public Component getTableCellRendererComponent(
JTable table, Object value, boolean isSelected,
boolean hasFocus, int row, int column)
// Set JProgressBar's percent complete value.
setValue((int) ((Float) value).floatValue());
return this;
The ProgressRenderer class takes advantage of the fact that Swing's JTable class
has a rendering system that can accept "plug-ins" for rendering table cells. To plug into
this rendering system, first, the ProgressRenderer class has to implement Swing's
TableCellRenderer interface. Second, a ProgressRenderer instance has to be registered
with a JTable instance; doing so instructs the JTable instance as to which cells should be
rendered with the "plug-in."
Implementing the TableCellRenderer interface requires the class to override the
getTableCellRendererComponent( ) method. The getTableCellRendererComponent( )
method is invoked each time a JTable instance goes to render a cell for which this class has
been registered. This method is passed several variables, but in this case, only the value
variable is used. The value variable holds the data for the cell being rendered and is passed
to JProgressBar's setValue( ) method. The getTableCellRendererComponent( ) method
wraps up by returning a reference to its class. This works because the ProgressRenderer
class is a subclass of JProgressBar, which is a descendent of the AWT Component class.
The DownloadsTableModel Class
The DownloadsTableModel class houses the Download Manager 's list of downloads
and is the backing data source for the GUI's "Downloads" JTable instance.
The DownloadsTableModel class is shown here. Notice that it extends
AbstractTableModel and implements the Observer interface:
import java.util.*;
import javax.swing.*;
import javax.swing.table.*;
// This class manages the download table's data.
class DownloadsTableModel extends AbstractTableModel
implements Observer
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