nov01_ericg.txt

TechTips j2me的常用技巧. 网络功能

 J 2 M E    T  E  C  H    T  I  P  S

                      TIPS, TECHNIQUES, AND SAMPLE CODE


WELCOME to the Java Developer Connection(sm) (JDC)
Java(tm) 2 Platform, Micro Edition (J2ME(tm)) 
Tech Tips, for November 14, 2001. This issue covers:

     * Dealing With Dates and Times in MIDP Applications
     * Developing Custom Components for the Mobile Information 
       Device Profile
         
The J2ME Tech Tips are written by Eric Giguere
(http://www.ericgiguere.com), an engineer at iAnywhere 
Solutions, inc. Eric is the author of the book "Java 2 Micro
Edition: Professional Developer's Guide" and co-author of the 
upcoming "Mobile Information Device Profile for Java 2 Micro 
Edition," both books in John Wiley & Sons' Professional 
Developer's Guide series.

You can view this issue of the J2ME Tech Tips on the Web at
http://java.sun.com/jdc/J2METechTips/2001/tt1114.html

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DEALING WITH DATES AND TIMES IN MIDP APPLICATIONS

Parsing and displaying dates and times is often complicated 
because of formatting and locale issues. Java 2 Platform, 
Standard Edition (J2SE(tm)) provides several classes to simplify
date and time handling -- classes such as java.util.Calendar, 
java.util.Date, java.util.TimeZone, and java.text.DateFormat. By 
comparison, the Mobile Information Device Profile (MIDP) defines
only subsets of the Calendar, Date and TimeZone classes, and does 
not include any form of DateFormat.  How, then, can your MIDP
applications properly handle dates and times?

The answer lies in the javax.microedition.lcdui.DateField class, 
part of the MIDP high-level user interface API. DateField is an 
interactive user interface component that displays a date, time,
or both. It also allows you to edit the date and time. DateField 
extends the Item class. This means that DateField components can 
be placed on Form objects. So the first step in using a DateField 
is to create a Form and place the DateField on the form:

    Form f = new Form( "A Form" );
    f.append( df );
    
As with any Item, the DateField is displayed only when the form 
is made active by calling Display.setCurrent.

The DateField class defines two constructors:

    public DateField( String label, int mode );
    public DateField( String label, int mode,
                      java.util.TimeZone zone );
                    
To properly display dates and times, a DateField instance needs 
to know which time zone to use. The two-argument constructor uses 
the device's default time zone. The three-argument constructor 
lets you specify an explicit time zone if the default is 
inappropriate. Note that you can't change the displayed time zone
without creating a new instance of DateField.

The first two arguments are identical in both constructors. The 
first argument is the label to display alongside the field -- use 
null if there is no label. The second argument is the input mode 
of the field. There are three possible modes, these are declared 
as constants in the DateField class:

    public static final int DATE = 1;
    public static final int TIME = 2;
    public static final int DATE_TIME = 3;
    
The input mode controls what the field displays: a date only, 
a time only, or a combined date and a time. You can change the 
input mode at any time by calling the setInputMode method.

When you create a new DateField instance, you do not have to set
a date or time. The following code, for example, displays an 
uninitialized date:

    Display display = ....; // initialized elsewhere
    Form f = new Form( "An Empty Date" );
    DateField df = new DateField( "Date:", 
                                  DateField.DATE );
    f.append( df );
    display.setCurrent( f );
    
To initialize the field to a particular date or time, call 
setDate and pass in a java.util.Date object initialized to the 
correct value:

    Calendar c = Calendar.getInstance();
    c.set( Calendar.MONTH, Calendar.OCTOBER );
    c.set( Calendar.DAY_OF_MONTH, 18 );
    c.set( Calendar.YEAR, 1996 );
    c.set( Calendar.HOUR_OF_DAY, 16 );
    c.set( Calendar.MINUTE, 39 );
    c.set( Calendar.SECOND, 45 );
    c.set( Calendar.MILLISECOND, 0 );
    
    Date moment = c.getTime();
    DateField df = new DateField( null, 
                                  DateField.DATE_TIME );
    df.setTime( moment );
  
A Date object represents a moment in time (in coordinated
universal time, or UTC, to be exact) as the number of 
milliseconds since midnight, January 1, 1970. Use a Calendar 
instance to create a Date instance, as shown above.  
  
Note that a DateField in TIME input mode requires the date 
portion to be set to January 1, 1970. Two useful routines for 
clearing out the date portion of a Date and for combining two 
Date objects into a single object are as follows:

    // Return a Date with the time intact but the date
    // set to January 1, 1970

    public static Date clearDate( Date d ){
        Calendar c = Calendar.getInstance();
        c.setTime( d );
        c.set( Calendar.MONTH, Calendar.JANUARY );
        c.set( Calendar.DAY_OF_MONTH, 1 );
        c.set( Calendar.YEAR, 1970 );
        return c.getTime();
    }

    // Combine a date and time into a single 
    // Date instance

    public static Date combineDateTime( 
                                Date date, Date time ){
        Calendar cd = Calendar.getInstance();
        Calendar ct = Calendar.getInstance();

        cd.setTime( date );
        ct.setTime( time );

        ct.set( Calendar.MONTH, 
                            cd.get( Calendar.MONTH ) );
        ct.set( Calendar.DAY_OF_MONTH, 
                     cd.get( Calendar.DAY_OF_MONTH ) );
        ct.set( Calendar.YEAR, 
                             cd.get( Calendar.YEAR ) );

        return ct.getTime();
    }

Always do your date manipulation using the Calendar class, not 
using the raw milliseconds value stored in a Date object.

After a DateField is displayed, the system will allow the user to 
select the object and edit the date, time or both, depending on 
the input mode. Whenever you need to obtain the new date/time, 
call the getDate method:

    DateField df = ....;
    Date editedDate = df.getDate();
    
Here is a simple MIDlet that lets you view and edit dates and 
times using all three input modes.

import java.util.*;
import javax.microedition.lcdui.*;
import javax.microedition.midlet.*;

/**
 * Demonstration of time/date editing using the MIDP
 * DateField class.
 */

public class DateFieldTest extends MIDlet {

    private Display display;
    
    // Define our Command objects
  
    private Command exitCommand =
             new Command( "Exit", Command.EXIT, 1 ); 
    private Command okCommand =
             new Command( "OK", Command.OK, 1 );
    private Command cancelCommand =
             new Command( 
                         "Cancel", Command.CANCEL, 1 );

    public DateFieldTest(){
    }

    protected void destroyApp( boolean unconditional )
                    throws MIDletStateChangeException {
        exitMIDlet();
    }

    protected void pauseApp(){
    }
 
    protected void startApp()
                    throws MIDletStateChangeException {
        if( display == null ){ // first time called...
            initMIDlet();
        }
    }

    private void initMIDlet(){
        display = Display.getDisplay( this );
        testList = new TestList();
        display.setCurrent( testList );
    }

    public void exitMIDlet(){
        notifyDestroyed();
    }
 
    // Return a Date with the time intact but the date
    // set to January 1, 1970

    public static Date clearDate( Date d ){
        Calendar c = Calendar.getInstance();
        c.setTime( d );
        c.set( Calendar.MONTH, Calendar.JANUARY );
        c.set( Calendar.DAY_OF_MONTH, 1 );
        c.set( Calendar.YEAR, 1970 );
        return c.getTime();
    }

    // Combine a date and time into a single 
    // Date instance

    public static Date combineDateTime( Date date, 
                                        Date time ){
        Calendar cd = Calendar.getInstance();
        Calendar ct = Calendar.getInstance();

        cd.setTime( date );
        ct.setTime( time );

        ct.set( Calendar.MONTH, 
              cd.get( Calendar.MONTH ) );
        ct.set( Calendar.DAY_OF_MONTH,  
                   cd.get( Calendar.DAY_OF_MONTH ) );
        ct.set( Calendar.YEAR, 
                   cd.get( Calendar.YEAR ) );

        return ct.getTime();
    }
    
    // The list of tests we can perform, arranged 
    // in threes so that ( index % 3 ) == one 
    // of DATE, TIME or DATE_TIME

    static final String[] testLabels = {
        "Current date",
        "Current time",
        "Current date/time",
        "Edit date",
        "Edit time",
        "Edit date/time",
    };

    private TestList testList;
    private Date     editDate;

    //
    // Displays the list of actions
    //

    class TestList extends List
                   implements CommandListener {
        public TestList(){
            super( "DateField Tests", IMPLICIT,
                   testLabels, null );
            addCommand( exitCommand );
            setCommandListener( this );
        }

        public void commandAction( Command c,
                                   Displayable d ){
            if( c == exitCommand ){
                exitMIDlet();
            } else if( c == List.SELECT_COMMAND ){
             
                // Figure out which date to display
                // and what the input mode is

                int     which = getSelectedIndex();
                String  label = getString( which );
                int     mode = ( which % 3 ) + 1;
                boolean save = ( which > 2 );

                display.setCurrent(
                   new Edit( save, label, mode ) );
            }
        }
    }

    //
    // Edit a date, time or date/time, optionally
    // saving the value.
    //

    class Edit extends Form
               implements CommandListener {

        public Edit( boolean save, String label,
                     int mode ){

            super( label );
            this.save = save;

            Date d = editDate;

            if( !save ){
                d = new Date();
            }

            dateField = new DateField( null, mode );
            append( dateField );

            if( d != null ){
                if( mode == DateField.TIME ){
                    d = clearDate( d );
                }

                dateField.setDate( d );
            }