chap01.html

Inside the java virtualMachine,深入研究java虚拟机

<P>Another area in which much work is being done is user interface. One of the tradeoffs listed in this chapter for writing platform-independent programs is the lowest-common-denominator problem. A major area in which this problem reveals itself is user interfaces. In an effort to provide a user-interface library that could map to native components on most platforms, Sun filled the AWT library in Java 1.0 and 1.1 with a lowest-common-denominator subset of components. The 1.0 AWT library included a button class, for example, because every platform had a native button. The library did not include more advanced components such as tab controls or spin controls, however, in part because of schedule constraints, but also because these kinds of controls weren韙 native to enough platforms.</P>
<P>The Java programmer was faced with an AWT library that directly supported the creation of rather simple user interfaces. With work, however, the programmer could build a fancier user interface on top of the AWT primitives. Many third party vendors built more advanced user interface libraries on top of AWT to help ease the programmer韘 burden. Microsoft韘 AFC (Application Foundation Classes) and Netscape韘 IFC (Internet Foundation Classes) are two good examples. These libraries add support for more advanced user-interface components and functionality to those directly supported by the AWT. They are, however, built on top of AWT, so programs that use them are still platform independent.</P>
<P>Sun has announced JFC (Java Foundation Classes), which is their approach to solving the lowest-common-denominator problem with the 1.0 and 1.1 AWT libraries. Rather than attempting to map more components to native counterparts, Sun韘 strategy is to provide what they call &quot;lightweight components.&quot; A lightweight component doesn韙 directly map to a native component. Instead, it is built out of the AWT primitives. So for example, instead of providing a tab control that maps to a native tab control on each platform that supports one, JFC would provide a &quot;Java Platform tab control&quot;. When such a control is used on Windows95, which supports tab controls natively, the control would not necessarily have the native Windows look and feel. It would have the Java Platform look and feel.</P>
<P>As Java user interface libraries evolve, they will reduce the pain of writing platform-independent user interfaces. Sun韘 lightweight component approach could enable the Java Platform to become more of a driving force in the evolution of user-interface. Rather than just trying to catch up with the user interfaces available on native platforms, Sun can develop the user interface of the Java Platform. With lightweight components, they need not be restrained by lowest-common-denominator choices between native user interfaces.</P>
<P>It韘 not clear to what extent users will accept a Java Platform look and feel over a native one, but user interface does seem to be evolving towards more heterogeneity. Back in the eighties, the Apple Macintosh established a principle that stated all Macintosh applications should adhere to certain user-interface guidelines. The theory was that software would be easier to use if all a user韘 applications were homogeneous: if they all used familiar metaphors and exhibited the same look and feel. Today, however, when a Macintosh user browses the World Wide Web, they don韙 expect every web page to look like a Macintosh page. When they go to the IBM site, they expect it to look like IBM. When they go to the Disney site, they expect it to look like Disney. This is similar to the real world in that when you go to New York, you expect it to look and feel like New York. When you go to Paris, you expect it to look and feel like Paris. You don韙 expect all cities to have the same look and feel. As users are exposed to the web, they are becoming accustomed to working with more heterogeneous user interfaces than they might have encountered on an isolated personal computer.</P>
<P>As Java user-interface libraries evolve, the lowest-common-denominator problem inherent in platform-independent user interfaces may gradually become less painful. It remains to be seen, however, the extent to which users will accept interfaces that do not look and feel 100% native. For links to the latest information about the evolution of user interface technologies for Java, visit the resources page for this chapter.</P>
<H3><EM><P>The Resources Page</P>
</EM></H3><P>For links to more information about the material presented in this chapter, visit the resources page at <FONT FACE="Courier New"><A HREF="http://www.artima.com/insidejvm/intro.html" tppabs="http://www.artima.com/insidejvm/intro.html">http://www.artima.com/insidejvm/intro.html</A></FONT>.</P>
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