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Swing Chapter 10. (The basics) List Boxes. Easy for reading, Click here!

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Swing Chapter 10. (The basics) List Boxes. Easy for reading, Click here!

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Page: 4/5 



Previous Page Previous Page (3/5) - Next Page (5/5) Next Page
Subpages: 1. JList API overview  
2.
Basic JList example 
3.
Custom rendering 
4. Processing keyboard input and searching 
5. List of check boxes 

10.4  Processing keyboard input and searching

In this section we will continue to enhance our JList states example by adding the ability to select an element whose text starts with a character corresponding to a key press. We will also show how to extend this functionality to search for an element whose text starts with a sequence of typed key characters.

To do this, we must use a KeyListener to listen for keyboard input, and accumulate this input in a String. Each time a key is pressed, the listener must search through the list and select the first element whose text matches the String that we have accumulated. If the time interval between two key presses exceeds a certain pre-defined value, the accumulated String must be cleared before appending a new character to avoid overflow.

Figure 10.3 JList allowing accumulated keyboard input to search for a matching item.

<<file figure10-3.gif>>

The Code: StatesList.java

see \Chapter10\3

import java.awt.*;

import java.awt.event.*;

import java.util.*;

import javax.swing.*;

import javax.swing.border.*;

import javax.swing.event.*;

public class StatesList extends JFrame

{

  protected JList m_statesList;

  public StatesList() {

    // Unchanged code from section 10.3

    m_statesList = new JList(states);

    TabListCellRenderer renderer = new TabListCellRenderer();

    renderer.setTabs(new int[] {50, 200, 300});

    m_statesList.setCellRenderer(renderer);

    m_statesList.addKeyListener(new ListSearcher(m_statesList));

    // Unchanged code from section 10.3

  }

}

class ListSearcher extends KeyAdapter

{

  protected JList m_list;

  protected ListModel m_model;

  protected String m_key = "";

  protected long m_time = 0;

  public static int CHAR_DELTA = 1000;

  public ListSearcher(JList list) {

    m_list = list;

    m_model = m_list.getModel();

  }

  public void keyTyped(KeyEvent e) {

    char ch = e.getKeyChar();

    if (!Character.isLetterOrDigit(ch))

      return;

    if (m_time+CHAR_DELTA < System.currentTimeMillis())

      m_key = "";

    m_time = System.currentTimeMillis();

    m_key += Character.toLowerCase(ch);

    for (int k=0; k<m_model.getSize(); k++) {

      String str = ((String)m_model.getElementAt(k)).toLowerCase();

      if (str.startsWith(m_key)){

        m_list.setSelectedIndex(k);

        m_list.ensureIndexIsVisible(k);

        break;

      }

    }

  }

}

Understanding the Code

Class StatesList

An instance of ListSearcher is added to the m_statesList component as a KeyListener. This is the only difference made to this class with respect to the previous example.

Class ListSearcher

Class ListSearcher extends the KeyAdapter class and defines on class variable:

int CHAR_DELTA: static variable to hold the maximum time interval in ms between two subsequent key presses before clearing the search key character String.

Instance variables:

JList m_list: list component to search and change selection based on keyboard input.

ListModel m_model: list model of m_list.

String m_key: key character String used to search for a match.

long m_time: time in ms of the last key press.

The ListSearcher constructor simply takes a reference to the parent JList component and stores it in instance variable m_list, and its model in m_model.

The keyTyped() method is called each time a new character is typed (i.e. a key is pressed and released). Our implementation first obtains a typed character and returns if that character is not letter or digit. keyTyped() then checks the time interval between now and the time when the previous key type event occurred. If this interval exceeds CHAR_DELTA, the m_key String is cleared. Finally, this method walks through the list and performs a case-insensitive comparison of the list Strings and searching String (m_key). If an element's text starts with m_key, this element is selected and it is forced to appear within our current JList view using the ensureIndexIsVisible() method.

Running the Code

Try out the search functionality. Figure 10.3 shows our list's selection after pressing "n" immediately followed by "j". As expected, New Jersey is selected.

UI Guideline : Extending Usability and List Size

This technique of allowing accumulated keyboard input to sift and select a List item, improves usability by making the task of search and locating an item in the list easier. This extends the number of items you can put in a list and still have a usable design. A technique like this can easily improve the usefulness of the list up to several thousand entries.

This is another good example of improved usability when the developer takes extra time to provide additional code to make the User's task easier.



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