SCROLL BARS

Scroll bars are used to select continuous values between a specified minimum and
maximum. Scroll bars may be oriented horizontally or vertically. A scroll bar is actually
a composite of several individual parts. Each end has an arrow that you can click to move
the current value of the scroll bar one unit in the direction of the arrow. The current value
of the scroll bar relative to its minimum and maximum values is indicated by the slider box
(or thumb) for the scroll bar. The slider box can be dragged by the user to a new position.
The scroll bar will then reflect this value. In the background space on either side of the thumb, the user can click to cause the thumb to jump in that direction by some increment
larger than 1. Typically, this action translates into some form of page up and page down.
Scroll bars are encapsulated by the Scrollbar class.
Scrollbar defines the following constructors:
Scrollbar( )
Scrollbar(int style)
Scrollbar(int style, int initialValue, int thumbSize, int min, int max)
The first form creates a vertical scroll bar. The second and third forms allow you to
specify the orientation of the scroll bar. If style is Scrollbar.VERTICAL, a vertical scroll
bar is created. If style is Scrollbar.HORIZONTAL, the scroll bar is horizontal. In the
third form of the constructor, the initial value of the scroll bar is passed in initialValue.
The number of units represented by the height of the thumb is passed in thumbSize.
The minimum and maximum values for the scroll bar are specified by min and max.
If you construct a scroll bar by using one of the first two constructors, then you
need to set its parameters by using setValues( ), shown here, before it can be used:
void setValues(int initialValue, int thumbSize, int min, int max)
The parameters have the same meaning as they have in the third constructor just
described.
To obtain the current value of the scroll bar, call getValue( ). It returns the current
setting. To set the current value, call setValue( ). These methods are as follows:
int getValue( )
void setValue(int newValue)
Here, newValue specifies the new value for the scroll bar. When you set a value, the
slider box inside the scroll bar will be positioned to reflect the new value.
You can also retrieve the minimum and maximum values via getMinimum( ) and
getMaximum( ), shown here:
int getMinimum( )
int getMaximum( )
They return the requested quantity.
By default, 1 is the increment added to or subtracted from the scroll bar each
time it is scrolled up or down one line. You can change this increment by calling
setUnitIncrement( ). By default, page-up and page-down increments are 10. You can
change this value by calling setBlockIncrement( ). These methods are shown here:
void setUnitIncrement(int newIncr)
void setBlockIncrement(int newIncr)

Handling Scroll Bars
To process scroll bar events, you need to implement the AdjustmentListener interface.
Each time a user interacts with a scroll bar, an AdjustmentEvent object is generated.
Its getAdjustmentType( ) method can be used to determine the type of the adjustment.
The types of adjustment events are as follows:
BLOCK_DECREMENT A page-down event has been generated.
BLOCK_INCREMENT A page-up event has been generated.
TRACK An absolute tracking event has been generated.
UNIT_DECREMENT The line-down button in a scroll bar has been pressed.
UNIT_INCREMENT The line-up button in a scroll bar has been pressed.
The following example creates both a vertical and a horizontal scroll bar. The
current settings of the scroll bars are displayed. If you drag the mouse while inside
the window, the coordinates of each drag event are used to update the scroll bars.
An asterisk is displayed at the current drag position.
// Demonstrate scroll bars.
import java.awt.*;
import java.awt.event.*;
import java.applet.*;
/*


*/
public class SBDemo extends Applet
implements AdjustmentListener, MouseMotionListener {
String msg = "";
Scrollbar vertSB, horzSB;
public void init() {
int width = Integer.parseInt(getParameter("width"));
int height = Integer.parseInt(getParameter("height"));
vertSB = new Scrollbar(Scrollbar.VERTICAL,
0, 1, 0, height);
horzSB = new Scrollbar(Scrollbar.HORIZONTAL,
0, 1, 0, width);

add(vertSB);
add(horzSB);
// register to receive adjustment events
vertSB.addAdjustmentListener(this);
horzSB.addAdjustmentListener(this);
addMouseMotionListener(this);
}
public void adjustmentValueChanged(AdjustmentEvent ae) {
repaint();
}
// Update scroll bars to reflect mouse dragging.
public void mouseDragged(MouseEvent me) {
int x = me.getX();
int y = me.getY();
vertSB.setValue(y);
horzSB.setValue(x);
repaint();
}
// Necessary for MouseMotionListener
public void mouseMoved(MouseEvent me) {
}
// Display current value of scroll bars.
public void paint(Graphics g) {
msg = "Vertical: " + vertSB.getValue();
msg += ", Horizontal: " + horzSB.getValue();
g.drawString(msg, 6, 160);
// show current mouse drag position
g.drawString("*", horzSB.getValue(),
vertSB.getValue());
}
}