list.java

用于移动设备上的java虚拟机源代码

/*
 * @(#)List.java	1.93 02/09/12 @(#)
 *
 * Copyright (c) 1999-2002 Sun Microsystems, Inc.  All rights reserved.
 * PROPRIETARY/CONFIDENTIAL
 * Use is subject to license terms.
 */

package javax.microedition.lcdui;

/**
 * A <code>Screen</code> containing list of choices. Most of its
 * behavior is common with
 * class {@link ChoiceGroup ChoiceGroup}, and their common API. The
 * different <code>List</code> types in particular, are defined in
 * interface {@link Choice
 * Choice}.  When a <code>List</code> is present on the display, the
 * user can interact with
 * it by selecting elements and possibly by traversing and scrolling among
 * them.  Traversing and scrolling operations do not cause application-visible
 * events. The system notifies the application only when a {@link Command
 * Command} is invoked by notifying its {@link CommandListener}.  The
 * <code>List</code>
 * class also supports a select command that may be invoked specially
 * depending upon the capabilities of the device.
 *
 * <p>The notion of a <em>select</em> operation on a <code>List</code>
 * element is central
 * to the user's interaction with the <code>List</code>.  On devices
 * that have a dedicated
 * hardware &quot;select&quot; or &quot;go&quot; key, the select
 * operation is implemented with
 * that key.  Devices that do not have a dedicated key must provide another
 * means to do the select operation, for example, using a soft key.  The
 * behavior of the select operation within the different types of lists is
 * described in the following sections.</p>
 *
 * <p><code>List</code> objects may be created with <code>Choice</code> types of
 * {@link Choice#EXCLUSIVE}, {@link Choice#MULTIPLE}, and
 * {@link Choice#IMPLICIT}.  The <code>Choice</code> type {@link Choice#POPUP}
 * is not allowed on <code>List</code> objects.</p>
 *
 * <h3>Selection in <code>EXCLUSIVE</code> and <code>MULTIPLE</code> Lists</h3>
 *
 * <p>The select operation is not associated with a
 * <code>Command</code> object, so the
 * application has no means of setting a label for it or being notified when
 * the operation is performed.  In <code>Lists</code> of type
 * <code>EXCLUSIVE</code>, the select
 * operation selects the target element and deselects the previously selected
 * element.  In <code>Lists</code> of type <code>MULTIPLE</code>, the
 * select operation toggles the
 * selected state of the target element, leaving the selected state of other
 * elements unchanged.  Devices that implement the select operation using a
 * soft key will need to provide a label for it.  The label should be something
 * similar to &quot;Select&quot; for <code>Lists</code> of type
 * <code>EXCLUSIVE</code>, and it should be something
 * similar to &quot;Mark&quot; or &quot;Unmark&quot; for
 * <code>Lists</code> of type <code>MULTIPLE</code>.</p>
 *
 * <h3>Selection in <code>IMPLICIT</code> Lists</h3>
 *
 * <p>The select operation is associated with a <code>Command</code>
 * object referred to as
 * the <em>select command</em>.  When the user performs the select operation,
 * the system will invoke the select command by notifying the
 * <code>List's</code> {@link
 * CommandListener CommandListener}.  The default select command is the
 * system-provided command <code>SELECT_COMMAND</code>.  The select
 * command may be modified
 * by the application through use of the {@link #setSelectCommand(Command
 * command) setSelectCommand} method.  Devices that implement the select
 * operation using a soft key will use the label from the select command.  If
 * the select command is <code>SELECT_COMMAND</code>, the device may
 * choose to provide its
 * own label instead of using the label attribute of
 * <code>SELECT_COMMAND</code>.
 * Applications should generally provide their own select command to replace
 * <code>SELECT_COMMAND</code>.  This allows applications to provide a
 * meaningful label,
 * instead of relying on the one provided by the system for
 * <code>SELECT_COMMAND</code>.
 * The implementation must <em>not</em> invoke the select command if there are
 * no elements in the <code>List</code>, because if the
 * <code>List</code> is empty the selection does
 * not exist.  In this case the implementation should remove or disable the
 * select command if it would appear explicitly on a soft button or in a menu.
 * Other commands can be invoked normally when the <code>List</code>
 * is empty.</p>
 *
 * <h3>Use of <code>IMPLICIT</code> Lists</h3>
 *
 * <p> <code>IMPLICIT</code> <code>Lists</code> can be used to
 * construct menus by providing operations
 * as <code>List</code> elements.  The application provides a
 * <code>Command</code> that is used to
 * select a <code>List</code> element and then defines this
 * <code>Command</code> to be used as the
 * select command.  The application must also register a
 * <code>CommandListener</code> that
 * is called when the user selects or activates the <code>Command</code>:</p>
 *
 * <TABLE BORDER="2">
 * <TR>
 * <TD ROWSPAN="1" COLSPAN="1">
 *    <pre><code>
 *     String[] elements = { ... }; //Menu items as List elements
 *     List menuList = new List("Menu", List.IMPLICIT, elements, null);
 *     Command selectCommand = new Command("Open", Command.ITEM, 1);
 *     menuList.setSelectCommand(selectCommand);
 *     menuList.setCommandListener(...);     </code></pre>
 * </TD>
 * </TR>
 * </TABLE>
 *
 * <p>The listener can query the <code>List</code> to determine which
 * element is selected
 * and then perform the corresponding action.  Note that setting a command as
 * the select command adds it to the <code>List</code> as a side effect.</p>
 *
 * <p> The select command should be considered as a <em>default operation</em>
 * that takes place when a select key is pressed.  For example, a
 * <code>List</code>
 * displaying email headers might have three operations: read, reply, and
 * delete. Read is considered to be the default operation.  </p>
 *
 * <TABLE BORDER="2">
 * <TR>
 * <TD ROWSPAN="1" COLSPAN="1">
 *    <pre><code>
 *     List list = new List("Email", List.IMPLICIT, headers);
 *     readCommand = new Command("Read", Command.ITEM, 1);
 *     replyCommand = new Command("Reply", Command.ITEM, 2);
 *     deleteCommand = new Command("Delete", Command.ITEM, 3);
 *     list.setSelectCommand(readCommand);
 *     list.addCommand(replyCommand);
 *     list.addCommand(deleteCommand);
 *     list.setCommandListener(...);     </code></pre>
 * </TD>
 * </TR>
 * </TABLE>
 * <p>On a device with a dedicated select key, pressing this key will invoke
 * <code>readCommand</code>.  On a device without a select key, the user is
 * still able to invoke the read command, since it is also provided as an
 * ordinary <code>Command</code>.</p>
 *
 * <p> It should be noted that this kind of default operation must be used
 * carefully, and the usability of the resulting user interface must always
 * kept in mind. The default operation should always be the most intuitive
 * operation on a particular List.  </p>
 *
 * @since MIDP 1.0
 */

public class List extends Screen implements Choice {

    /**
     * The default select command for <code>IMPLICIT</code> <code>Lists</code>.
     * Applications using an <code>IMPLICIT</code> <code>List</code>
     * should set their own select command
     * using 
     * {@link #setSelectCommand(Command command) setSelectCommand}.
     * 
     * <p>
     * The field values of <code>SELECT_COMMAND</code> are:<br>
     * - <code>label = &quot;&quot;</code> (an empty string)<br>
     * - <code>type = SCREEN</code><br>
     * - <code>priority = 0</code><br>
     * </p>
     * <p>(It would be more appropriate if the type were
     * <code>ITEM</code>, but the type of
     * <code>SCREEN</code> is retained for historical purposes.)</p>
     * <p>
     * The application should not use these values for recognizing
     * the <code>SELECT_COMMAND</code>. Instead, object identities of
     * the <code>Command</code> and
     * <code>Displayable</code> (<code>List</code>) should be used.
     * </p>
     * 
     * <p><code>SELECT_COMMAND</code> is treated as an ordinary
     * <code>Command</code> if it is used with other <code>Displayable</code>
     * types.</p>
     */
    public final static Command SELECT_COMMAND =
        new Command("", Command.SCREEN, 0);

    // private members //

    /**
     * An internal Form to render this List (basically, a List is
     * just a Form with a choiceGroup in it
     */
    private Form form;

    // SYNC NOTE: The List constructor establishes 'cg' as non-null
    // and which remains constant for the lifetime of this object.
    // All public api calls are delegated to the 'cg' object and
    // therefore no synchronization is necessary.
    /**
     * An internal choicegroup to handle the selections
     */
    private ChoiceGroup cg;

    /**
     * This is an internal Command which represents the callback
     * to a selection event of an IMPLICIT list. By default, this
     * command is the predefined List.SELECT_COMMAND. This can be
     * overridden however using the setSelectCommand().
     */
    private Command selectCommand = SELECT_COMMAND;

    // constructors //

    /**
     * Creates a new, empty <code>List</code>, specifying its title
     * and the type of the
     * list. 
     * @param title the screen's title (see {@link Displayable Displayable})
     * @param listType one of <code>IMPLICIT</code>, <code>EXCLUSIVE</code>,
     * or <code>MULTIPLE</code>
     * @throws IllegalArgumentException if <code>listType</code> is not
     * one of
     * <code>IMPLICIT</code>,
     * <code>EXCLUSIVE</code>, or <code>MULTIPLE</code>
     * @see Choice
     */
    public List(String title, int listType) {
        this(title, listType, new String[] {}, new Image[] {});
    }

    /**
     * Creates a new <code>List</code>, specifying its title, the type
     * of the <code>List</code>, and
     * an array of <code>Strings</code> and <code>Images</code> to be
     * used as its initial contents.
     *
     * <p>The <code>stringElements</code> array must be non-null and
     * every array element
     * must also be non-null.  The length of the
     * <code>stringElements</code> array
     * determines the number of elements in the <code>List</code>.
     * The <code>imageElements</code> array
     * may be <code>null</code> to indicate that the <code>List</code>
     * elements have no images.  If the
     * <code>imageElements</code> array is non-null, it must be the
     * same length as the
     * <code>stringElements</code> array.  Individual elements of the
     * <code>imageElements</code> array
     * may be <code>null</code> in order to indicate the absence of an
     * image for the
     * corresponding <code>List</code> element. Non-null elements of the
     * <code>imageElements</code> array may refer to mutable or
     * immutable images.</p>
     *
     * @param title the screen's title (see {@link Displayable Displayable})
     * @param listType one of <code>IMPLICIT</code>, <code>EXCLUSIVE</code>,
     * or <code>MULTIPLE</code>
     * @param stringElements set of strings specifying the string parts of the
     * <code>List</code> elements
     * @param imageElements set of images specifying the image parts of
     * the <code>List</code> elements
     *
     * @throws NullPointerException if <code>stringElements</code> is
     * <code>null</code>
     * @throws NullPointerException if the <code>stringElements</code>
     * array contains any null elements
     * @throws IllegalArgumentException if the <code>imageElements</code>
     * array is non-null
     * and has a different length from the <code>stringElements</code> array
     * @throws IllegalArgumentException if <code>listType</code> is not one 
     * of <code>IMPLICIT</code>,
     * <code>EXCLUSIVE</code>, or <code>MULTIPLE</code>
     *
     * @see Choice#EXCLUSIVE
     * @see Choice#MULTIPLE
     * @see Choice#IMPLICIT
     */
    public List(String title, int listType, String[] stringElements,
		Image[] imageElements) {

        super(title);