翻译文献-简单的动画及其实现

外文原文SIMPLEANIMATIONANDTHREADSTHEFIRSTTHINGIEVERSAWJAVADOWASANANIMATIONALARGERED“HITHERE”THATRANACROSSTHESCREENFROMTHERIGHTTOLEFTEVENTHATSIMPLEFORMOFANIMATIONWASENOUGHTOMAKEMESTOPANDTHINK,“THISISREALLYCOOL”THATSORTOFSIMPLEANIMATIONTAKESONLYAFEWMETHODSTOIMPLEMENTINJAVA,BUTTHOSEFEWMETHODSARETHEBASISFORANYJAVAAPPLETTHATYOUWANTTOUPDATETHESCREENDYNAMICALLYSTARTINGWITHSIMPLEANIMATIONSISAGOODWAYTOBUILDUPTOTHEMORECOMPLICATEDAPPLETSTODAY,YOULLLEARNTHEFUNDAMENTALSOFANIMATIONINJAVAHOWTHEVARIOUSPARTSOFTHESYSTEMALLWORKTOGETHERSOTHATYOUCANCREATEMOVINGFIGURESANDDYNAMICUPDATEABLEAPPLETSSPECIFICALLY,YOULLEXPLORETHEFOLLOWINGHOWJAVAANIMATIONSWORKTHEPAINTANDREPAINTMETHODS,STARTINGANDSTOPPINGDYNAMICAPPLETS,ANDHOWTOUSEANDOVERRIDETHESEMETHODSINYOUROWNAPPLETSTHREADSWHATTHEYAREANDHOWTHEYCANMAKEYOURAPPLETSMOREWELLBEHAVEDWITHOTHERAPPLETSANDWITHTHEJAVASYSTEMINGENERALREDUCINGANIMATIONFLICKER,ACOMMONPROBLEMWITHANIMATIONINJAVATHROUGHOUTTODAY,YOULLALSOWORKWITHLOTSOFEXAMPLESOFREALAPPLETSTHATCREATEANIMATIONSORPERFORMSOMEKINDOFDYNAMICMOVEMENTCREATINGANIMATIONINJAVAANIMATIONINJAVAINVOLVESTWOSTEPSCONSTRUCTINGAFRAMEOFANIMATION,ANDTHENASKINGJAVATOPAINTTHATFRAMEREPEATASNECESSARYTOCREATETHEILLUSIONOFMOVEMENTTHEBASIC,STATICAPPLETSTHATYOUCREATEDYESTERDAYTAUGHTYOUHOWTOACCOMPLISHTHEFIRSTPARTALLTHATSLEFTISHOWTOTELLJAVATOPAINTAFRAMEPAINTINGANDREPAINTINGTHEPAINTMETHOD,ASYOULEARNEDYESTERDAY,ISCALLEDBYJAVAWHENEVERTHEAPPLETNEEDSTOBEPAINTEDWHENTHEAPPLETISINITIALLYDRAWN,WHENTHEWINDOWCONTAININGITISMOVED,ORWHENANOTHERWINDOWISMOVEDFROMOVERITYOUCANALSO,HOWEVER,ASKJAVATOREPAINTTHEAPPLETATATIMEYOUCHOOSESO,TOCHANGETHEAPPEARANCEOFWHATISONTHESCREEN,YOUCONSTRUCTTHEIMAGEOR“FRAME”YOUWANTTOPAINT,ANDTHENASKJAVATOPAINTTHISFRAMEIFYOUDOTHISREPEATEDLY,ANDFASTENOUGH,YOUGETANIMATIONINSIDEYOURJAVAAPPLETTHATSALLTHEREISTOITWHEREDOESALLTHISTAKEPLACENOTINTHEPAINTMETHODITSELFALLPAINTDOESISPUTDOTSONTHESCREENPAINT,INOTHERWORDS,ISRESPONSIBLEONLYFORTHECURRENTFRAMEOFTHEANIMATIONATATIMETHEREALWORKOFCHANGINGWHATPAINTDOES,OFMODIFYINGTHEFRAMEFORANANIMATION,ACTUALLYOCCURSSOMEWHEREELSEINTHEDEFINITIONOFYOURAPPLETINTHAT“SOMEWHEREELSE,”YOUCONSTRUCTTHEFRAMESETVARIABLESFORPAINTTOUSE,CREATECOLORORFONTOROTHEROBJECTSTHATPAINTWILLNEED,ANDTHENCALLTHEREPAINTMETHODREPAINTISTHETRIGGERTHATCAUSESJAVATOCALLPAINTANDCAUSESYOURFRAMETOGETDRAWNTECHNICALNOTEBECAUSEAJAVAAPPLETCANCONTAINMANYDIFFERENTCOMPONENTSTHATALLNEEDTOBEPAINTEDASYOULLLEARNLATERONTHISWEEK,ANDINFACT,APPLETSAREEMBEDDEDINSIDEALARGERJAVAAPPLICATIONTHATALSOPAINTSTOTHESCREENINSIMILARWAYS,WHENYOUCALLREPAINTANDTHEREFOREPAINTYOURENOTACTUALLYIMMEDIATELYDRAWINGTOTHESCREENASYOUDOINOTHERWINDOWORGRAPHICSTOOLKITSINSTEAD,REPAINTISAREQUESTFORJAVATOREPAINTYOURAPPLETASSOONASITCANMUCHOFTHETIME,THEDELAYBETWEENTHECALLANDTHEACTUALREPAINTISNEGLIGIBLESTARTINGANDSTOPPINGANAPPLETSEXECUTIONREMEMBERSTARTANDSTOPFROMDAY8THESEARETHEMETHODSTHATTRIGGERYOURAPPLETTOSTARTANDSTOPRUNNINGYOUDIDNTUSESTARTANDSTOPYESTERDAY,BECAUSETHEAPPLETSONTHATDAYDIDNOTHINGEXCEPTPAINTONCEWITHANIMATIONSANDOTHERJAVAAPPLETSTHATAREACTUALLYPROCESSINGANDRUNNINGOVERTIME,YOULLNEEDTOMAKEUSEOFSTARTANDSTOPTOTRIGGERTHESTARTOFYOURAPPLETSEXECUTION,ANDTOSTOPITFROMRUNNINGWHENYOULEAVETHEPAGETHATCONTAINSTHATAPPLETFORMOSTAPPLETS,YOULLWANTTOOVERRIDESTARTANDSTOPFORJUSTTHISREASONTHESTARTMETHODTRIGGERSTHEEXECUTIONOFTHEAPPLETYOUCANEITHERDOALLTHEAPPLETSWORKINSIDETHATMETHOD,ORYOUCANCALLOTHEROBJECTSMETHODSINORDERTODOSOUSUALLY,STARTISUSEDTOCREATEANDBEGINEXECUTIONOFATHREADSOTHEAPPLETCANRUNINITSOWNTIMESTOP,ONTHEOTHERHAND,SUSPECTSANAPPLETSEXECUTIONSOWHENYOUMOVEOFFTHEPAGEONWHICHTHEAPPLETISDISPLAYING,ITDOESNTKEEPRUNNINGANDUSINGUPSYSTEMRESOURCESMOSTOFTHETIMEWHENYOUCREATEASTARTMETHOD,YOUSHOULDALSOCREATEACORRESPONDINGSTOPPUTTINGITTOGETHEREXPLAININGHOWTODOJAVAANIMATIONINTEXTISMOREOFATASKTHANACTUALLYSHOWINGYOUHOWITWORKSINCODEANEXAMPLEORTWOWILLHELPMAKETHERELATIONSHIPBETWEENALLTHESEMETHODSCLEARERLISTING101SHOWSASAMPLEAPPLETTHAT,ATFIRSTGLANCE,USESBASICAPPLETANIMATIONTODISPLAYTHEDATEANDTIMEANDCONSTANTLYUPDATESITEVERYSECOND,CREATINGAVERYSIMPLEANIMATEDDIGITALCLOCKAFRAMEFROMTHATCLOCKISSHOWNINFIGURE101THEWORDS“ATFIRSTGLANCE”INTHEPREVIOUSPARAGRAPHAREVERYIMPORTANTTHISAPPLETDOESNTWORKHOWEVER,DESPITETHEFACTTHATITDOESNTWORK,YOUCANSTILLLEARNALOTABOUTBASICANIMATIONWITHIT,SOWORKINGTHROUGHTHECODEWILLSTILLBEVALUABLEINTHENEXTSECTION,YOULLLEARNJUSTWHATSWRONGWITHITSEEWHETHERYOUCANFIGUREOUTWHATSGOINGONWITHTHISCODEBEFOREYOUGOONTOTHEANALYSISTYPELISTING101THEDATEAPPLET1IMPORTJAVAAWTGRAPHICS2IMPORTJAVAAWTFONT3IMPORTJAVAUTILDATE45PUBLICCLASSDIGITALCLOCKEXTENDSJAVAAPPLETAPPLET67FONTTHEFONTNEWFONT“TIMESROMAN”,FONTBOLD,248DATETHEDATE910PUBLICVOIDSTART11WHILETRUE12THEDATENEWDATE13REPAINT14TRYTHREADSLEEP100015CATCHINTERRUPTEDEXCEPTIONE16171819PUBLICVOIDPAINTGRAPHICSG20GSETFONTTHEFONT21GDRAWSTRINGTHEDATETOSTRING,10,502223FIGURE101THEDIGITALCLOCKANALYSISTHINKYOUVEGOTTHEBASICIDEALETSGOTHROUGHIT,LINEBYLINELINES7AND8DEFINETWOBASICINSTANCEVARIABLESTHEFONTANDTHEDATE,WHICHHOLDOBJECTSREPRESENTINGTHECURRENTFONTANDTHECURRENTDATE,RESPECTIVELYMOREABOUTTHESELATERTHESTARTMETHODTRIGGERSTHEAC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为一次性的动画的当前画面负责。改变PAINT操作及为一个动画修饰画面的实际工作实际上发生在你的程序的定义的其他什么地方。（在所谓的其它什么地方，你创建了画面为PAINT设置变量，创建颜色或则字体，及PAINT需要的对象），并且调用REPAINT方法。REPAINT是导致JAVA调用PAINT并关闭你的画面的触发。技术注释因为一个JAVAAPPLET能够包含很多需要被描绘的组件，并且事实上，APPLETS被嵌入在一个大的JAVA的应用之中，此应用将用同样的方法在屏幕中绘图，当你调用REPAINT时你其实并不是立刻就在屏幕上按照你在其它窗口或则绘图工具栏所做的那样。相反，REPAINT是一个对于JAVA来说尽可能快的重新绘制你的APPLET的请求。大多数情况下，在调用命令和实际的REPAINT之间的时延时是可忽略的。开始并停止一个程序的执行记得在第八天所说的START和STOP命令吗它们是能够触发你的程序开始及停止运行的方法。上一次你并没有使用START和STOP命令，因为那次的APPLET除了一次性的PAINT之外并没有做其它工作。在动画及JAVA的其它程序运行的过程中，你将需要利用START和STOP命令去触发你的程序执行的开始以及在你想离开包含APPLET的页面时停止程序的运行。对于大多数的APPLET中，为此原因你将希望不考虑START及STOP命令。START命令触发APPLET的执行。你可以在此命令中做所有APPLET的工作，或则可以调用其它对象的命令。通常，START被用来创建并开始一个思路的执行为的是APPLET可以在它自己的时间内运行。另一方面，STOP停止一个程序的执行当你移开正在显示的APPLET的页时，STOP并不持续运行并使用系统资源。大多数的情况下在你创造一个START的同时，你同样也应该创建一个相应的STOP。把它放在一起用语言解释JAVA是如何做一个动画的不仅仅是表现出它在编码上是如何实现的。一两个例子可使得所有这些命令的关系更清楚。列表101显示了一个例子APPLET，这个APPLET使用基础的动画显示了日期和时间并持续的以一秒的时间间隔更新此状态，这个例子创建了一个很简单的动画数字时钟,这个时钟的画面显示于FIGURE101。在前面的段落中“ATFIRSTGLANCE”是非常重要的这个APPLET确实工作然而，即使它不工作，你仍然能够学到基本的动画的很多东西，所以此编码的学习仍是有价值的。在下一段中，你将要学习还有什么问题。看一下在你继续分析此编码的同时能够考虑出将会发生什么。列表101这个APPLETIMPORTJAVAAWTGRAPHICSIMPORTJAVAAWTFONTIMPORTJAVAUTILDATEPUBLICCLASSDIGITALCLOCKEXTENDSJAVAAPPLETAPPLETFONTTHEFONTNEWFONT“TIMESROMAN”,FONTBOLD,24DATETHEDATEPUBLICVOIDSTARTWHILETRUETHEDATENEWDATEREPAINTTRYTHREADSLEEP1000CATCHINTERRUPTEDEXCEPTIONEPUBLICVOIDPAINTGRAPHICSGGSETFONTTHEFONTGDRAWSTRINGTHEDATETOSTRING,10,50想一下你得到基本的概念了吗让我们逐行的开始吧第7，8行定义了两个基本的中间变量THEFONT和THEDATE,它含有表现出当前字体和当前日期的对象。START命令触发实际程序的执行。注意在这个方法中的WHILE循环；考虑到TESTTRUE）永远返回TRUE,所以此循环从没存在过。在下面的叙述中，一个动画画面将在WHILE循环中被建立类DATE代表一个日期和时间。第12行创建了一个包含当前日期和时间的新的类DATE，并且把它付给变量THEDATE。REPAINT命令被调用。看起来很复杂的第14，15行除了在循环结束前延时1000微秒之外并没做什么别的。SLEEP命令作为类THREAD的一部分是使程序停止的原因。没有一个特别的SLEEP命令，APPLET将尽可能快地运行，在一个高速的计算机系统中他可能运行得太快以至于我们的肉眼无法看到它。使用SLEEP使得你能够准确的控制此动画多快地发生。在它周围的TRY及CATCH使得JAVA处理错误。TRY和CATCH被称作例外并将在下周被描述。这里，在PAINT之内，所有的当前字体（在变量THEFONT）被设置，并且日期也被打印在屏幕上（注意你已经调用TOSTRING命令把日期转换为一个字符串）。无论什么数据出现在THEDATE中，字符串总是一秒钟更新一次以反映新的日期。对于这个例子有一些事情需要注意。首先你应该认为在PAINT命令中创建新的DATE将变得更容易。在这个方法中你可以使用一个局部变量而不需要一个全局变量运行于DATA中。虽然那样做可以创建更加清晰的代码，但却导致程序的效率下降。每当一个画面需要被改变的时候，PAINT命令都需要被调用。在这个例子中，虽说这不是那样的重要，但在一个需要快速切换画面的动画中，PAINT命令不得不每次都停下来创建新的对象。如果我们使PAINT工作的很好即在屏幕上绘图并计算新的对象，你可以使得绘画更具效率。这确实是FONT也在中间变量中的原因。THREADS它们是什么并且为什么我们需要它们从你从前对操作系统及在那些系统中的环境的经验，你或许已经或许还没有进入到THREADS的概念中。让我们从一些基本定义开始吧。当一个程序开始运行，它开始执行它的初始代码，调用命令或则进程，并且将继续运行直到程序全部完成或则退出运行。运行的程序使用一个单独的THREAD在此程序中THREAD是一个控制的焦点。MULTITHREADING在JAVA中使得不同的操作THREADS能够在同一时间，在同一个程序中以并行且不互相干扰的方式运行。这里是个简单的例子。假设你要在程序的开始处做一个很长的计算。这个很长的计算可能会再后来程序执行的时候才被需要它确实与程序的主要内容相切，但还是最终要完成的。但在一个单主线的程序中，你不得不在剩余的程序能够继续运行之前等待计算的结束。在一个多主线的系统中，你可以把计算放到它自己的THREAD中，使得剩下的程序能继续单独的运行。在JAVA中使用THREADS你可以创建一个运行于它自己的THREAD的APPLET，并且它将很幸运的在不影响系统的其它部分的情况下独立运行自己的所有内容。使用THREADS你可以自一个页面中同时拥有很多APPLETS。根据你拥有APPLETS的数量，你可以最终消耗所有的系统资源以至于它们将运行得较慢，但它们仍将单独运行。即使你没有很多APPLET，在你的APPLET中使用THREADS仍是一个很好的JAVA编程的练习。对于运行良好的APPLETS的THUMB规则无论你何时有