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SHAPE ="RECT" HREF ="/reference/author.html" COORDS ="361,0,440,29"><AREA SHAPE ="RECT" HREF ="/reference/basic.html" COORDS ="254,0,360,29"><AREA SHAPE ="RECT" HREF ="/reference/help.html" COORDS ="183,0,251,29"><AREA SHAPE ="RECT" HREF ="/reference/cool.html" COORDS ="100,0,181,29"><AREA SHAPE ="RECT" HREF ="/reference/mybook.html" COORDS ="0,0,100,29"></MAP><MAP NAME ="header_r1.gif.map"><AREA SHAPE ="RECT" HREF ="/reference/personalize.html" COORDS ="339,0,439,29"></MAP><p><font face="Arial,Helvetica" size="-1" color="#006666"><A HREF="/reference/r_library.html"><B>All Categories</A> :</B><b><A HREF="/reference/r_java.html">Java</A></b></font><p> <META NAME="GENERATOR" CONTENT="Mozilla/3.0b5aGold (WinNT; I) [Netscape]"><H1><FONT COLOR=#FF0000>Chapter 11</FONT></H1><H1><B><FONT SIZE=5 COLOR=#FF0000>Building a Live Data Applet</FONT></B></H1><P><HR WIDTH="100%"></P><P><H3 ALIGN=CENTER><FONT COLOR="#000000"><FONT SIZE=+2>CONTENTS<A NAME="CONTENTS"></A></FONT></FONT></H3><UL><LI><A HREF="#ObserversandtheModelViewParadigm" >Observers and the Model-View Paradigm</A><LI><A HREF="#ChapterProject" >Chapter Project</A><UL><LI><A HREF="#GeneralArchitectureoftheProject" >General Architecture of the Project</A><LI><A HREF="#ChangestotheServer" >Changes to the Server</A><LI><A HREF="#AppletClient" >Applet Client</A></UL><LI><A HREF="#Summary" >Summary</A></UL><HR><P>This chapter concludes Part IV with an application that bringstogether most of the JDK features you have seen throughout thisbook. The application is an Election Night applet that shows electionreturns as they arrive. It can do this because the applet getsdatagrams from a back-end Web server that broadcasts returns asthey are entered into a server database. This client-server applicationis Internet-ready! You can run the server on the host and havemultiple clients connect to your site to see the live returns.<P>This application brings in many of the JDK's major components,such as AWT, threads and synchronization, streams, exception handling,native methods, and sockets. Because of the project's size, manyless critical but useful components are also applied-such as theHashtable class, static methods, and string parsing. Some newconcepts are also introduced; for example, the model-view paradigmis covered briefly before you get into the heart of the chapterproject.<H2><A NAME="ObserversandtheModelViewParadigm"><FONT SIZE=5 COLOR=#FF0000>Observersand the Model-View Paradigm</FONT></A></H2><P>One problem that programmers often face is showing different viewsof data that may change over time. A classic example of this isa spreadsheet with multiple graphs (this sounds familiar!). Ifthe spreadsheet were tied to a relational database, traditionalprogrammers would represent the graph by making SQL calls to thesame set of tables the spreadsheet uses. However, if the spreadsheetchanges (and, therefore, its underlying table structure), thegraph programs would need to be modified, also. In other words,the spreadsheet structure is closely tied to both the spreadsheetprogram and the graphs that use its data.<P>The problem can get even worse if the graphs must update themselvesas the spreadsheet data changes. How do the graphs monitor thedata? Should the graphs keep querying the data to see whetherit has changed? This would be inefficient from a performance standpointsince such data will probably not change much from second to second,although it can change dramatically over longer periods of time.Furthermore, with multiple graphs and a spreadsheet constantlyaccessing the data, the database host's performance will be strained.<P>The <I>model-view paradigm</I> offers a solution to these problemswith two fundamental tenets: Separate the underlying data modelfrom the programs that view (use) this data, and let the underlyingdata model inform the views that something has changed, whichprevents the view from having to constantly query the data. Toillustrate the first tenet being applied, take a look again atthe spreadsheet applet developed in Part II. The data was maintainedin the CellContainer class, whose only function was the properupkeep of the spreadsheet data. The CellContainer class provideda "model" for the spreadsheet data. There were three"views" on the data-the two graphs and the spreadsheetitself. The latter was represented by the SpreadsheetContainerclass, which simply uses the CellContainer model. For example,the SpreadsheetContainer didn't have any responsibility for verifyinga formula or evaluating its results-that was the job of the model.In a sense, the SpreadsheetContainer was little more than a "dumb"view of the model.<P>The second tenet of the model-view paradigm is how to let a view⌨️ 快捷键说明
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