Swing Chapter 9. (The basics) Combo Boxes. Easy for reading, Click here!

Subpages: 1. Combo Boxes: JComboBox 
2. Basic JComboBox example 
3. Custom model and renderer 
4. Comboboxes with memory 
5. Custom editing 

9.5    Custom editing

In this section we will discuss a custom editing feature to make the example from the last section even more convenient and similar to modern browser applications. We will attach a key event listener to our combo box's editor and search for previously viosited URLs with matching beginning strings. If a match occurs the remainder of that URL is displayed in the editor, and by pressing Enter we can accept the suggestion. Most modern browsers also provide this functionality.

Note that the caret position will remain unchanged as well as the text on the left side of the caret (i.e. the text most likely typed by the user). The text on the right side of the caret represents the browser's suggestion which may or may not correspond to the user's intentions. To avoid distracting the user, this portion of the text is highlighted, so any newly typed character will replace that suggested text.

Figure 9.4 JComboBox with custom editor suggesting previously visited URLs.

<<file figure9-4.gif>>

The Code: Browser.java

see \Chapter9\4

public class Browser extends JFrame

{

  // Unchanged code from section 9.4

  public Browser() {

    super("HTML Browser [Advanced Editor]");

    // Unchanged code from section 9.4

    MemComboAgent agent = new MemComboAgent(m_locator);

    // Unchanged code from section 9.4

  }

  // Unchanged code from section 9.4

}

class MemComboAgent extends KeyAdapter

{

  protected JComboBox   m_comboBox;

  protected JTextField  m_editor;

  public MemComboAgent(JComboBox comboBox) {

    m_comboBox = comboBox;

    m_editor = (JTextField)comboBox.getEditor().

      getEditorComponent();

    m_editor.addKeyListener(this);

  }

  public void keyReleased(KeyEvent e) {

    char ch = e.getKeyChar();

    if (ch == KeyEvent.CHAR_UNDEFINED || Character.isISOControl(ch))

      return;

    int pos = m_editor.getCaretPosition();

    String str = m_editor.getText();

    if (str.length() == 0)

      return;

    for (int k=0; k<m_comboBox.getItemCount(); k++) {

      String item = m_comboBox.getItemAt(k).toString();

      if (item.startsWith(str)) {

        m_editor.setText(item);

        m_editor.setCaretPosition(item.length());

        m_editor.moveCaretPosition(pos);

        break;

      }

    }

  }

}

Understanding the Code

Class Browser

This class has only one change in comparison with the previous example: it creates an instance of our custom MemComboAgent class and passes it a reference to our m_locator combo box.

Class MemComboAgent

This class extends KeyAdapter to listen for keyboard activity. It takes a reference to a JComboBox component and stores it in an instance variable along with the JTextField component used as that combo box's editor. Finally, a MemComboAgent object adds itself to that editor as a KeyListener to be notified of all keyboard input that is passed to the editor component.

Method keyReleased() is the only method we implement. First this method retrieves the pressed characters and verifies that they are not control characters. We also retrieve the contents of the text field and check that it is not empty (to avoid annoying the user with suggestions in an empty field). Note that when this method is invoked the pressed key will already have been included in this text.

This method then walks through the list of combo box items and searches for an item starting with the combo box editor text. If such an item is found it is set as the combo box editor's text. Then we place the caret at the end of that string using setCaretPosition(), and move it back to its initial position in the backward direction using the moveCaretPosition() method. This final JTextComponent method places the caret in its original position and highlights all text to its right (see chapters 11 and 19).

Running the Code

Figure 9.4 shows our custom combo box's editor displaying a portion of a URL address taken from its list. Try entering some new addresses and browsing to them. After some experimentation, try typing in an address that you have already visited with this application. Notice that the enhanced combo box suggests the remainder of this address from its pull-down list. Press "Enter" as soon as an address matches your intended selection to avoid typing the complete URL.