Swing Chapter 13. (The basics) Progress Bars, Sliders, and Scroll Bars. Easy for reading, Click here!

Subpages: 1. JBounded-range components overview 
2. Basic JScrollBar example 
3. JSlider date chooser 
4. JSliders in a JPEG image editor 
5. JProgressBar in an FTP client application 

13.2  Basic JScrollBar example

The JScrollBar component is most often seen as part of a JScrollPane. We rarely use this component alone, unless customized scrolling is desired. In this section we'll show how to use JScrollBar to create a simple custom scrolling pane from scratch.

Figure 13.2 Running ScrollDemo example showing an image in the custom scroll pane.

<<file figure13-2.gif>>

The Code: ScrollDemo.java

see \Chapter13\2

import java.awt.*;

import java.awt.event.*;

import javax.swing.*;

import javax.swing.event.*;

public class ScrollDemo extends JFrame

{

  public ScrollDemo() {

    super("JScrollBar Demo");

    setSize(300,250);

    ImageIcon ii = new ImageIcon("earth.jpg");

    CustomScrollPane sp = new CustomScrollPane(new JLabel(ii));

    getContentPane().add(sp);

    WindowListener wndCloser = new WindowAdapter() {

      public void windowClosing(WindowEvent e) {

        System.exit(0);

      }

    };

    addWindowListener(wndCloser);

    setVisible(true);

  }

  public static void main(String[] args) {

    new ScrollDemo();

  }

}

class CustomScrollPane extends JPanel

{

  protected JScrollBar m_vertSB;

  protected JScrollBar m_horzSB;

  protected CustomViewport m_viewport;

  protected JComponent m_comp;

  public CustomScrollPane(JComponent comp) {

    setLayout(null);

    m_viewport = new CustomViewport();

    m_viewport.setLayout(null);

    add(m_viewport);

    m_comp = comp;

    m_viewport.add(m_comp);

    m_vertSB = new JScrollBar(

      JScrollBar.VERTICAL, 0, 0, 0, 0);

    m_vertSB.setUnitIncrement(5);

    add(m_vertSB);

    m_horzSB = new JScrollBar(

      JScrollBar.HORIZONTAL, 0, 0, 0, 0);

    m_horzSB.setUnitIncrement(5);

    add(m_horzSB);

    AdjustmentListener lst = new AdjustmentListener() {

      public void adjustmentValueChanged(AdjustmentEvent e) {

        m_viewport.doLayout();

      }

    };

    m_vertSB.addAdjustmentListener(lst);

    m_horzSB.addAdjustmentListener(lst);

  }

  public void doLayout() {

    Dimension d = getSize();

    Dimension d0 = m_comp.getPreferredSize();

    Dimension d1 = m_vertSB.getPreferredSize();

    Dimension d2 = m_horzSB.getPreferredSize();

    int w = Math.max(d.width - d1.width-1, 0);

    int h = Math.max(d.height - d2.height-1, 0);

    m_viewport.setBounds(0, 0, w, h);

    m_vertSB.setBounds(w+1, 0, d1.width, h);

    m_horzSB.setBounds(0, h+1, w, d2.height);

    int xs = Math.max(d0.width - w, 0);

    m_horzSB.setMaximum(xs);

    m_horzSB.setBlockIncrement(xs/5);

    m_horzSB.setEnabled(xs > 0);

    int ys = Math.max(d0.height - h, 0);

    m_vertSB.setMaximum(ys);

    m_vertSB.setBlockIncrement(ys/5);

    m_vertSB.setEnabled(ys > 0);

    m_horzSB.setVisibleAmount(m_horzSB.getBlockIncrement());

    m_vertSB.setVisibleAmount(m_vertSB.getBlockIncrement());

  }

  public Dimension getPreferredSize() {

    Dimension d0 = m_comp.getPreferredSize();

    Dimension d1 = m_vertSB.getPreferredSize();

    Dimension d2 = m_horzSB.getPreferredSize();

    Dimension d = new Dimension(d0.width+d1.width,

      d0.height+d2.height);

    return d;

  }

  class CustomViewport extends JPanel

  {

    public void doLayout() {

      Dimension d0 = m_comp.getPreferredSize();

      int x = m_horzSB.getValue();

      int y = m_vertSB.getValue();

      m_comp.setBounds(-x, -y, d0.width, d0.height);

    }

  }

}

Understanding the Code

Class ScrollDemo

This simple frame-based class creates a CustomScrollPane instance to scroll a large image. This class is very similar to the first example in the chapter 7 and does not require additional explanation.

Class CustomScrollPane

This class extends JPanel to represent a simple custom scroll pane. Four instance variables are declared:

JScrollBar m_vertSB: vertical scroll bar.

JScrollBar m_horzSB: horizontal scroll bar.

CustomViewport m_viewport: custom viewport component.

JComponent m_comp: component to be placed in our custom viewport.

The CustomScrollPane constructor takes a component to be scrolled as parameter. It instantiates the instance variables described above and adds them to itself using a null layout (because this component acts as its own layout manager). Note that the JScrollBars are created with proper orientation and zero values accross the board (because these are meaningless if not based on the size of the component being scrolled).

An AdjustmentListener is created and added to both scroll bars. The adjustmentValueChanged() method calls the doLayout() method on the m_viewport component to perform the actual component scrolling according to the new scroll bars values.

The doLayout() method sets the bounds for the viewport (in the center), vertical scroll bar (on the right), and horizontal scroll bar (on the bottom). New maximum values and block increment values are set for the scroll bars based on the sizes of the scrolling pane and component to be scrolled. Note that if the maximum value reaches zero, the corresponding scroll bar is disabled. The visibleAmount property of each is set to the corresponding blockIncrement value to provide proportional thumb sizes.

The getPreferredSize() method simply calculates the preferred size of this component based on the preferred sizes of it's children.

Class CustomViewport

This class extends JPanel and represents a simple realization of a viewport for our custom scrolling pane. The only implemented method, doLayout(), reads the current scroll bar values and assigns bounds to the scrolling component accordingly.

Running the Code

Figure 13.2 shows an image in the custom scroll pane. Use the horizontal and vertical scroll bars to verify that scrolling works as expected. Resize the frame component to verify that the scroll bar values and thumbs are adjusted correctly as the container's size is changed.