java语言程序设计基础篇原书第八版课件PPT(第十七章) 64页

'1Chapter8ObjectsandClasses 2MotivationsAfterlearningtheprecedingchapters,youarecapableofsolvingmanyprogrammingproblemsusingselections,loops,methods,andarrays.However,theseJavafeaturesarenotsufficientfordevelopinggraphicaluserinterfacesandlargescalesoftwaresystems.Supposeyouwanttodevelopagraphicaluserinterfaceasshownbelow.Howdoyouprogramit? 3ObjectivesTodescribeobjectsandclasses,anduseclassestomodelobjects(§8.2).TouseUMLgraphicalnotationstodescribeclassesandobjects(§8.2).Todemonstratedefiningclassesandcreatingobjects(§8.3).Tocreateobjectsusingconstructors(§8.4).Toaccessobjectsviaobjectreferencevariables(§8.5).Todefineareferencevariableusingareferencetype(§8.5.1).Toaccessanobject’sdataandmethodsusingtheobjectmemberaccessoperator(.)(§8.5.2).Todefinedatafieldsofreferencetypesandassigndefaultvaluesforanobject’sdatafields(§8.5.3).Todistinguishbetweenobjectreferencevariablesandprimitivedatatypevariables(§8.5.4).TouseclassesDate,Random,andJFrameintheJavalibrary(§8.6).Todistinguishbetweeninstanceandstaticvariablesandmethods(§8.7).Todefineprivatedatafieldswithappropriategetandsetmethods(§8.8).Toencapsulatedatafieldstomakeclasseseasytomaintain(§8.9).Todevelopmethodswithobjectargumentsanddifferentiatebetweenprimitive-typeargumentsandobject-typearguments(§8.10).Tostoreandprocessobjectsinarrays(§8.11). 4OOProgrammingConceptsObject-orientedprogramming(OOP)involvesprogrammingusingobjects.Anobjectrepresentsanentityintherealworldthatcanbedistinctlyidentified.Forexample,astudent,adesk,acircle,abutton,andevenaloancanallbeviewedasobjects.Anobjecthasauniqueidentity,state,andbehaviors.Thestateofanobjectconsistsofasetofdatafields(alsoknownasproperties)withtheircurrentvalues.Thebehaviorofanobjectisdefinedbyasetofmethods. 5ObjectsAnobjecthasbothastateandbehavior.Thestatedefinestheobject,andthebehaviordefineswhattheobjectdoes. 6ClassesClassesareconstructsthatdefineobjectsofthesametype.AJavaclassusesvariablestodefinedatafieldsandmethodstodefinebehaviors.Additionally,aclassprovidesaspecialtypeofmethods,knownasconstructors,whichareinvokedtoconstructobjectsfromtheclass. 7Classes 8UMLClassDiagram 9Example:DefiningClassesandCreatingObjectsObjective:Demonstratecreatingobjects,accessingdata,andusingmethods.TestCircle1Run 10Example:DefiningClassesandCreatingObjectsObjective:Demonstratecreatingobjects,accessingdata,andusingmethods.TestTVRunTV 11ConstructorsCircle(){}Circle(doublenewRadius){radius=newRadius;}Constructorsareaspecialkindofmethodsthatareinvokedtoconstructobjects. 12Constructors,cont.Aconstructorwithnoparametersisreferredtoasano-argconstructor.·      Constructorsmusthavethesamenameastheclassitself.·      Constructorsdonothaveareturntype—notevenvoid.·      Constructorsareinvokedusingthenewoperatorwhenanobjectiscreated.Constructorsplaytheroleofinitializingobjects. 13CreatingObjectsUsingConstructorsnewClassName();Example:newCircle();newCircle(5.0); 14DefaultConstructorAclassmaybedeclaredwithoutconstructors.Inthiscase,ano-argconstructorwithanemptybodyisimplicitlydeclaredintheclass.Thisconstructor,calledadefaultconstructor,isprovidedautomaticallyonlyifnoconstructorsareexplicitlydeclaredintheclass. 15DeclaringObjectReferenceVariablesToreferenceanobject,assigntheobjecttoareferencevariable.Todeclareareferencevariable,usethesyntax:ClassNameobjectRefVar;Example:CirclemyCircle; 16Declaring/CreatingObjectsinaSingleStepClassNameobjectRefVar=newClassName();Example:CirclemyCircle=newCircle();CreateanobjectAssignobjectreference 17AccessingObjectsReferencingtheobject’sdata:objectRefVar.datae.g.,myCircle.radiusInvokingtheobject’smethod:objectRefVar.methodName(arguments)e.g.,myCircle.getArea() 18TraceCodeCirclemyCircle=newCircle(5.0);SCircleyourCircle=newCircle();yourCircle.radius=100;DeclaremyCirclenovaluemyCircleanimation 19TraceCode,cont.CirclemyCircle=newCircle(5.0);CircleyourCircle=newCircle();yourCircle.radius=100;novaluemyCircleCreateacircleanimation 20TraceCode,cont.CirclemyCircle=newCircle(5.0);CircleyourCircle=newCircle();yourCircle.radius=100;referencevaluemyCircleAssignobjectreferencetomyCircleanimation 21TraceCode,cont.CirclemyCircle=newCircle(5.0);CircleyourCircle=newCircle();yourCircle.radius=100;referencevaluemyCirclenovalueyourCircleDeclareyourCircleanimation 22TraceCode,cont.CirclemyCircle=newCircle(5.0);CircleyourCircle=newCircle();yourCircle.radius=100;referencevaluemyCirclenovalueyourCircleCreateanewCircleobjectanimation 23TraceCode,cont.CirclemyCircle=newCircle(5.0);CircleyourCircle=newCircle();yourCircle.radius=100;referencevaluemyCirclereferencevalueyourCircleAssignobjectreferencetoyourCircleanimation 24TraceCode,cont.CirclemyCircle=newCircle(5.0);CircleyourCircle=newCircle();yourCircle.radius=100;referencevaluemyCirclereferencevalueyourCircleChangeradiusinyourCircleanimation 25CautionRecallthatyouuseMath.methodName(arguments)(e.g.,Math.pow(3,2.5))toinvokeamethodintheMathclass.CanyouinvokegetArea()usingCircle1.getArea()?Theanswerisno.Allthemethodsusedbeforethischapterarestaticmethods,whicharedefinedusingthestatickeyword.However,getArea()isnon-static.ItmustbeinvokedfromanobjectusingobjectRefVar.methodName(arguments)(e.g.,myCircle.getArea()).Moreexplanationswillbegiveninthesectionon“StaticVariables,Constants,andMethods.” 26ReferenceDataFieldsThedatafieldscanbeofreferencetypes.Forexample,thefollowingStudentclasscontainsadatafieldnameoftheStringtype.publicclassStudent{Stringname;//namehasdefaultvaluenullintage;//agehasdefaultvalue0booleanisScienceMajor;//isScienceMajorhasdefaultvaluefalsechargender;//chasdefaultvalue"u0000"} 27ThenullValueIfadatafieldofareferencetypedoesnotreferenceanyobject,thedatafieldholdsaspecialliteralvalue,null. 28DefaultValueforaDataFieldThedefaultvalueofadatafieldisnullforareferencetype,0foranumerictype,falseforabooleantype,and"u0000"forachartype.However,Javaassignsnodefaultvaluetoalocalvariableinsideamethod.publicclassTest{publicstaticvoidmain(String[]args){Studentstudent=newStudent();System.out.println("name?"+student.name);System.out.println("age?"+student.age);System.out.println("isScienceMajor?"+student.isScienceMajor);System.out.println("gender?"+student.gender);}} 29ExamplepublicclassTest{publicstaticvoidmain(String[]args){intx;//xhasnodefaultvalueStringy;//yhasnodefaultvalueSystem.out.println("xis"+x);System.out.println("yis"+y);}}Compilationerror:variablesnotinitializedJavaassignsnodefaultvaluetoalocalvariableinsideamethod. 30DifferencesbetweenVariablesofPrimitiveDataTypesandObjectTypes 31CopyingVariablesofPrimitiveDataTypesandObjectTypes 32GarbageCollectionAsshowninthepreviousfigure,aftertheassignmentstatementc1=c2,c1pointstothesameobjectreferencedbyc2.Theobjectpreviouslyreferencedbyc1isnolongerreferenced.Thisobjectisknownasgarbage.GarbageisautomaticallycollectedbyJVM. 33GarbageCollection,contTIP:Ifyouknowthatanobjectisnolongerneeded,youcanexplicitlyassignnulltoareferencevariablefortheobject.TheJVMwillautomaticallycollectthespaceiftheobjectisnotreferencedbyanyvariable. 34TheDateClassJavaprovidesasystem-independentencapsulationofdateandtimeinthejava.util.Dateclass.YoucanusetheDateclasstocreateaninstanceforthecurrentdateandtimeanduseitstoStringmethodtoreturnthedateandtimeasastring. 35TheDateClassExampleForexample,thefollowingcodejava.util.Datedate=newjava.util.Date();System.out.println(date.toString());displaysastringlikeSunMar0913:50:19EST2003. 36TheRandomClassYouhaveusedMath.random()toobtainarandomdoublevaluebetween0.0and1.0(excluding1.0).Amoreusefulrandomnumbergeneratorisprovidedinthejava.util.Randomclass. 37TheRandomClassExampleIftwoRandomobjectshavethesameseed,theywillgenerateidenticalsequencesofnumbers.Forexample,thefollowingcodecreatestwoRandomobjectswiththesameseed3.Randomrandom1=newRandom(3);System.out.print("Fromrandom1:");for(inti=0;i<10;i++)System.out.print(random1.nextInt(1000)+"");Randomrandom2=newRandom(3);System.out.print("nFromrandom2:");for(inti=0;i<10;i++)System.out.print(random2.nextInt(1000)+"");Fromrandom1:734660210581128202549564459961Fromrandom2:734660210581128202549564459961 38DisplayingGUIComponentsWhenyoudevelopprogramstocreategraphicaluserinterfaces,youwilluseJavaclassessuchasJFrame,JButton,JRadioButton,JComboBox,andJListtocreateframes,buttons,radiobuttons,comboboxes,lists,andsoon.HereisanexamplethatcreatestwowindowsusingtheJFrameclass.TestFrameRun 39TraceCodeJFrameframe1=newJFrame();frame1.setTitle("Window1");frame1.setSize(200,150);frame1.setVisible(true);JFrameframe2=newJFrame();frame2.setTitle("Window2");frame2.setSize(200,150);frame2.setVisible(true);Declare,create,andassigninonestatementreferenceframe1:JFrametitle:width:height:visible:animation 40TraceCodeJFrameframe1=newJFrame();frame1.setTitle("Window1");frame1.setSize(200,150);frame1.setVisible(true);JFrameframe2=newJFrame();frame2.setTitle("Window2");frame2.setSize(200,150);frame2.setVisible(true);referenceframe1:JFrametitle:"Window1"width:height:visible:Settitlepropertyanimation 41TraceCodeJFrameframe1=newJFrame();frame1.setTitle("Window1");frame1.setSize(200,150);frame1.setVisible(true);JFrameframe2=newJFrame();frame2.setTitle("Window2");frame2.setSize(200,150);frame2.setVisible(true);referenceframe1:JFrametitle:"Window1"width:200height:150visible:Setsizepropertyanimation 42TraceCodeJFrameframe1=newJFrame();frame1.setTitle("Window1");frame1.setSize(200,150);frame1.setVisible(true);JFrameframe2=newJFrame();frame2.setTitle("Window2");frame2.setSize(200,150);frame2.setVisible(true);referenceframe1:JFrametitle:"Window1"width:200height:150visible:trueSetvisiblepropertyanimation 43TraceCodeJFrameframe1=newJFrame();frame1.setTitle("Window1");frame1.setSize(200,150);frame1.setVisible(true);JFrameframe2=newJFrame();frame2.setTitle("Window2");frame2.setSize(200,150);frame2.setVisible(true);referenceframe1:JFrametitle:"Window1"width:200height:150visible:trueDeclare,create,andassigninonestatementreferenceframe2:JFrametitle:width:height:visible:animation 44TraceCodeJFrameframe1=newJFrame();frame1.setTitle("Window1");frame1.setSize(200,150);frame1.setVisible(true);JFrameframe2=newJFrame();frame2.setTitle("Window2");frame2.setSize(200,150);frame2.setVisible(true);referenceframe1:JFrametitle:"Window1"width:200height:150visible:truereferenceframe2:JFrametitle:"Window2"width:height:visible:Settitlepropertyanimation 45TraceCodeJFrameframe1=newJFrame();frame1.setTitle("Window1");frame1.setSize(200,150);frame1.setVisible(true);JFrameframe2=newJFrame();frame2.setTitle("Window2");frame2.setSize(200,150);frame2.setVisible(true);referenceframe1:JFrametitle:"Window1"width:200height:150visible:truereferenceframe2:JFrametitle:"Window2"width:200height:150visible:Setsizepropertyanimation 46TraceCodeJFrameframe1=newJFrame();frame1.setTitle("Window1");frame1.setSize(200,150);frame1.setVisible(true);JFrameframe2=newJFrame();frame2.setTitle("Window2");frame2.setSize(200,150);frame2.setVisible(true);referenceframe1:JFrametitle:"Window1"width:200height:150visible:truereferenceframe2:JFrametitle:"Window2"width:200height:150visible:trueSetvisiblepropertyanimation 47AddingGUIComponentstoWindowYoucanaddgraphicaluserinterfacecomponents,suchasbuttons,labels,textfields,comboboxes,lists,andmenus,tothewindow.Thecomponentsaredefinedusingclasses.Hereisanexampletocreatebuttons,labels,textfields,checkboxes,radiobuttons,andcomboboxes.GUIComponentsRun 48InstanceVariables,andMethodsInstancevariablesbelongtoaspecificinstance.Instancemethodsareinvokedbyaninstanceoftheclass. 49StaticVariables,Constants,andMethodsStaticvariablesaresharedbyalltheinstancesoftheclass.Staticmethodsarenottiedtoaspecificobject.Staticconstantsarefinalvariablessharedbyalltheinstancesoftheclass. 50StaticVariables,Constants,andMethods,cont.Todeclarestaticvariables,constants,andmethods,usethestaticmodifier. 51StaticVariables,Constants,andMethods,cont. 52ExampleofUsingInstanceandClassVariablesandMethodObjective:Demonstratetherolesofinstanceandclassvariablesandtheiruses.ThisexampleaddsaclassvariablenumberOfObjectstotrackthenumberofCircleobjectscreated.TestCircle2RunCircle2 53VisibilityModifiersandAccessor/MutatorMethodsBydefault,theclass,variable,ormethodcanbeaccessedbyanyclassinthesamepackage.publicTheclass,data,ormethodisvisibletoanyclassinanypackage.privateThedataormethodscanbeaccessedonlybythedeclaringclass.Thegetandsetmethodsareusedtoreadandmodifyprivateproperties. 54Theprivatemodifierrestrictsaccesstowithinaclass,thedefaultmodifierrestrictsaccesstowithinapackage,andthepublicmodifierenablesunrestrictedaccess. 55NOTEAnobjectcannotaccessitsprivatemembers,asshownin(b).ItisOK,however,iftheobjectisdeclaredinitsownclass,asshownin(a). 56WhyDataFieldsShouldBeprivate?Toprotectdata.Tomakeclasseasytomaintain. 57ExampleofDataFieldEncapsulationCircle3RunTestCircle3 58PassingObjectstoMethodsPassingbyvalueforprimitivetypevalue(thevalueispassedtotheparameter)Passingbyvalueforreferencetypevalue(thevalueisthereferencetotheobject)TestPassObjectRun 59PassingObjectstoMethods,cont. 60ArrayofObjectsCircle[]circleArray=newCircle[10];Anarrayofobjectsisactuallyanarrayofreferencevariables.SoinvokingcircleArray[1].getArea()involvestwolevelsofreferencingasshowninthenextfigure.circleArrayreferencestotheentirearray.circleArray[1]referencestoaCircleobject. 61ArrayofObjects,cont.Circle[]circleArray=newCircle[10]; 62ArrayofObjects,cont.SummarizingtheareasofthecirclesTotalAreaRun 服务理念中的“点点”◆理解多一点真情浓一点◆学习勤一点品质高一点◆理由少一点效率高一点◆处理问题灵活点工作过程用心点◆对待同事宽容点互相协作快乐点 放映结束！敬请各位的批评指导！谢谢观看'