javax.microedition.lcdui
Class Alert

java.lang.Object
  |
  +--javax.microedition.lcdui.Displayable
        |
        +--javax.microedition.lcdui.Screen
              |
              +--javax.microedition.lcdui.Alert

public class Alert
extends Screen

An alert is a screen that shows data to the user and waits for a certain period of time before proceeding to the next Displayable. An alert can contain a text string and an image. The intended use of Alert is to inform the user about errors and other exceptional conditions.

The application can set the alert time to be infinity with setTimeout(Alert.FOREVER) in which case the Alert is considered to be modal and the implementation provide a feature that allows the user to "dismiss" the alert, whereupon the next Displayable is displayed as if the timeout had expired immediately.

If an application specifies an alert to be of a timed variety and gives it too much content such that it must scroll, then it automatically becomes a modal alert.

An alert may have an AlertType associated with it to provide an indication of the nature of the alert. The implementation may use this type to play an appropriate sound when the Alert is presented to the user. See AlertType.playSound.

An alert may contain an optional Image. The Image may be mutable or immutable. If the Image is mutable, the effect is as if a snapshot of its contents is taken at the time the Alert is constructed with this Image and when setImage is called with an Image. This snapshot is used whenever the contents of the Alert are to be displayed. Even if the application subsequently draws into the Image, the snapshot is not modified until the next call to setImage. The snapshot is not updated when the Alert becomes current or becomes visible on the display. (This is because the application does not have control over exactly when Displayables appear and disappear from the display.) Gauge object that is used as an activity or progress indicator. By default, an Alert has no activity indicator; one may be set with the setIndicator(javax.microedition.lcdui.Gauge) method. The Gauge object used for the activity indicator must conform to all of the following restrictions:

It is an error for the application to attempt to use a Gauge object that violates any of these restrictions. In addition, when the Gauge object is being used as the indicator within an Alert, the application is prevented from modifying any of these pieces of the Gauge's state.

Commands and Listeners

Like the other Displayable classes, an Alert can accept Commands, which can be delivered to a CommandListener set by the application. The Alert class adds some special behavior for Commands and listeners.

When it is created, an Alert implicitly has the special Command DISMISS_COMMAND present on it. If the application adds any other Commands to the Alert, DISMISS_COMMAND is implicitly removed. If the application removes all other Commands, DISMISS_COMMAND is implicitly restored. Attempts to add or remove DISMISS_COMMAND explicitly are ignored. Thus, there is always at least one Command present on an Alert.

If there are two or more Commands present on the Alert, it is automatically turned into a modal Alert, and the timeout value is always FOREVER. The Alert remains on the display until a Command is invoked. If the Alert has one Command (whether it is DISMISS_COMMAND or it is one provided by the application), the Alert may have the timed behavior as described above. When a timeout occurs, the effect is the same as if the user had invoked the Command explicitly.

When it is created, an Alert implicitly has a CommandListener called the default listener associated with it. This listener may be replaced by an application-provided listener through use of the setCommandListener(javax.microedition.lcdui.CommandListener) method. If the application removes its listener by passing null to the setCommandListener method, the default listener is implicitly restored.

The Display.setCurrent(Alert, Displayable) method and the Display.setCurrent(Displayable) method (when called with an Alert) define special behavior for automatically advancing to another Displayable after the Alert is dismissed. This special behavior occurs only when the default listener is present on the Alert at the time it is dismissed or when a command is invoked. If the user invokes a Command and the default listener is present, the default listener ignores the Command and implements the automatic-advance behavior.

If the application has set its own CommandListener, the automatic- advance behavior is disabled. The listener code is responsible for advancing to another Displayable. When the application has provided a listener, Commands are invoked normally by passing them to the listener's commandAction method. The Command passed will be one of the Commands present on the Alert: either DISMISS_COMMAND or one of the application- provided Commands.

The application can restore the default listener by passing null to the setCommandListener method.

Note: An application may set a Ticker with Displayable.setTicker on an Alert, however it may not be displayed due to implementation restrictions.

Since:
MIDP 1.0

See Also:
AlertType

Field Summary
static Command DISMISS_COMMAND
          A Command delivered to a listener to indicate that the Alert has been dismissed.
static int FOREVER
          FOREVER indicates that an Alert is kept visible until the user dismisses it.
 
Constructor Summary
Alert(String title)
          Constructs a new, empty Alert object with the given title.
Alert(String title, String alertText, Image alertImage, AlertType alertType)
          Constructs a new Alert object with the given title, content string and image, and alert type.
 
Method Summary
 void addCommand(Command cmd)
          Commands are not allowed on Alerts, so this method will always throw IllegalStateException whenever it is called.
 int getDefaultTimeout()
          Gets the default time for showing an Alert.
 Image getImage()
          Gets the Image used in the Alert.
 Gauge getIndicator()
          Gets the activity indicator for this Alert.
 String getString()
          Gets the text string used in the Alert.
 int getTimeout()
          Gets the time this Alert will be shown.
 String getTitle()
          Retrieve the title of the Alert
 AlertType getType()
          Gets the type of the Alert.
 void removeCommand(Command cmd)
          Similar to Displayable.removeCommand(javax.microedition.lcdui.Command), however when the application removes the last command from an Alert, DISMISS_COMMAND is implicitly added.
 void setCommandListener(CommandListener l)
          Listeners are not allowed on Alerts, so this method will always throw IllegalStateException whenever it is called.
 void setImage(Image img)
           Sets the Image used in the Alert.
 void setIndicator(Gauge indicator)
          Sets an activity indicator on this Alert.
 void setString(String str)
           Sets the text string used in the Alert.
 void setTimeout(int time)
          Set the time for which the Alert is to be shown.
 void setTitle(String title)
          Set the title of the Alert
 void setType(AlertType type)
          Sets the type of the Alert.
 
Methods inherited from class javax.microedition.lcdui.Displayable
getHeight, getTicker, getWidth, isShown, setTicker, sizeChanged
 
Methods inherited from class java.lang.Object
equals, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait
 

Field Detail

DISMISS_COMMAND

public static final Command DISMISS_COMMAND
A Command delivered to a listener to indicate that the Alert has been dismissed. This Command is implicitly present an on Alert whenever there are no other Commands present. The field values of DISMISS_COMMAND are as follows:
  • label = "" (an empty string)
  • type = Command.OK
  • priority = 0

The label value visible to the application must be as specified above. However, the implementation may display DISMISS_COMMAND to the user using an implementation-specific label.

Attempting to add or remove DISMISS_COMMAND from an Alert has no effect. However, DISMISS_COMMAND is treated as an ordinary Command if it is used with other Displayable types.

Since:
MIDP 2.0

FOREVER

public static final int FOREVER

FOREVER indicates that an Alert is kept visible until the user dismisses it. It is used as a value for the parameter to setTimeout() to indicate that the alert is modal. Instead of waiting for a specified period of time, a modal Alert will wait for the user to take some explicit action, such as pressing a button, before proceeding to the next screen.

Value -2 is assigned to FOREVER.

Constructor Detail

Alert

public Alert(String title)

Constructs a new, empty Alert object with the given title. If null is passed, the Alert will have no title. Calling this constructor is equivalent to calling

    Alert(title, null, null, null)
 
Parameters:
title - the title string, or null
See Also:
Alert(String, String, Image, AlertType)

Alert

public Alert(String title,
             String alertText,
             Image alertImage,
             AlertType alertType)

Constructs a new Alert object with the given title, content string and image, and alert type. The layout of the contents is implementation dependent. The timeout value of this new alert is the same value that is returned by getDefaultTimeout(). If an image is provided it must be immutable. The handling and behavior of specific AlertTypes is described in AlertType. Null is allowed as the value of the alertType parameter and indicates that the Alert is not to have a specific alert type. DISMISS_COMMAND is the only Command present on the new Alert. The CommandListener associated with the new Alert is the default listener. Its behavior is described in more detail in the section Commands and Listeners.

Parameters:
title - the title string, or null if there is no title
alertText - the string contents, or null if there is no string
alertImage - the image contents, or null if there is no image
alertType - the type of the Alert, or null if the Alert has no specific type
Throws:
IllegalArgumentException - if the image is mutable
Method Detail

addCommand

public void addCommand(Command cmd)
Commands are not allowed on Alerts, so this method will always throw IllegalStateException whenever it is called.
Overrides:
addCommand in class Displayable
Parameters:
cmd - the Command
Throws:
IllegalStateException - always

getDefaultTimeout

public int getDefaultTimeout()

Gets the default time for showing an Alert. This is either a positive value, which indicates a time in milliseconds, or the special value FOREVER, which indicates that Alerts are modal by default. The value returned will vary across implementations and is presumably tailored to be suitable for each.

Returns:
default timeout in milliseconds, or FOREVER

getImage

public Image getImage()
Gets the Image used in the Alert.
Returns:
the Alert's image, or null if there is no image

getIndicator

public Gauge getIndicator()
Gets the activity indicator for this Alert.
Returns:
a reference to this Alert's activity indicator, or null if there is none
Since:
MIDP 2.0
See Also:
setIndicator(javax.microedition.lcdui.Gauge)

getString

public String getString()
Gets the text string used in the Alert.
Returns:
the Alert's text string, or null if there is no text

getTimeout

public int getTimeout()

Gets the time this Alert will be shown. This is either a positive value, which indicates a time in milliseconds, or the special value FOREVER, which indicates that this Alert is modal.

Returns:
timeout in milliseconds, or FOREVER

getTitle

public String getTitle()
Retrieve the title of the Alert
Overrides:
getTitle in class Displayable
Returns:
the Alert's title, or null if there is no title

getType

public AlertType getType()
Gets the type of the Alert.
Returns:
a reference to an instance of AlertType, or null if the Alert has no specific type

removeCommand

public void removeCommand(Command cmd)
Similar to Displayable.removeCommand(javax.microedition.lcdui.Command), however when the application removes the last command from an Alert, DISMISS_COMMAND is implicitly added. Calling this method with DISMISS_COMMAND as the parameter has no effect.
Overrides:
removeCommand in class Displayable
Parameters:
cmd - the command to be removed
Since:
JDE 4.0.0

setCommandListener

public void setCommandListener(CommandListener l)
Listeners are not allowed on Alerts, so this method will always throw IllegalStateException whenever it is called.
Overrides:
setCommandListener in class Displayable
Parameters:
l - the Listener
Throws:
IllegalStateException - always

setImage

public void setImage(Image img)

Sets the Image used in the Alert. The Image may be mutable or immutable. If img is null, specifies that this Alert has no image. If img is mutable, the effect is as if a snapshot is taken of img's contents immediately prior to the call to setImage. This snapshot is used whenever the contents of the Alert are to be displayed. If img is already the Image of this Alert, the effect is as if a new snapshot of img's contents is taken. Thus, after painting into a mutable image contained by an Alert, the application can call
alert.setImage(alert.getImage());
to refresh the Alert's snapshot of its Image.

If the Alert is visible on the display when its contents are updated through a call to setImage, the display will be updated with the new snapshot as soon as it is feasible for the implementation to do so.

Parameters:
img - the Alert's image, or null if there is no image
Throws:
IllegalArgumentException - if img is mutable

setIndicator

public void setIndicator(Gauge indicator)
Sets an activity indicator on this Alert. The activity indicator is a Gauge object. It must be in a restricted state in order for it to be used as the activity indicator for an Alert. The restrictions are listed above. If the Gauge object violates any of these restrictions, IllegalArgumentException is thrown.

If indicator is null, this removes any activity indicator present on this Alert.

Parameters:
indicator - the activity indicator for this Alert, or null if there is to be none
Throws:
IllegalArgumentException - if indicator does not meet the restrictions for its use in an Alert
Since:
MIDP 2.0
See Also:
getIndicator()

setString

public void setString(String str)

Sets the text string used in the Alert.

If the Alert is visible on the display when its contents are updated through a call to setString, the display will be updated with the new contents as soon as it is feasible for the implementation to do so.

Parameters:
str - the Alert's text string, or null if there is no text

setTimeout

public void setTimeout(int time)

Set the time for which the Alert is to be shown. This must either be a positive time value in milliseconds, or the special value FOREVER.

Parameters:
time - timeout in milliseconds, or FOREVER
Throws:
IllegalArgumentException - if time is not positive and is not FOREVER

setTitle

public void setTitle(String title)
Set the title of the Alert
Overrides:
setTitle in class Displayable
Parameters:
title - the title string, or null if there is no title

setType

public void setType(AlertType type)
Sets the type of the Alert. The handling and behavior of specific AlertTypes is described in AlertType.
Parameters:
type - an AlertType, or null if the Alert has no specific type



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