Introduction
Scrolling is nothing but allowing the user to completely view the window by moving up and down (or) left <-> right. AWT Scrolling chapter gives clear idea about
- ScrollBar
- AdjustmentEvent
Description
ScrollBar scrolls vertically and horizontally from its default value by setting the minimum and maximum values. It consists of a box, which does not have any value to display except scrolling.
Conceptual
figure
Properties
| Property | Description |
|---|---|
| Initial value | The slider is set to the initial value. Default value is set when being used. |
| Alignment | This decides whether to scrol vertically or horizontally. |
| Scroll box size | It represents the size of the slider. It is the result of difference between the maximum value and the amount visible. |
| Unit increment, Unit decrement | This value gets incremented or decremented when clicked on the end arrows. The default value is 1. |
| Block Increment, Block Decrement | The scrolling value changes and moves as per the changed value. The default value is 10. |
| Minimum value and Maximum value | These values indicates the range of scrollbar. Exceeding these values, the scroll bar cannot be scrolled. |
Declaration
java.awt.Scrollbar class can be declared as follows:
public class Scrollbar extends Component implements Adjustable, Accessible Description
AdjustmentEvent is generated when scroll bar action is performed. This is handled by the AdjustmentListener, which overrides the adjustValueChanged() method.
Examples
[java]
import java.awt.*;
import java.awt.event.*;
import java.awt.event.WindowAdapter;
import java.awt.event.WindowEvent;
class Splesson extends Frame implements AdjustmentListener
{
Scrollbar s;
public Splesson()
{
createAndShowGUI();
}
private void createAndShowGUI()
{
setTitle("Scrollbar with AdjustmentListener Demo");
setLayout(new FlowLayout());
// Create and add scrollbar
s=new Scrollbar();
add(s);
s.setBackground(Color.cyan);
s.setPreferredSize(new Dimension(50,250));
// Add adjustment listener
s.addAdjustmentListener(this);
setSize(400,400);
setVisible(true);
}
// Called whenever the scrollbar value changes
public void adjustmentValueChanged(AdjustmentEvent ae)
{
// Update the title
setTitle("Current value: "+ae.getValue());
}
public static void main(String args[])
{
Splesson s = new Splesson();
s.addWindowListener ( new WindowAdapter () {
public void windowClosing ( WindowEvent evt )
{
System.exit(0);
}
});
}
}[/java]
Output
Example showing change of colors when scrolled vertically
[java]
import java.awt.*;
import java.awt.event.*;
import java.awt.event.WindowAdapter;
import java.awt.event.WindowEvent;
public class Splesson extends Frame implements AdjustmentListener
{
Scrollbar redScroll, greenScroll, blueScroll;
Label redLabel, greenLabel, blueLabel;
Panel p1;
public Splesson()
{
setBackground(Color.cyan);
p1 = new Panel();
p1.setLayout(new GridLayout(3, 2, 5, 5));
redScroll = new Scrollbar(Scrollbar.HORIZONTAL, 0, 0, 0, 255);
redScroll.setUnitIncrement(5); //default is 1
redScroll.setBlockIncrement(15); //default is 10
p1.add(redLabel = new Label("RED"));
p1.add(redScroll);
// similarly set for green scroll bar
greenScroll = new Scrollbar(Scrollbar.HORIZONTAL, 0, 0, 0, 255);
greenScroll.setUnitIncrement(5);
greenScroll.setBlockIncrement(15);
p1.add(greenLabel = new Label("GREEN"));
p1.add(greenScroll);
blueScroll = new Scrollbar();
blueScroll.setOrientation(Scrollbar.HORIZONTAL);
blueScroll.setValue(0);
blueScroll.setVisibleAmount(0);
blueScroll.setUnitIncrement(5);
blueScroll.setBlockIncrement(10);
blueScroll.setMinimum(0);
blueScroll.setMaximum(255);
p1.add(blueLabel = new Label("BLUE"));
p1.add(blueScroll);
redScroll.addAdjustmentListener(this);
greenScroll.addAdjustmentListener(this);
blueScroll.addAdjustmentListener(this);
add(p1,"South");
setTitle("Playing With Colors");
setSize(450,325);
setVisible(true);
}
public Insets getInsets()
{
Insets is1 = new Insets(5, 8, 10, 25);
return is1;
}
public void adjustmentValueChanged(AdjustmentEvent e)
{
int rv = redScroll.getValue();
int gv = greenScroll.getValue();
int bv = blueScroll.getValue();
redLabel.setText("RED: "+ rv);
greenLabel.setText("GREEN: "+ gv);
blueLabel.setText("BLUE: "+ bv);
Color clr1 = new Color(rv, gv, bv);
setBackground(clr1);
}
public static void main(String args[])
{
Splesson s = new Splesson();
s.addWindowListener ( new WindowAdapter () {
public void windowClosing ( WindowEvent evt )
{
System.exit(0);
}
});
}
}[/java]
Output
Key Points
- AWT Scrolling is done vertically or horizontally.
- Slider moves between the minimum and maximum values.
- AdjustmentListener handles scrolling events.