Learning Java, 4th Edition

Scrollbars and Sliders

JScrollPane is such a handy component that you may not ever need to use scrollbars by themselves. In fact, if you ever do find yourself using a scrollbar by itself, chances are that you really want to use another component called a slider.

There’s not much point in describing the appearance and functionality of scrollbars and sliders. Instead, let’s jump right in with an example that includes both components. Figure 17-12 shows a simple example with both a scrollbar and a slider.

Figure 17-12. Using a scrollbar and a slider

Here is the source code for this example:

    //file: Slippery.java
    import java.awt.*;
    import java.awt.event.*;
    import javax.swing.*;
    import javax.swing.event.*;

    public class Slippery {
      public static void main(String[] args)
      {
        JFrame frame = new JFrame("Slippery v1.0");
        Container content = frame.getContentPane();  // unnecessary  in 5.0+

        JPanel main = new JPanel(new GridLayout(2, 1));
        JPanel scrollBarPanel = new JPanel();
        final JScrollBar scrollBar =
            new JScrollBar(JScrollBar.HORIZONTAL, 0, 48, 0, 255);
        int height = scrollBar.getPreferredSize().height;
        scrollBar.setPreferredSize(new Dimension(175, height));
        scrollBarPanel.add(scrollBar);
        main.add(scrollBarPanel);

        JPanel sliderPanel = new JPanel();
        final JSlider slider =
            new JSlider(JSlider.HORIZONTAL, 0, 255, 128);
        slider.setMajorTickSpacing(48);
        slider.setMinorTickSpacing(16);
        slider.setPaintTicks(true);
        sliderPanel.add(slider);
        main.add(sliderPanel);

        content.add(main, BorderLayout.CENTER);

        final JLabel statusLabel =
            new JLabel("Welcome to Slippery v1.0");
        content.add(statusLabel, BorderLayout.SOUTH);

        // wire up the event handlers
        scrollBar.addAdjustmentListener(new AdjustmentListener() {
          public void adjustmentValueChanged(AdjustmentEvent e) {
            statusLabel.setText("JScrollBar's current value = "
                                + scrollBar.getValue());
          }
        });

        slider.addChangeListener(new ChangeListener() {
          public void stateChanged(ChangeEvent e) {
            statusLabel.setText("JSlider's current value = "
                                + slider.getValue());
          }
        });

        frame.pack();
        frame.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
        frame.setVisible(true);
      }
    }

All we’ve really done here is added a JScrollBar and a JSlider to our main window. If the user adjusts either of these components, the current value of the component is displayed in a JLabel at the bottom of the window.

You create both the JScrollBar and JSlider by specifying an orientation, either HORIZONTAL or VERTICAL. You can also specify the minimum and maximum values for the components, as well as the initial value. The JScrollBar supports one additional parameter, the extent. The extent simply refers to what range of values is represented by the slider within the scroll bar. For example, in a scrollbar that runs from 0 to 255, an extent of 128 means that the slider will be half the width of the scrollable area of the scrollbar.

JSlider supports the idea of tick marks, lines drawn at certain values along the slider’s length. Major tick marks are slightly larger than minor tick marks. To draw tick marks, just specify an interval for major and minor tick marks, and then paint the tick marks:

    slider.setMajorTickSpacing(48);
    slider.setMinorTickSpacing(16);
    slider.setPaintTicks(true);

JSlider also supports labeling the ticks with text strings, using the setLabelTable() method.

Responding to events from the two components is straightforward. The JScrollBar sends out AdjustmentEvents every time something happens; the JSlider fires off ChangeEvents when its value changes. In our simple example, we display the new value of the changed component in the JLabel at the bottom of the window.