Creating and using a Progress Bar

Letıs take a look at a small sample application, ProgressBarExample.java which can be readily adapted to any time demanding process of your own.

The program implements a simple panel containing a progress bar, a label, and a button as shown in the figure above:

• When the Start button is pressed, the application simulates some time consuming task.
• At the completion of each task, the program updates its progress bar, showing the user the relative time remaining for the process to complete.
• Notice that the actual code responsible for creating the progress bar is only 7 lines long.

The Code listing forProgressBarExample.java is as follows:

import java.util.*;
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.event.*;

class ProgressBarExample
		extends 	JFrame
		implements  ActionListener
 {
	private	JProgressBar    progress;
	private	JButton         button;
	private	JLabel          label1;
	private	JPanel          topPanel;


	public ProgressBarExample()
	{
		setTitle( "Progress Bar Application" );
		setSize( 310, 130 );
		setBackground( Color.gray );

		topPanel = new JPanel();
		topPanel.setPreferredSize( new Dimension( 310, 130 ) );
		getContentPane().add( topPanel );

		// Create a label and progress bar
		label1 = new JLabel( "Waiting to start tasks..." );
		label1.setPreferredSize( new Dimension( 280, 24 ) );
		topPanel.add( label1 );

		progress = new JProgressBar();
		progress.setPreferredSize( new Dimension( 300, 20 ) );
		progress.setMinimum( 0 );
		progress.setMaximum( 20 );
		progress.setValue( 0 );
		progress.setBounds( 20, 35, 260, 20 );
		topPanel.add( progress );

		button = new JButton( "Start" );
		topPanel.add( button );
		button.addActionListener( this );
	}


	public void actionPerformed( ActionEvent event )
	{
		if( event.getSource() == button )
		{
			// Prevent more button presses
			button.setEnabled( false );

			// Perform all of our bogus tasks
			for( int iCtr = 1; iCtr < 21; iCtr++ )
			{
				// Do some sort of simulated task
				DoBogusTask( iCtr );

				// Update the progress indicator and label
				label1.setText( "Performing task " + iCtr + " of 20" );
				Rectangle labelRect = label1.getBounds();
				labelRect.x = 0;
				labelRect.y = 0;
				label1.paintImmediately( labelRect );

				progress.setValue( iCtr );
				Rectangle progressRect = progress.getBounds();
				progressRect.x = 0;
				progressRect.y = 0;
				progress.paintImmediately( progressRect );
			}
 		}
	}

	public void DoBogusTask( int iCtr )
	{
		Random random = new Random( iCtr );

		// Waste some time
		for( int iValue = 0; iValue < random.nextFloat() * 10000; iValue++ )
		{
			System.out.println( "iValue=" + iValue );
		}
	}

	public static void main( String args[] )
	{
		// Create an instance of the test application
		ProgressBarExample mainFrame	= new ProgressBarExample();
		mainFrame.setVisible( true );
		mainFrame.pack();
	}
}

The program should be fairly straight forward to understand except for the action handler code:

• This is different since we need to update the progress bar as the task we are timing moves on.
• After the value of the label and progress indicator are changed, the application executes these four lines of code:

  Rectangle progressRect 
      = progress.getBounds();
  progressRect.x = 0;
  progressRect.y = 0;
  progress.paintImmediately( progressRect );
  

• The Swing architects adds a paintImmediately(Rectangle) method.
• In our example, we determine the rectangle for the progress bar, progressRect and call the paintImmediately() method to refresh the component after its value is changed.
• Without these lines of code, the progress bar would repaint only after all of our time consuming tasks have finished since it would receive only a single cached paint message.

Another technique to solve this problem would be to start a thread to execute our time consuming tasks.

This frees up execution time on the main thread, but Swing components cannot be accessed reliably on the secondary thread.

If component access is a requirement, then you will have to use a procedure similar to the one above