Use is subject to License Terms. Your use of this web site or any of its contents or software indicates your agreement to be bound by these License Terms.

Copyright © 2006 Sun Microsystems, Inc. and Motorola, Inc. All rights reserved.

MID Profile

Package javax.microedition.lcdui

The UI API provides a set of features for implementation of user interfaces for MIDP applications.


Interface Summary
Choice Choice defines an API for a user interface components implementing selection from predefined number of choices.
CommandListener This interface is used by applications which need to receive high-level events from the implementation.
ItemCommandListener A listener type for receiving notification of commands that have been invoked on Item objects.
ItemStateListener This interface is used by applications which need to receive events that indicate changes in the internal state of the interactive items within a Form screen.

Class Summary
Alert 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.
AlertType The AlertType provides an indication of the nature of alerts.
Canvas The Canvas class is a base class for writing applications that need to handle low-level events and to issue graphics calls for drawing to the display.
ChoiceGroup A ChoiceGroup is a group of selectable elements intended to be placed within a Form.
Command The Command class is a construct that encapsulates the semantic information of an action.
CustomItem A CustomItem is customizable by subclassing to introduce new visual and interactive elements into Forms.
DateField A DateField is an editable component for presenting date and time (calendar) information that may be placed into a Form.
Display Display represents the manager of the display and input devices of the system.
Displayable An object that has the capability of being placed on the display.
Font The Font class represents fonts and font metrics.
Form A Form is a Screen that contains an arbitrary mixture of items: images, read-only text fields, editable text fields, editable date fields, gauges, choice groups, and custom items.
Gauge Implements a graphical display, such as a bar graph, of an integer value.
Graphics Provides simple 2D geometric rendering capability.
Image The Image class is used to hold graphical image data.
ImageItem An item that can contain an image.
Item A superclass for components that can be added to a Form.
List A Screen containing list of choices.
Screen The common superclass of all high-level user interface classes.
Spacer A blank, non-interactive item that has a settable minimum size.
StringItem An item that can contain a string.
TextBox The TextBox class is a Screen that allows the user to enter and edit text.
TextField A TextField is an editable text component that may be placed into a Form.
Ticker Implements a "ticker-tape", a piece of text that runs continuously across the display.

Package javax.microedition.lcdui Description

The UI API provides a set of features for implementation of user interfaces for MIDP applications.

User Interface

The main criteria for the MIDP have been drafted with mobile information devices in mind (i.e., mobile phones and pagers). These devices differ from desktop systems in many ways, especially how the user interacts with them. The following UI-related requirements are important when designing the user interface API:

Given the capabilities of devices that will implement the MIDP and the above requirements, the MIDPEG decided not to simply subset the existing Java UI, which is the Abstract Windowing Toolkit (AWT). Reasons for this decision include:

Structure of the MIDP UI API

The MIDP UI is logically composed of two APIs: the high-level and the low-level.

The high-level API is designed for business applications whose client parts run on MIDs. For these applications, portability across devices is important. To achieve this portability, the high-level API employs a high level of abstraction and provides very little control over look and feel. This abstraction is further manifested in the following ways:

In other words, when using the high-level API, it is assumed that the underlying implementation will do the necessary adaptation to the device's hardware and native UI style. The classes that provide the high-level API are the subclasses of Screen.

The low-level API, on the other hand, provides very little abstraction. This API is designed for applications that need precise placement and control of graphic elements, as well as access to low-level input events. Some applications also need to access special, device-specific features. A typical example of such an application would be a game.

Using the low-level API, an application can:

The classes that provide the low-level API are Canvas and Graphics.

Applications that program to the low-level API are not guaranteed to be portable, since the low-level API provides the means to access details that are specific to a particular device. If the application does not use these features, it will be portable. It is recommended that applications use only the platform-independent part of the low-level API whenever possible. This means that the applications should not directly assume the existence of any keys other than those defined in the Canvas class, and they should not depend on a specific screen size. Rather, the application game-key event mapping mechanism should be used instead of concrete keys, and the application should inquire about the size of the display and adjust itself accordingly.

Class Hierarchy

The central abstraction of the MIDP's UI is a Displayable object, which encapsulates device-specific graphics rendering with user input. Only one Displayable may be visible at a time, and and the user can see and interact with only contents of that Displayable.

The Screen class is a subclass of Displayable that takes care of all user interaction with high-level user interface component. The Screen subclasses handle rendering, interaction, traversal, and scrolling, with only higher-level events being passed on to the application.

The rationale behind this design is based on the different display and input solutions found in MIDP devices. These differences imply that the component layout, scrolling, and focus traversal will be implemented differently on different devices. If an application were required to be aware of these issues, portability would be compromised. Simple screenfuls also organize the user interface into manageable pieces, resulting in user interfaces that are easy to use and learn.

There are three categories of Displayable objects:

Each Displayable can have a title, a Ticker and a set of Commands attached to it.

The class Display acts as the display manager that is instantiated for each active MIDlet and provides methods to retrieve information about the device's display capabilities. A Displayable is made visible by calling the setCurrent() method of Display. When a Displayable is made current, it replaces the previous Displayable.

Class Overview

It is anticipated that most applications will utilize screens with predefined structures like List , TextBox , and Alert . These classes are used in the following ways:

A special class Form is defined for cases where screens with a predefined structure are not sufficient. For example, an application may have two TextFields, or a TextField and a simple ChoiceGroup . Although this class (Form ) allows creation of arbitrary combinations of components, developers should keep the limited display size in mind and create only simple Forms .

Form is designed to contain a small number of closely related UI elements. These elements are the subclasses of Item: ImageItem, StringItem, TextField, ChoiceGroup, Gauge, and CustomItem. The classes ImageItem and StringItem are convenience classes that make certain operations with Form and Alert easier. By subclassing CustomItem application developers can introduce Items with a new visual representation and interactive elements. If the components do not all fit on the screen, the implementation may either make the form scrollable or implement some components so that they can either popup in a new screen or expand when the user edits the element.

Interplay with Application Manager

The user interface, like any other resource in the API, is to be controlled according to the principle of MIDP application management. The UI expects the following conditions from the application management software:

The application manager assumes that the application behaves as follows with respect to the MIDlet events:

Event Handling

User interaction causes events, and the implementation notifies the application of the events by making corresponding callbacks. There are four kinds of UI callbacks:

All UI callbacks are serialized, so they will never occur in parallel. That is, the implementation will never call an callback before a prior call to any other callback has returned. This property enables applications to be assured that processing of a previous user event will have completed before the next event is delivered. If multiple UI callbacks are pending, the next is called as soon as possible after the previous UI callback returns. The implementation also guarantees that the call to run() requested by a call to callSerially() is made after any pending repaint requests have been satisfied.

There is one exception to the callback serialization rule, which occurs when the Canvas.serviceRepaints method is called. This method causes the the Canvas.paint method to be called and waits for it to complete. This occurs even if the caller of serviceRepaints is itself within an active callback. There is further discussion of this issue below.

The following callbacks are all serialized with respect to each other:

Note that Timer events are not considered UI events. Timer callbacks may run concurrently with UI event callbacks, although TimerTask callbacks scheduled on the same Timer are serialized with each other. Applications that use timers must guard their data structures against concurrent access from timer threads and UI event callbacks. Alternatively, applications may have their timer callbacks use Display.callSerially so that work triggered by timer events can be serialized with the UI event callbacks.

Abstract Commands

Since MIDP UI is highly abstract, it does not dictate any concrete user interaction technique like soft buttons or menus. Also, low-level user interactions such as traversal or scrolling are not visible to the application. MIDP applications define Commands , and the implementation may manifest these via either soft buttons, menus, or whatever mechanisms are appropriate for that device.

Commands are installed to a Displayable (Canvas or Screen ) with a method addCommand of class Displayable .

The native style of the device may assume that certain types of commands are placed on standard places. For example, the "go-back" operation may always be mapped to the right soft button. The Command class allows the application to communicate such a semantic meaning to the implementation so that these standard mappings can be effected.

The implementation does not actually implement any of the semantics of the Command. The attributes of a Command are used only for mapping it onto the user interface. The actual semantics of a Command are always implemented by the application in a CommandListener.

Command objects have attributes:

Device-Provided Operations

In many high-level UI classes there are also some additional operations available in the user interface. The additional operations are not visible to applications, only to the end-user. The set of operations available depends totally on the user interface design of the specific device. For example, an operation that allows the user to change the mode for text input between alphabetic and numeric is needed in devices that have only an ITU-T keypad. More complex input systems will require additional operations. Some of operations available are presented in the user interface in the same way the application-defined commands are. End-users need not understand which operations are provided by the application and which provided by the system. Not all operations are available in every implementation. For example, a system that has a word-lookup-based text input scheme will generally provide additional operations within the TextBox class. A system that lacks such an input scheme will also lack the corresponding operations.

Some operations are available on all devices, but the way the operation is implemented may differ greatly from device to device. Examples of this kind of operation are: the mechanism used to navigate between List elements and Form items, the selection of List elements, moving an insertion position within a text editor, and so forth. Some devices do not allow the direct editing of the value of an Item, but instead require the user to switch to an off-screen editor. In such devices, there must be a dedicated selection operation that can be used to invoke the off-screen editor. The selection of a List elements could be, for example, implemented with a dedicated "Go" or "Select" or some other similar key. Some devices have no dedicated selection key and must select elements using some other means.

On devices where the selection operation is performed using a dedicated select key, this key will often not have a label displayed for it. It is appropriate for the implementation to use this key in situations where its meaning is obvious. For example, if the user is presented with a set of mutually exclusive options, the selection key will obviously select one of those options. However, in a device that doesn't have a dedicated select key, it is likely that the selection operation will be performed using a soft key that requires a label. The ability to set the select-command for a List of type IMPLICIT and the ability to set the default command for an Item are provided so that the application can set the label for this operation and so it can receive notification when this operation occurs.

High-Level API for Events

The handling of events in the high-level API is based on a listener model. Screens and Canvases may have listeners for commands. An object willing to be a listener should implement an interface CommandListener that has one method:

    void commandAction(Command c, Displayable d);    

The application gets these events if the Screen or Canvas has attached Commands and if there is a registered listener. A unicast-version of the listener model is adopted, so the Screen or Canvas can have one listener at a time.

There is also a listener interface for state changes of the Items in a Form . The method

    void itemStateChanged(Item item);    

defined in interface ItemStateListener is called when the value of an interactive Gauge , ChoiceGroup , or TextField changes. It is not expected that the listener will be called after every change. However, if the value of an Item has been changed, the listener will be called for the change sometime before it is called for another item or before a command is delivered to the Form's CommandListener. It is suggested that the change listener is called at least after focus (or equivalent) is lost from field. The listener should only be called if the field's value has actually changed.

Low-Level API for Events

Low-level graphics and events have the following methods to handle low-level key events:

    public void keyPressed(int keyCode);    
    public void keyReleased(int keyCode);    
    public void keyRepeated(int keyCode); 

The last call, keyRepeated , is not necessarily available in all devices. The applications can check the availability of repeat actions by calling the following method of the Canvas :

    public static boolean hasRepeatEvents();     

The API requires that there be standard key codes for the ITU-T keypad (0-9, *, #), but no keypad layout is required by the API. Although an implementation may provide additional keys, applications relying on these keys are not portable.

In addition, the class Canvas has methods for handling abstract game events. An implementation maps all these key events to suitable keys on the device. For example, a device with four-way navigation and a select key in the middle could use those keys, but a simpler device may use certain keys on the numeric keypad (e.g., 2, 4, 5, 6, 8). These game events allow development of portable applications that use the low-level events. The API defines a set of abstract key-events: UP, DOWN, LEFT, RIGHT, FIRE, GAME_A, GAME_B, GAME_C, and GAME_D.

An application can get the mapping of the key events to abstract key events by calling:

    public static int getGameAction(int keyCode);    

If the logic of the application is based on the values returned by this method, the application is portable and run regardless of the keypad design.

It is also possible to map an abstract event to a key with:

    public static int getKeyCode(int gameAction);    

where gameAction is UP,DOWN, LEFT, RIGHT, FIRE, etc. On some devices, more than one key is mapped to the same game action, in which case the getKeyCode method will return just one of them. Properly-written applications should map the key code to an abstract key event and make decisions based on the result.

The mapping between keys and abstract events does not change during the execution of the game.

The following is an example of how an application can use game actions to interpret keystrokes.

    class MovingBlocksCanvas extends Canvas {
        public void keyPressed(int keyCode) {
            int action = getGameAction(keyCode);    
            switch (action) {
            case LEFT:
            case RIGHT:

The low-level API also has support for pointer events, but since the following input mechanisms may not be present in all devices, the following callback methods may never be called in some devices:

    public void pointerPressed(int x, int y);
    public void pointerReleased(int x, int y);
    public void pointerDragged(int x, int y);    

The application may check whether the pointer is available by calling the following methods of class Canvas :

    public static boolean hasPointerEvents();
    public static boolean hasPointerMotionEvents();    

Interplay of High-Level Commands and the Low-Level API

The class Canvas , which is used for low-level events and drawing, is a subclass of Displayable , and applications can attach Commands to it. This is useful for jumping to an options setup Screen in the middle of a game. Another example could be a map-based navigation application where keys are used for moving in the map but commands are used for higher-level actions.

Some devices may not have the means to invoke commands when Canvas and the low-level event mechanism are in use. In that case, the implementation may provide a means to switch to a command mode and back. This command mode might pop up a menu over the contents of the Canvas. In this case, the Canvas methods hideNotify() and showNotify() will be called to indicate when the Canvas has been obscured and unobscured, respectively.

The Canvas may have a title and a Ticker like the Screen objects. However, Canvas also has a full-screen mode where the title and the Ticker are not displayed. Setting this mode indicates that the application wishes for the Canvas to occupy as much of the physical display as is possible. In this mode, the title may be reused by the implementation as the title for pop-up menus. In normal (not full-screen) mode, the appearance of the Canvas should be similar to that of Screen classes, so that visual continuity is retained when the application switches between low-level Canvas objects and high-level Screen objects.

Graphics and Text in Low-Level API

The Redrawing Scheme

Repainting is done automatically for all Screens , but not for Canvas ; therefore, developers utilizing the low-level API must ; understand its repainting scheme.

In the low-level API, repainting of Canvas is done asynchronously so that several repaint requests may be implemented within a single call as an optimization. This means that the application requests the repainting by calling the method repaint() of class Canvas . The actual drawing is done in the method paint() -- which is provided by the subclass Canvas -- and does not necessarily happen synchronously to repaint() . It may happen later, and several repaint requests may cause one single call to paint() . The application can flush the repaint requests by calling serviceRepaints() .

As an example, assume that an application moves a box of width wid and height ht from coordinates (x1,y1 ) to coordinates (x2,y2 ), where x2>x1 and y2>y1 :

    // move coordinates of box
    box.x = x2;
    box.y = y2;
    // ensure old region repainted (with background)    
    canvas.repaint(x1,y1, wid, ht);
    // make new region repainted
    canvas.repaint(x2,y2, wid, ht);
    // make everything really repainted

The last call causes the repaint thread to be scheduled. The repaint thread finds the two requests from the event queue and repaints the region that is a union of the repaint area:

    graphics.clipRect(x1,y1, (x2-x1+wid), (y2-y1+ht));      

In this imaginary part of an implementation, the call canvas.paint() causes the application-defined paint() method to be called.

Drawing Model

The primary drawing operation is pixel replacement, which is used for geometric rendering operations such as lines and rectangles. With offscreen images, support for full transparency is required, and support for partial transparency (alpha blending) is optional.

A 24-bit color model is provided with 8 bits each for the red, green, and blue components of a color. Not all devices support 24-bit color, so they will map colors requested by the application into colors available on the device. Facilities are provided in the Display class for obtaining device characteristics, such as whether color is available and how many distinct gray levels are available. This enables applications to adapt their behavior to a device without compromising device independence.

Graphics may be rendered either directly to the display or to an off-screen image buffer. The destination of rendered graphics depends on the origin of the graphics object. A graphics object for rendering to the display is passed to the Canvas object's paint() method. This is the only way to obtain a graphics object whose destination is the display. Furthermore, applications may draw by using this graphics object only for the duration of the paint() method.

A graphics object for rendering to an off-screen image buffer may be obtained by calling the getGraphics() method on the desired image. These graphics objects may be held indefinitely by the application, and requests may be issued on these graphics objects at any time.

The Graphics class has a current color that is set with the setColor() method. All geometric rendering, including lines, rectangles, and arcs, uses the current color. The pixel representing the current color replaces the destination pixel in these operations. There is no background color. Painting of any background be performed explicitly by the application using the setColor() and rendering calls.

Support for full transparency is required, and support for partial transparency (alpha blending) is optional. Transparency (both full and partial) exists only in off-screen images loaded from PNG files or from arrays of ARGB data. Images created in such a fashion are immutable in that the application is precluded from making any changes to the pixel data contained within the image. Rendering is defined in such a way that the destination of any rendering operation always consists entirely of fully opaque pixels.

Coordinate System

The origin (0,0) of the available drawing area and images is in the upper-left corner of the display. The numeric values of the x-coordinates monotonically increase from left to right, and the numeric values of the y-coordinates monotonically increase from top to bottom. Applications may assume that horizontal and vertical distances in the coordinate system represent equal distances on the actual device display. If the shape of the pixels of the device is significantly different from square, the implementation of the UI will do the required coordinate transformation. A facility is provided for translating the origin of the coordinate system. All coordinates are specified as integers.

The coordinate system represents locations between pixels, not the pixels themselves. Therefore, the first pixel in the upper left corner of the display lies in the square bounded by coordinates (0,0), (1,0), (0,1), (1,1).

An application may inquire about the available drawing area by calling the following methods of Canvas :

    public static final int getWidth();
    public static final int getHeight();    

Font Support

An application may request one of the font attributes specified below. However, the underlying implementation may use a subset of what is specified. So it is up to the implementation to return a font that most closely resembles the requested font.

Each font in the system is implemented individually. A programmer will call the static getFont() method instead of instantiating new Font objects. This paradigm eliminates the garbage creation normally associated with the use of fonts.

The Font class provides calls that access font metrics. The following attributes may be used to request a font (from the Font class):


The UI API has been designed to be thread-safe. The methods may be called from callbacks, TimerTasks, or other threads created by the application. Also, the implementation generally does not hold any locks on objects visible to the application. This means that the applications' threads can synchronize with themselves and with the event callbacks by locking any object according to a synchronization policy defined by the application. One exception to this rule occurs with the Canvas.serviceRepaints method. This method calls and awaits completion of the paint method. Strictly speaking, serviceRepaints might not call paint directly, but instead it might cause another thread to call paint. In either case, serviceRepaints blocks until paint has returned. This is a significant point because of the following case. Suppose the caller of serviceRepaints holds a lock that is also needed by the paint method. Since paint might be called from another thread, that thread will block trying to acquire the lock. However, this lock is held by the caller of serviceRepaints, which is blocked waiting for paint to return. The result is deadlock. In order to avoid deadlock, the caller of serviceRepaints must not hold any locks needed by the paint method.

The UI API includes also a mechanism similar to other UI toolkits for serializing actions with the event stream. The method Display.callSerially requests that the run method of a Runnable object be called, serialized with the event stream. Code that uses serviceRepaints() can usually be rewritten to use callSerially(). The following code illustrates this technique:

    class MyCanvas extends Canvas {    
        void doStuff() {
            // <code fragment 1>    
            // <code fragment 2>    

The following code is an alternative way of implementing the same functionality:

    class MyClass extends Canvas 
       implements Runnable {            
        void doStuff() {
            // <code fragment 1>

        // called only after all pending repaints served    
        public void run() {
            // <code fragment 2>;

Implementation Notes

The implementation of a List or ChoiceGroup may include keyboard shortcuts for focusing and selecting the choice elements, but the use of these shortcuts is not visible to the application program.

In some implementations the UI components -- Screens and Items -- will be based on native components. It is up to the implementation to free the used resources when the Java objects are not needed anymore. One possible implementation scenario is a hook in the garbage collector of KVM.

MIDP 1.0

MID Profile

Copyright © 2006 Sun Microsystems, Inc. and Motorola, Inc. All rights reserved. Use is subject to License Terms. Your use of this web site or any of its content or software indicates your agreement to be bound by these License Terms.

For more information, please consult the JSR 118 specification.