Java语言程序设计10E LiangPPT习题案例19slide.ppt

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1、1Chapter19Generics2ObjectivesToknowthebenefitsofgenerics(§19.1).Tousegenericclassesandinterfaces(§19.2).Todeclaregenericclassesandinterfaces(§19.3).Tounderstandwhygenerictypescanimprovereliabilityandreadability(§19.3).Todeclareandusegenericmethodsandboundedgener

2、ictypes(§19.4).Touserawtypesforbackwardcompatibility(§19.5).Toknowwildcardtypesandunderstandwhytheyarenecessary(§19.6).ToconvertlegacycodeusingJDK1.5generics(§19.7).Tounderstandthatgenerictypeinformationiserasedbythecompilerandallinstancesofagenericclasssharethe

3、sameruntimeclassfile(§19.8).Toknowcertainrestrictionsongenerictypescausedbytypeerasure(§19.8).Todesignandimplementgenericmatrixclasses(§19.9).3WhyDoYouGetaWarning?publicclassShowUncheckedWarning{publicstaticvoidmain(String[]args){java.util.ArrayListlist=newjava.

4、util.ArrayList();list.add("JavaProgramming");}}Tounderstandthecompilewarningonthisline,youneedtolearnJDK1.6generics.4FixtheWarningpublicclassShowUncheckedWarning{publicstaticvoidmain(String[]args){java.util.ArrayListlist=newjava.util.ArrayList();

5、list.add("JavaProgramming");}}Nocompilewarningonthisline.`5WhatisGenerics?Genericsisthecapabilitytoparameterizetypes.Withthiscapability,youcandefineaclassoramethodwithgenerictypesthatcanbesubstitutedusingconcretetypesbythecompiler.Forexample,youmaydefineageneric

6、stackclassthatstorestheelementsofagenerictype.Fromthisgenericclass,youmaycreateastackobjectforholdingstringsandastackobjectforholdingnumbers.Here,stringsandnumbersareconcretetypesthatreplacethegenerictype.6WhyGenerics?Thekeybenefitofgenericsistoenableerrorstobed

7、etectedatcompiletimeratherthanatruntime.Agenericclassormethodpermitsyoutospecifyallowabletypesofobjectsthattheclassormethodmayworkwith.Ifyouattempttousetheclassormethodwithanincompatibleobject,acompileerroroccurs.7GenericTypeGenericInstantiationRuntimeerrorCompi

8、leerrorImprovesreliability8GenericArrayListinJDK1.59NoCastingNeededArrayListlist=newArrayList<>();list.add(5.5);//5.5isautomaticallyconvertedtonewDouble(5.5)l