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MID Profile: Package javax.microedition.lcdui
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<H2>
Package javax.microedition.lcdui
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The UI API provides a set of features for implementation of user interfaces for MIDP applications.
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<B>See: </B>
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<A HREF="#package_description"><B>Description</B></A>
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<B>Interface Summary</B></FONT></TD>
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<TD WIDTH="15%"><B><A HREF="Choice.html"><I>Choice</I></A></B></TD>
<TD>Choice defines an API for a user interface components implementing selection from predefined number of choices.</TD>
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<TD WIDTH="15%"><B><A HREF="CommandListener.html"><I>CommandListener</I></A></B></TD>
<TD>This interface is used by applications which need to receive high-level events from the implementation.</TD>
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<TD WIDTH="15%"><B><A HREF="ItemCommandListener.html"><I>ItemCommandListener</I></A></B></TD>
<TD>A listener type for receiving notification of commands that have been invoked on <A HREF="../../../javax/microedition/lcdui/Item.html"><CODE>Item</CODE></A> objects.</TD>
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<TD WIDTH="15%"><B><A HREF="ItemStateListener.html"><I>ItemStateListener</I></A></B></TD>
<TD>This interface is used by applications which need to receive events that indicate changes in the internal state of the interactive items within a <A HREF="../../../javax/microedition/lcdui/Form.html"><CODE>Form</CODE></A> screen.</TD>
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<B>Class Summary</B></FONT></TD>
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<TD WIDTH="15%"><B><A HREF="Alert.html">Alert</A></B></TD>
<TD>An alert is a screen that shows data to the user and waits for a certain period of time before proceeding to the next <code>Displayable</code>.</TD>
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<TD WIDTH="15%"><B><A HREF="AlertType.html">AlertType</A></B></TD>
<TD>The <code>AlertType</code> provides an indication of the nature of alerts.</TD>
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<TD WIDTH="15%"><B><A HREF="Canvas.html">Canvas</A></B></TD>
<TD>The <code>Canvas</code> 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.</TD>
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<TD WIDTH="15%"><B><A HREF="ChoiceGroup.html">ChoiceGroup</A></B></TD>
<TD>A <code>ChoiceGroup</code> is a group of selectable elements intended to be placed within a <A HREF="../../../javax/microedition/lcdui/Form.html"><CODE>Form</CODE></A>.</TD>
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<TD WIDTH="15%"><B><A HREF="Command.html">Command</A></B></TD>
<TD>The <code>Command</code> class is a construct that encapsulates the semantic information of an action.</TD>
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<TD WIDTH="15%"><B><A HREF="CustomItem.html">CustomItem</A></B></TD>
<TD>A CustomItem is customizable by subclassing to introduce new visual and interactive elements into <code>Forms</code>.</TD>
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<TD WIDTH="15%"><B><A HREF="DateField.html">DateField</A></B></TD>
<TD>A <code>DateField</code> is an editable component for presenting date and time (calendar) information that may be placed into a <code>Form</code>.</TD>
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<TD WIDTH="15%"><B><A HREF="Display.html">Display</A></B></TD>
<TD><code>Display</code> represents the manager of the display and input devices of the system.</TD>
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<TD WIDTH="15%"><B><A HREF="Displayable.html">Displayable</A></B></TD>
<TD>An object that has the capability of being placed on the display.</TD>
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<TD WIDTH="15%"><B><A HREF="Font.html">Font</A></B></TD>
<TD>The <code>Font</code> class represents fonts and font metrics.</TD>
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<TD WIDTH="15%"><B><A HREF="Form.html">Form</A></B></TD>
<TD>A <code>Form</code> is a <code>Screen</code> that contains an arbitrary mixture of items: images, read-only text fields, editable text fields, editable date fields, gauges, choice groups, and custom items.</TD>
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<TD WIDTH="15%"><B><A HREF="Gauge.html">Gauge</A></B></TD>
<TD>Implements a graphical display, such as a bar graph, of an integer value.</TD>
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<TD WIDTH="15%"><B><A HREF="Graphics.html">Graphics</A></B></TD>
<TD>Provides simple 2D geometric rendering capability.</TD>
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<TD WIDTH="15%"><B><A HREF="Image.html">Image</A></B></TD>
<TD>The <code>Image</code> class is used to hold graphical image data.</TD>
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<TD WIDTH="15%"><B><A HREF="ImageItem.html">ImageItem</A></B></TD>
<TD>An item that can contain an image.</TD>
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<TD WIDTH="15%"><B><A HREF="Item.html">Item</A></B></TD>
<TD>A superclass for components that can be added to a <A HREF="../../../javax/microedition/lcdui/Form.html"><CODE>Form</CODE></A>.</TD>
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<TD WIDTH="15%"><B><A HREF="List.html">List</A></B></TD>
<TD>A <code>Screen</code> containing list of choices.</TD>
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<TD WIDTH="15%"><B><A HREF="Screen.html">Screen</A></B></TD>
<TD>The common superclass of all high-level user interface classes.</TD>
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<TD WIDTH="15%"><B><A HREF="Spacer.html">Spacer</A></B></TD>
<TD>A blank, non-interactive item that has a settable minimum size.</TD>
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<TD WIDTH="15%"><B><A HREF="StringItem.html">StringItem</A></B></TD>
<TD>An item that can contain a string.</TD>
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<TD WIDTH="15%"><B><A HREF="TextBox.html">TextBox</A></B></TD>
<TD>The <code>TextBox</code> class is a <code>Screen</code> that allows the user to enter and edit text.</TD>
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<TD WIDTH="15%"><B><A HREF="TextField.html">TextField</A></B></TD>
<TD>A <code>TextField</code> is an editable text component that may be placed into a <A HREF="../../../javax/microedition/lcdui/Form.html"><CODE>Form</CODE></A>.</TD>
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<TR BGCOLOR="white" CLASS="TableRowColor">
<TD WIDTH="15%"><B><A HREF="Ticker.html">Ticker</A></B></TD>
<TD>Implements a "ticker-tape", a piece of text that runs continuously across the display.</TD>
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</TABLE>
<P>
<A NAME="package_description"><!-- --></A><H2>
Package javax.microedition.lcdui Description
</H2>
<P>
The UI API provides a set of features for implementation of user interfaces for MIDP applications. <H2>User Interface</H2> <P> 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:</P> <UL> <LI> The devices and applications should be useful to users who are not necessarily experts in using computers. </LI> <LI> The devices and applications should be useful in situations where the user cannot pay full attention to the application. For example, many phone-type devices will be operated with one hand.</LI> <LI> The form factors and UI concepts of the device differ between devices, especially from desktop systems. For example, the display sizes are smaller, and the input devices do not always include pointing devices.</LI> <LI> The applications run on MIDs should have UIs that are compatible to the native applications so that the user finds them easy to use.</LI> </UL> <P> 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:</P> <UL> <LI> Although AWT was designed for desktop computers and optimized to these devices, it also suffers from assumptions based on this heritage.</LI> <LI> When a user interacts with AWT, event objects are created dynamically. These objects are short-lived and exist only until each associated event is processed by the system. At this point, the event object becomes garbage and must be reclaimed by the system's garbage collector. The limited CPU and memory subsystems of a MID typically cannot handle this behavior.</LI> <LI> AWT has a rich but desktop-based feature set. This feature set includes support for features not found on MIDs. For example, AWT has extensive support for window management (e.g., overlapping windows, window resize, etc.). MIDs have small displays which are not large enough to display multiple overlapping windows. The limited display size also makes resizing a window impractical. As such, the windowing and layout manager support within AWT is not required for MIDs.</LI> <LI> AWT assumes certain user interaction models. The component set of AWT was designed to work with a pointer device (e.g., a mouse or pen input). As mentioned earlier, this assumption is valid for only a small subset of MIDs since many of these devices have only a keypad for user input.</LI> </UL> <H3>Structure of the MIDP UI API</H3> <P> The MIDP UI is logically composed of two APIs: the high-level and the low-level.</P> <P> 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:</P> <UL> <LI> The actual drawing to the MID's display is performed by the implementation. Applications do not define the visual appearance (e.g., shape, color, font, etc.) of the components.</LI> <LI> Navigation, scrolling, and other primitive interaction is encapsulated by the implementation, and the application is not aware of these interactions.</LI> <LI> Applications cannot access concrete input devices like specific individual keys. </LI> </UL> <P> 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 <A HREF="../../../javax/microedition/lcdui/Screen.html"><CODE>Screen</CODE></A>.</P> <P> 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⌨️ 快捷键说明
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