J2ME: The Complete Reference in Java

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Each key on an ITU-T keypad, which is used on cellular telephones, is mapped to a standard set of key codes that are shown in Table 7-1 J2ME associates key code values with constants; however, use the constant instead of the constant value You ll find that using constants within your code clarifies the reference to a key because the name of the constant contains the name of the key associated with the key code All small computing devices that use the ITU-T keypad adhere to these key codes Some of these devices also have other keys on the keypad, each of which is also assigned a key code The manufacturer s specification for the device usually lists key codes used by the device for keys outside of those on the standard ITU-T keypad Your MIDlet can detect and process any key code including keys other than those found on the standard ITU-T keypad using techniques described later in this section However, keys outside of the standard ITU-T keypad may not be available in all small computing devices, resulting in a possible portability problem with your MIDlet So avoid using keys other than those on the standard ITU-T keypad unless your MIDlet will run exclusively on a specific make and model device that contains those keys

KEY_NUM0 KEY_NUM1 KEY_NUM2 KEY_NUM3 KEY_NUM4 KEY_NUM5 KEY_NUM6 KEY_NUM7 KEY_NUM8 KEY_NUM9 KEY_STAR KEY_POUND Table 7-1

48 49 50 51 52 53 54 55 56 57 42 35

7:

There are three empty methods that are called when a particular key event occurs while your MIDlet is running You should override these methods if your application needs to call them These methods are keyPressed(), keyReleased(), and keyRepeated() The keyPressed() method is called by the application manager whenever a key is pressed by the user Likewise, the keyReleased() method is called when the key selected by the user is released And the keyRepeated() method is called by the application manager when the user holds down the key, causing the key to be automatically repeated A word of caution: not all devices support repeated keys Your MIDlet can inquire whether or not the repeated key feature is supported by calling the hasRepeatEvents() method (See Quick Reference Guide at the end of this chapter) All of these methods have empty implementation You must override each method if your application needs to process the related key events However, many of the applications you create that implement a low-level user interface will only need to override the keyPressed() method because you ll need to know which key was selected by the user The keyRelease() method and the keyRepeated() method are overridden only for applications that have special processing whenever a person releases a key or holds down a key for an extended period All three methods require one parameter, which is an integer that represents the value of the key code passed to the method by the device s application manager An if statement or switch case statement is used to compare the incoming key code with key code constants that are processed by the MIDlet Listing 7-1 illustrates the basic way in which you detect and process key codes of keys selected by the person using the MIDlet This example designates the 2, 8, 4, and 6 keys as directional keys, where the 2 and 8 are up and down and 4 and 6 are left and right The MIDlet displays text that describes the direction selected by the user Of course, in a real application you would likely reposition an image, such as a game piece, on the screen whenever a direction key is detected rather than display text You ll see how this is done later in this chapter