Inheritance.ppt

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1、InheritanceInheritanceInheritanceallowsasoftwaredevelopertoderiveanewclassfromanexistingoneTheexistingclassiscalledtheparentclassorsuperclassThederivedclassiscalledthechildclassorsubclass.Createsanis-arelationshipThesubclassisamorespecificversionoftheOriginal(Rememberhas-a

2、isaggregation.)BookNovelDictionaryMysteryRomance2InheritanceThechildclassinheritsthemethodsanddatadefinedfortheparentclassTotailoraderivedclass,theprogrammercanaddnewvariablesormethods,orcanmodifytheinheritedonesSoftwarereuseisattheheartofinheritanceByusingexistingsoftware

3、componentstocreatenewones,wecapitalizeonalltheeffortthatwentintothedesign,implementation,andtestingoftheexistingsoftwareDerivingSubclassesInJava,weusethereservedwordextendstoestablishaninheritancerelationshipclassDictionaryextendsBook{//classcontents}4Dictionarywebster=new

4、Dictionary();webster.message();webster.defMessage();publicclassBook{protectedintpages=1500;publicStringmessage(){System.out.println(“Numberofpages:”+pages);}}publicclassDictionaryextendsBook{privateintdefinitions=52500;publicvoiddefMessage(){System.out.println(“Numberofdef

5、initions”+definitions);System.out.println(“Definitionsperpage:”+(definitions/pages));}}Numberofpages:1500Numberofdefinitions:52500Definitionsperpage:35SomeInheritanceDetailsAninstanceofachildclassdoesnotrelyonaninstanceofaparentclassHencewecouldcreateaDictionaryobjectwitho

6、uthavingtocreateaBookobjectfirstInheritanceisaone-waystreetTheBookclasscannotusevariablesormethodsdeclaredexplicitlyintheDictionaryclassTheprotectedModifierVisibilitymodifiersdeterminewhichclassmembersareinheritedandwhicharenotVariablesandmethodsdeclaredwithpublicvisibilit

7、yareinherited;thosewithprivatevisibilityarenotButpublicvariablesviolatetheprincipleofencapsulationThereisathirdvisibilitymodifierthathelpsininheritancesituations:protected7TheprotectedModifierTheprotectedmodifierallowsamemberofabaseclasstobeinheritedintoachildProtectedvisi

8、bilityprovidesmoreencapsulationthanpublicvisibilitydoesthebestpossibleencapsulationthatpe