5.2.4 The `Scrollbar` Class

The following example is an applet that contains a scrollbar (or slider) and a canvas, i.e., a drawing area. The scrollbar points at values for gray shadings of the drawing area that range from black to white.

The Applet

The Java Code

import java.applet.Applet;
import java.awt.*;

public class GraySlider extends Applet {

     private Scrollbar slider;
     private Canvas canvas;

     public void init() {
       //  create the horizontal scrollbar and the canvas
       slider = new Scrollbar(Scrollbar.VERTICAL, 0, 51, 0, 255);
       //  start with black canvas
       canvas = new Canvas();
       canvas.setBackground(new Color(0,0,0));
       //  choose border layout and add components
       setLayout(new BorderLayout());
       add("West", slider);
       add("Center", canvas);
    }

    public boolean handleEvent(Event evt) {
      if (evt.target == slider) {    // slider moved?
         int gl = slider.getValue(); // get graylevel
         canvas.setBackground(new Color(gl,gl,gl)); // change background color
      }
      return true;
    }
}

The way we have created the scrollbar needs some explanation. A vertical scrollbar looks as follows:

A scrollbar allows the user to select an integer value from a range of values. The position of the tab specifies the currently selected value. The user can move the scrollbar in three ways:

dragging the tab
clicking in the page-up or page-down region
clicking on the up- or down-arrow

The most important instance variables of a Scrollbar are:

orientation, which specifies whether the scrollbar is vertical or horizontal;
minimum, which is the smallest in the range of possible values;
maximum, which is the largest in the range of possible values;
value, which is the currently selected value;
sVisible, which is the size of the tab;
lineIncrement , which is the increment or decrement of the current value when you click on the up- or down-arrow of the scrollbar;
pageIncrement , which is the increment or decrement of the current value when you click in the paging area of the scrollbar;

You can specify one or more of these values when the scrollbar is created. In our applet,

slider = new Scrollbar(Scrollbar.VERTICAL, 0, 51, 0, 255);

sets up a vertical scrollbar, with initial value 0, with a tab of size 51, that increments or decrements by 51 when you click in the paging area of the scrollbar, and that has minimum value of 0 and maximum value of 255.

The current value of the scrollbar is used to define a gray color for the drawing area: the red, green and blue components in the RGB-color scheme are given the the current value of the scrollbar between 0 and 255 (8-bit color specification), where 0 means black, and 255 means white.

The current value of the scrollbar is obtained via the getValue() method. Of course the opposite can be done as well: the setValue(int value) method sets the value of the scrollbar to the specified one. The following table summarizes all constructors and main methods for scrollbars.

Constructors Meaning

Scrollbar() creates a vertical scrollbar

Scrollbar(int orientation) creates a scrollbar with specified orientation; available orientations are Scrollbar.HORIZONTAL (0) and Scrollbar.VERTICAL (1)

Scrollbar(int orientation, int value, int sVisible, int minimum, int maximum) creates a scrollbar with the specified orientation, initial value, tab size, and minimum and maximum values

Methods Meaning

getLineIncrement() returns the line increment (default: 1)

getMaximum() returns the maximum value

getMinimum() returns the minimum value

getOrientation() returns the orientation (default: 1, vertical)

getPageIncrement() returns the page increment (default: 10)

getValue() returns the current value

getVisible() returns the size of the tab

setLineIncrement(int lineIncrement) changes the line increment

setMaximum(int maximum) changes the maximum value

setMinimum(int minimum) changes the minimum value

setOrientation(int orientation) sets the orientation

setPageIncrement(int lineIncrement) changes the page increment

setValue(int value) changes the current value

setValues(int value, int sVisible, int minimum, int maximum) changes the values (value, sVisible, minimum, maximum) that define the scrollbar

setVisible(int sVisible) changes the size of the tab

*Constructors*	*Meaning*
`Scrollbar()`	creates a vertical scrollbar
`Scrollbar(int orientation)`	creates a scrollbar with specified orientation; available orientations are `Scrollbar.HORIZONTAL` (0) and `Scrollbar.VERTICAL` (1)
`Scrollbar(int orientation, int value, int sVisible, int minimum, int maximum)`	creates a scrollbar with the specified orientation, initial value, tab size, and minimum and maximum values
*Methods*	*Meaning*
`getLineIncrement()`	returns the line increment (default: 1)
`getMaximum()`	returns the maximum value
`getMinimum()`	returns the minimum value
`getOrientation()`	returns the orientation (default: 1, vertical)
`getPageIncrement()`	returns the page increment (default: 10)
`getValue()`	returns the current value
`getVisible()`	returns the size of the tab
`setLineIncrement(int lineIncrement)`	changes the line increment
`setMaximum(int maximum)`	changes the maximum value
`setMinimum(int minimum)`	changes the minimum value
`setOrientation(int orientation)`	sets the orientation
`setPageIncrement(int lineIncrement)`	changes the page increment
`setValue(int value)`	changes the current value
`setValues(int value, int sVisible, int minimum, int maximum)`	changes the values (`value`, `sVisible`, `minimum`, `maximum`) that define the scrollbar
`setVisible(int sVisible)`	changes the size of the tab

5.2.4 The Scrollbar Class

5.2.4 The `Scrollbar` Class