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1、文件编号：YM/WI-19文件版本：A版文件页码：pageofjava课程实践指导书练习册——JAVA篇类的多态和继承（2）一、练习目标:1．方法的重载和覆盖。2．类的多态，继承。二、练习时间与作业要求：本次练习一共需要180分钟。翻译练习写在JAVA笔记本上，书面作业写在JAVA家庭作业本上。后面的书面作业不包括在这个时间范围内。三、翻译练习：将下面要求翻译的内容，写在记事本上（阴影部分不写出来，要求看懂）OrderofconstructorcallsAconstructorforthebaseclas
2、sisalwayscalledduringtheconstructionprocessforaderivedclass,chaininguptheinheritancehierarchysothataconstructorforeverybaseclassiscalled.Thismakessensebecausetheconstructorhasaspecialjob:toseethattheobjectisbuiltproperly.Aderivedclasshasaccesstoitsownmemb
3、ersonly,andnottothoseofthebaseclass(whosemembersaretypicallyprivate).Onlythebase-classconstructorhastheproperknowledgeandaccesstoinitializeitsownelements.Therefore,it’sessentialthatallconstructorsgetcalled,otherwisetheentireobjectwouldn’tbeconstructed.Tha
4、t’swhythecompilerenforcesaconstructorcallforeveryportionofaderivedclass.Itwillsilentlycallthedefaultconstructorifyoudon’texplicitlycallabase-classconstructorinthederived-classconstructorbody.Ifthereisnodefaultconstructor,thecompilerwillcomplain.(Inthecase
5、whereaclasshasnoconstructors,thecompilerwillautomaticallysynthesizeadefaultconstructor.)Theorderoftheconstructorcallsisimportant.Whenyouinherit,youknowallaboutthebaseclassandcanaccessanypublicandprotectedmembersofthebaseclass.Thismeansthatyoumustbeabletoa
6、ssumethatallthemembersofthebaseclassarevalidwhenyou’reinthederivedclass.Inanormalmethod,constructionhasalreadytakenplace,soallthemembersofallpartsoftheobjecthavebeenbuilt.Insidetheconstructor,however,youmustbeabletoassumethatallmembersthatyouusehavebeenbu
7、ilt.Theonlywayto文件编号：YM/WI-19文件版本：A版文件页码：pageofjava课程实践指导书guaranteethisisforthebase-classconstructortobecalledfirst.Thenwhenyou’reinthederived-classconstructor,allthemembersyoucanaccessinthebaseclasshavebeeninitialized.Knowingthatallmembersarevalidinsidet
8、heconstructorisalsothereasonthat,wheneverpossible,youshouldinitializeallmemberobjects(thatis,objectsplacedintheclassusingcomposition)attheirpointofdefinitionintheclass(e.g.,b,c,andlintheprecedingexample).Ifyoufollow
