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Swing Chapter 24. (Special topics) Accessibility. Easy for reading, Click here!

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Swing Chapter 24. (Special topics) Accessibility. Easy for reading, Click here!

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Page: 3/3 



Previous Page Previous Page (2/3)
Subpages: 1. Java 2 Accessibility API overview
2. Following accessibility guidelines
3. Accessibility support for custom components

24.3  Accessibility for custom components

In developing professional custom Swing components, special care should be taken to implement the Accessible interface and provide a custom extension of the AccessibleContext class. The following example extends the functionality of our OpenList component, developed and used in the examples of chapter 20, to comply with the accessibility standards.

Figure 24.2 Custom font dialog using accessible OpenList components, and monitored using the Explorer utility .

<<file figure24-2.gif>>

The Code: Access2.java

see \Chapter24\2

import javax.accessibility.*;

public class Access2 extends JDialog

{

  // added mnemonics and tooltip text to all relevant components

  // ...this class is used to set up a font dialog and has been

  // extracted from code in chapter 20. It requires little

  // presentation or explanation here.

  public static void main(String argv[]) {

    GraphicsEnvironment ge = GraphicsEnvironment.

      getLocalGraphicsEnvironment();

    m_fontNames = ge.getAvailableFontFamilyNames();

    m_fontSizes = new String[] {"8", "9", "10", "11", "12", "14",

      "16", "18", "20", "22", "24", "26", "28", "36", "48", "72"};

    Access2 dlg = new Access2(new JFrame());

    SimpleAttributeSet a = new SimpleAttributeSet();

    StyleConstants.setFontFamily(a, "Monospaced");

    StyleConstants.setFontSize(a, 12);

    dlg.setAttributes(a);

    dlg.show();

  }

}

class OpenList extends JPanel

 implements ListSelectionListener, ActionListener

{

  protected JLabel m_title;

  protected JTextField m_text;

  protected JList m_list;

  protected JScrollPane m_scroll;

  public OpenList(String[] data, String title)

  {

    setLayout(null);

    m_title = new OpenListLabel(title, JLabel.LEFT);

    add(m_title);

    m_text = new OpenListText();

    m_text.addActionListener(this);

    m_title.setLabelFor(m_text);

    add(m_text);

    m_list = new OpenListList(data);

    m_list.setVisibleRowCount(4);

    m_list.addListSelectionListener(this);

    m_scroll = new JScrollPane(m_list);

    add(m_scroll);

  }

  public OpenList(String title, int numCols) {

    setLayout(null);

    m_title = new OpenListLabel(title, JLabel.LEFT);

    add(m_title);

    m_text = new OpenListText(numCols);

    m_text.addActionListener(this);

    m_title.setLabelFor(m_text);

    add(m_text);

    m_list = new OpenListList();

    m_list.setVisibleRowCount(4);

    m_list.addListSelectionListener(this);

    m_scroll = new JScrollPane(m_list);

    add(m_scroll);

  }

  public void setToolTipText(String text) {

    super.setToolTipText(text);

    m_title.setToolTipText(text);

    m_text.setToolTipText(text);

    m_list.setToolTipText(text);

  }

  public void setDisplayedMnemonic(char ch) {

    m_title.setDisplayedMnemonic(ch);

  }

  public AccessibleContext getAccessibleContext() {

    if (accessibleContext == null)

      accessibleContext = new AccessibleOpenList();

    return accessibleContext;

  }

  // Unchanged code from section 20.9

  class OpenListLabel extends JLabel {

    public OpenListLabel(String text, int alignment) {

      super(text, alignment);

    }

    public AccessibleContext getAccessibleContext() {

      return OpenList.this.getAccessibleContext();

    }

  }

  class OpenListText extends JTextField

  {

    public OpenListText() {}

    public OpenListText(int numCols) {

      super(numCols);

    }

    public AccessibleContext getAccessibleContext() {

      return OpenList.this.getAccessibleContext();

    }

  }

  class OpenListList extends JList

  {

    public OpenListList() {}

    public OpenListList(String[] data) { super(data); }

    public AccessibleContext getAccessibleContext() {

      return OpenList.this.getAccessibleContext();

    }

  }

  protected class AccessibleOpenList extends AccessibleJComponent

  {

    public String getAccessibleName() {

      if (accessibleName != null)

        return accessibleName;

      return m_title.getText();

    }

    public AccessibleRole getAccessibleRole() {

      return AccessibleRole.LIST;

    }

  }

}

Understanding the Code

Class Access2

This class is mainly extracted from the Editor6 class of chapter 20 and represents an extension of our FontDialog component. We have simply added tooltip text and mnemonics to each of its children and have defined a main method to construct and display an instance of this component.

Class OpenList

Compared to the example in chapter 20 we haven't changed much. We now sub-class three components to build inner class, accessible counterparts: OpenListLabel extends JLabel, OpenListText extends JTextField, and OpenListList extends JList (see below for more about these sub-classes). Also note that the label component is linked with the text component using the setLabelFor() method as discussed above.


The setToolTipText() method is overridden to assign a given tooltip text to all three sub-components. The setDisplayedMnemonic() method is exposed assign the displayedMnemonic property of the label component. In conjunction with the setLabelFor() method call included in the constructors, this provides additional keyboard access to our OpenList component.

The getAccessibleContext() method is the only remaining change made to our OpenList class. It is required for implementation of the Accessible interface inherited from its JPanel ancestor. Our implementation fills the accessibleContext instance variable (inherited from JComponent) by creating an instance of our AccessibleOpenList inner class, and returns it as an AccessibleContext.

Class OpenList.OpenListLabel

This inner class extends JLabel to override the getAccessibleContext() method and delegates the call to OpenList's getAccessibleContext() method. The same is true for inner OpenListText and OpenListList sub-classes of JTextField and JList respectively. This guarantees that the AccessibleOpenList will be used as the accessible context, no matter which sub-component currently has the focus. In this way, information about OpenList and each of its constituent child components is always grouped together and treated as one component by assistive technologies.

Class OpenList.AccessibleOpenList

This inner class extends the AccessibleJComponent class (an extension of AccessibleContext provided by the JComponent ancestor) to provide accessible information about our component. The getAccessibleName() method returns the text of the m_title label as the accessible name (unless this has been set directly with the setAccessibleName() method). The getAccessibleRole() method returns AccessibleRole.LIST as the closest pre-defined accessible role (see 24.1.5).

Note: We could have gone much further in developing a custom accessible context. However, in the absense of real assistive technologies this does not make much sense. We hope to have provided a sufficient background to get you started when these are made available.

Running the Code

At this point you can compile and execute this example. Figure 24.2 shows the font dialog from our word processor application of chapter 20 now using an accessible variant of the OpenList component. Notice that when focus is transferred to an OpenList component an instance of AccessibleOpenList, described above, is used as the accessible context (the Explorer window in figure 24.2 illustrates).



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