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javax.microedition.lcdui
Class List

java.lang.Object
  javax.microedition.lcdui.Displayable
      javax.microedition.lcdui.Screen
          javax.microedition.lcdui.List

All Implemented Interfaces:

Choice

public class List

extends Screen

implements Choice

A Screen containing list of choices. Most of its behavior is common with class ChoiceGroup, and their common API. The different List types in particular, are defined in interface Choice. When a List 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 Command is invoked by notifying its CommandListener. The List class also supports a select command that may be invoked specially depending upon the capabilities of the device.

The notion of a select operation on a List element is central to the user's interaction with the List. On devices that have a dedicated hardware "select" or "go" 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.

List objects may be created with Choice types of Choice.EXCLUSIVE, Choice.MULTIPLE, and Choice.IMPLICIT. The Choice type Choice.POPUP is not allowed on List objects.

Selection in EXCLUSIVE and MULTIPLE Lists

The select operation is not associated with a Command object, so the application has no means of setting a label for it or being notified when the operation is performed. In Lists of type EXCLUSIVE, the select operation selects the target element and deselects the previously selected element. In Lists of type MULTIPLE, 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 "Select" for Lists of type EXCLUSIVE, and it should be something similar to "Mark" or "Unmark" for Lists of type MULTIPLE.

Selection in IMPLICIT Lists

The select operation is associated with a Command object referred to as the select command. When the user performs the select operation, the system will invoke the select command by notifying the List's CommandListener. The default select command is the system-provided command SELECT_COMMAND. The select command may be modified by the application through use of the 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 SELECT_COMMAND, the device may choose to provide its own label instead of using the label attribute of SELECT_COMMAND. Applications should generally provide their own select command to replace SELECT_COMMAND. This allows applications to provide a meaningful label, instead of relying on the one provided by the system for SELECT_COMMAND. The implementation must not invoke the select command if there are no elements in the List, because if the List 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 List is empty.

Use of IMPLICIT Lists

IMPLICIT Lists can be used to construct menus by providing operations as List elements. The application provides a Command that is used to select a List element and then defines this Command to be used as the select command. The application must also register a CommandListener that is called when the user selects or activates the Command:

     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(...);     

The listener can query the List 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 List as a side effect.

The select command should be considered as a default operation that takes place when a select key is pressed. For example, a List displaying email headers might have three operations: read, reply, and delete. Read is considered to be the default operation.

     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(...);     

On a device with a dedicated select key, pressing this key will invoke readCommand. 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 Command.

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.

Since:

MIDP 1.0

Field Summary

static Command

SELECT_COMMAND The default select command for IMPLICIT Lists.

Fields inherited from interface javax.microedition.lcdui.Choice

EXCLUSIVE, IMPLICIT, MULTIPLE, POPUP, TEXT_WRAP_DEFAULT, TEXT_WRAP_OFF, TEXT_WRAP_ON

Constructor Summary

List(String title, int listType)
Creates a new, empty List, specifying its title and the type of the list.

List(String title, int listType, String[] stringElements, Image[] imageElements)
Creates a new List, specifying its title, the type of the List, and an array of Strings and Images to be used as its initial contents.

Method Summary

int	append(String stringPart, Image imagePart) Appends an element to the List.
void	delete(int elementNum) Deletes the element referenced by elementNum.
void	deleteAll() Deletes all elements from this List.
int	getFitPolicy() Gets the application's preferred policy for fitting Choice element contents to the available screen space.
Font	getFont(int elementNum) Gets the application's preferred font for rendering the specified element of this Choice.
Image	getImage(int elementNum) Gets the Image part of the element referenced by elementNum.
int	getSelectedFlags(boolean[] selectedArray_return) Queries the state of a List and returns the state of all elements in the boolean array selectedArray_return.
int	getSelectedIndex() Returns the index number of an element in the List that is selected.
String	getString(int elementNum) Gets the String part of the element referenced by elementNum.
void	insert(int elementNum, String stringPart, Image imagePart) Inserts an element into the List just prior to the element specified.
boolean	isSelected(int elementNum) Gets a boolean value indicating whether this element is selected.
void	removeCommand(Command cmd) The same as Displayable.removeCommand but with the following additional semantics.
void	set(int elementNum, String stringPart, Image imagePart) Sets the String and Image parts of the element referenced by elementNum, replacing the previous contents of the element.
void	setFitPolicy(int fitPolicy) Sets the application's preferred policy for fitting Choice element contents to the available screen space.
void	setFont(int elementNum, Font font) Sets the application's preferred font for rendering the specified element of this Choice.
void	setSelectCommand(Command command) Sets the Command to be used for an IMPLICIT List selection action.
void	setSelectedFlags(boolean[] selectedArray) Sets the selected state of all elements of the List.
void	setSelectedIndex(int elementNum, boolean selected) Sets the selected state of an element.
void	setTicker(Ticker ticker) Sets a ticker for use with this Displayable, replacing any previous ticker.
void	setTitle(String s) Sets the title of the Displayable.
int	size() Gets the number of elements in the List.

Methods inherited from class javax.microedition.lcdui.Displayable

addCommand, getHeight, getTicker, getTitle, getWidth, isShown, setCommandListener, sizeChanged

Methods inherited from class java.lang.Object

equals, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

SELECT_COMMAND

public static final Command SELECT_COMMAND

The default select command for IMPLICIT Lists. Applications using an IMPLICIT List should set their own select command using setSelectCommand.

The field values of SELECT_COMMAND are:
- label = "" (an empty string)
- type = SCREEN
- priority = 0

(It would be more appropriate if the type were ITEM, but the type of SCREEN is retained for historical purposes.)

The application should not use these values for recognizing the SELECT_COMMAND. Instead, object identities of the Command and Displayable (List) should be used.

SELECT_COMMAND is treated as an ordinary Command if it is used with other Displayable types.

Constructor Detail

List

public List(String title,
            int listType)

Creates a new, empty List, specifying its title and the type of the list.

Parameters:

title - the screen's title (see Displayable)

listType - one of IMPLICIT, EXCLUSIVE, or MULTIPLE

Throws:

IllegalArgumentException - if listType is not one of IMPLICIT, EXCLUSIVE, or MULTIPLE

See Also:

Choice

List

public List(String title,
            int listType,
            String[] stringElements,
            Image[] imageElements)

Creates a new List, specifying its title, the type of the List, and an array of Strings and Images to be used as its initial contents.

The stringElements array must be non-null and every array element must also be non-null. The length of the stringElements array determines the number of elements in the List. The imageElements array may be null to indicate that the List elements have no images. If the imageElements array is non-null, it must be the same length as the stringElements array. Individual elements of the imageElements array may be null in order to indicate the absence of an image for the corresponding List element. Non-null elements of the imageElements array may refer to mutable or immutable images.

Parameters:

title - the screen's title (see Displayable)

listType - one of IMPLICIT, EXCLUSIVE, or MULTIPLE

stringElements - set of strings specifying the string parts of the List elements

imageElements - set of images specifying the image parts of the List elements

Throws:

NullPointerException - if stringElements is null

NullPointerException - if the stringElements array contains any null elements

IllegalArgumentException - if the imageElements array is non-null and has a different length from the stringElements array

IllegalArgumentException - if listType is not one of IMPLICIT, EXCLUSIVE, or MULTIPLE

See Also:

Choice.EXCLUSIVE, Choice.MULTIPLE, Choice.IMPLICIT

Method Detail

setTicker

public void setTicker(Ticker ticker)

Sets a ticker for use with this Displayable, replacing any previous ticker. If null, removes the ticker object from this Displayable. The same ticker may be shared by several Displayable objects within an application. This is done by calling setTicker() with the same Ticker object on several different Displayable objects. If the Displayable is actually visible on the display, the implementation should update the display as soon as it is feasible to do so.

The existence of a ticker may affect the size of the area available for Displayable's contents. If the application adds, removes, or sets the ticker text at runtime, this can dynamically change the size of the content area. This is most important to be aware of when using the Canvas class.

Overrides:

setTicker in class Displayable

Parameters:

ticker - the ticker object used on this screen

Since:

MIDP 2.0

See Also:

Displayable.getTicker()

setTitle

public void setTitle(String s)

Sets the title of the Displayable. If null is given, removes the title.

If the Displayable is actually visible on the display, the implementation should update the display as soon as it is feasible to do so.

The existence of a title may affect the size of the area available for Displayable content. If the application adds, removes, or sets the title text at runtime, this can dynamically change the size of the content area. This is most important to be aware of when using the Canvas class.

Overrides:

setTitle in class Displayable

Parameters:

s - the new title, or null for no title

Since:

MIDP 2.0

See Also:

Displayable.getTitle()

size

public int size()

Gets the number of elements in the List.

Specified by:

size in interface Choice

Returns:

the number of elements in the List

getString

public String getString(int elementNum)

Gets the String part of the element referenced by elementNum.

Specified by:

getString in interface Choice

Parameters:

elementNum - the index of the element to be queried

Returns:

the string part of the element

Throws:

IndexOutOfBoundsException - if elementNum is invalid

See Also:

getImage(int)

getImage

public Image getImage(int elementNum)

Gets the Image part of the element referenced by elementNum.

Specified by:

getImage in interface Choice

Parameters:

elementNum - the number of the element to be queried

Returns:

the image part of the element, or null if there is no image

Throws:

IndexOutOfBoundsException - if elementNum is invalid

See Also:

getString(int)

append

public int append(String stringPart,
                  Image imagePart)

Appends an element to the List.

Specified by:

append in interface Choice

Parameters:

stringPart - the string part of the element to be added

imagePart - the image part of the element to be added, or null if there is no image part

Returns:

the assigned index of the element

Throws:

NullPointerException - if stringPart is null

insert

public void insert(int elementNum,
                   String stringPart,
                   Image imagePart)

Inserts an element into the List just prior to the element specified.

Specified by:

insert in interface Choice

Parameters:

elementNum - the index of the element where insertion is to occur

stringPart - the string part of the element to be inserted

imagePart - the image part of the element to be inserted, or null if there is no image part

Throws:

IndexOutOfBoundsException - if elementNum is invalid

NullPointerException - if stringPart is null

delete

public void delete(int elementNum)

Deletes the element referenced by elementNum.

Specified by:

delete in interface Choice

Parameters:

elementNum - the index of the element to be deleted

Throws:

IndexOutOfBoundsException - if elementNum is invalid

deleteAll

public void deleteAll()

Deletes all elements from this List.

Specified by:

deleteAll in interface Choice

set

public void set(int elementNum,
                String stringPart,
                Image imagePart)

Sets the String and Image parts of the element referenced by elementNum, replacing the previous contents of the element.

Specified by:

set in interface Choice

Parameters:

elementNum - the index of the element to be set

stringPart - the string part of the new element

imagePart - the image part of the element, or null if there is no image part

Throws:

IndexOutOfBoundsException - if elementNum is invalid

NullPointerException - if stringPart is null

isSelected

public boolean isSelected(int elementNum)

Gets a boolean value indicating whether this element is selected.

Specified by:

isSelected in interface Choice

Parameters:

elementNum - index to element to be queried

Returns:

selection state of the element

Throws:

IndexOutOfBoundsException - if elementNum is invalid

getSelectedIndex

public int getSelectedIndex()

Returns the index number of an element in the List that is selected.

Specified by:

getSelectedIndex in interface Choice

Returns:

index of selected element, or -1 if none

See Also:

setSelectedIndex(int, boolean)

getSelectedFlags

public int getSelectedFlags(boolean[] selectedArray_return)

Queries the state of a List and returns the state of all elements in the boolean array selectedArray_return.

Specified by:

getSelectedFlags in interface Choice

Parameters:

selectedArray_return - array to contain the results

Returns:

the number of selected elements in the Choice

Throws:

IllegalArgumentException - if selectedArray_return is shorter than the size of the List

NullPointerException - if selectedArray_return is null

See Also:

setSelectedFlags(boolean[])

setSelectedIndex

public void setSelectedIndex(int elementNum,
                             boolean selected)

Sets the selected state of an element.

Specified by:

setSelectedIndex in interface Choice

Parameters:

elementNum - the index of the element, starting from zero

selected - the state of the element, where true means selected and false means not selected

Throws:

IndexOutOfBoundsException - if elementNum is invalid

See Also:

getSelectedIndex()

setSelectedFlags

public void setSelectedFlags(boolean[] selectedArray)

Sets the selected state of all elements of the List.

Specified by:

setSelectedFlags in interface Choice

Parameters:

selectedArray - an array in which the method collect the selection status

Throws:

IllegalArgumentException - if selectedArray is shorter than the size of the List

NullPointerException - if selectedArray is null

See Also:

getSelectedFlags(boolean[])

removeCommand

public void removeCommand(Command cmd)

The same as Displayable.removeCommand but with the following additional semantics.

If the command to be removed happens to be the select command, the List is set to have no select command, and the command is removed from the List.

The following code:

     // Command c is the select command on List list    
     list.removeCommand(c);     

is equivalent to the following code:

     // Command c is the select command on List list    
     list.setSelectCommand(null);    
     list.removeCommand(c);     

Overrides:

removeCommand in class Displayable

Parameters:

cmd - the command to be removed

Since:

MIDP 2.0

setSelectCommand

public void setSelectCommand(Command command)

Sets the Command to be used for an IMPLICIT List selection action. By default, an implicit selection of a List will result in the predefined List.SELECT_COMMAND being used. This behavior may be overridden by calling the List.setSelectCommand() method with an appropriate parameter value. If a null reference is passed, this indicates that no "select" action is appropriate for the contents of this List.

If a reference to a command object is passed, and it is not the special command List.SELECT_COMMAND, and it is not currently present on this List object, the command object is added to this List as if addCommand(command) had been called prior to the command being made the select command. This indicates that this command is to be invoked when the user performs the "select" on an element of this List.

The select command should have a command type of ITEM to indicate that it operates on the currently selected object. It is not an error if the command is of some other type. (List.SELECT_COMMAND has a type of SCREEN for historical purposes.) For purposes of presentation and placement within its user interface, the implementation is allowed to treat the select command as if it were of type ITEM.

If the select command is later removed from the List with removeCommand(), the List is set to have no select command as if List.setSelectCommand(null) had been called.

The default behavior can be reestablished explicitly by calling setSelectCommand() with an argument of List.SELECT_COMMAND.

This method has no effect if the type of the List is not IMPLICIT.

Parameters:

command - the command to be used for an IMPLICIT list selection action, or null if there is none

Since:

MIDP 2.0

setFitPolicy

public void setFitPolicy(int fitPolicy)

Sets the application's preferred policy for fitting Choice element contents to the available screen space. The set policy applies for all elements of the Choice object. Valid values are Choice.TEXT_WRAP_DEFAULT, Choice.TEXT_WRAP_ON, and Choice.TEXT_WRAP_OFF. Fit policy is a hint, and the implementation may disregard the application's preferred policy.

Specified by:

setFitPolicy in interface Choice

Parameters:

fitPolicy - preferred content fit policy for choice elements

Throws:

IllegalArgumentException - if fitPolicy is invalid

Since:

MIDP 2.0

See Also:

getFitPolicy()

getFitPolicy

public int getFitPolicy()

Gets the application's preferred policy for fitting Choice element contents to the available screen space. The value returned is the policy that had been set by the application, even if that value had been disregarded by the implementation.

Specified by:

getFitPolicy in interface Choice

Returns:

one of Choice.TEXT_WRAP_DEFAULT, Choice.TEXT_WRAP_ON, or Choice.TEXT_WRAP_OFF

Since:

MIDP 2.0

See Also:

setFitPolicy(int)

setFont

public void setFont(int elementNum,
                    Font font)

Sets the application's preferred font for rendering the specified element of this Choice. An element's font is a hint, and the implementation may disregard the application's preferred font.

The elementNum parameter must be within the range [0..size()-1], inclusive.

The font parameter must be a valid Font object or null. If the font parameter is null, the implementation must use its default font to render the element.

Specified by:

setFont in interface Choice

Parameters:

elementNum - the index of the element, starting from zero

font - the preferred font to use to render the element

Throws:

IndexOutOfBoundsException - if elementNum is invalid

Since:

MIDP 2.0

See Also:

getFont(int)

getFont

public Font getFont(int elementNum)

Gets the application's preferred font for rendering the specified element of this Choice. The value returned is the font that had been set by the application, even if that value had been disregarded by the implementation. If no font had been set by the application, or if the application explicitly set the font to null, the value is the default font chosen by the implementation.

The elementNum parameter must be within the range [0..size()-1], inclusive.

Specified by:

getFont in interface Choice

Parameters:

elementNum - the index of the element, starting from zero

Returns:

the preferred font to use to render the element

Throws:

IndexOutOfBoundsException - if elementNum is invalid

Since:

MIDP 2.0

See Also:

setFont(int elementNum, Font font)