java语言程序设计-基础篇-原书第八版-课件-PPT(第十七章).ppt

《java语言程序设计-基础篇-原书第八版-课件-PPT(第十七章).ppt》由会员分享，可在线阅读，更多相关《java语言程序设计-基础篇-原书第八版-课件-PPT(第十七章).ppt（62页珍藏版）》请在淘文阁 - 分享文档赚钱的网站上搜索。

1、Chapter8ObjectsandClasses1MotivationsAfterlearningtheprecedingchapters,youarecapableofsolvingmanyprogrammingproblemsusingselections,loops,methods,andarrays.However,theseJavafeaturesarenotsufficientfordevelopinggraphicaluserinterfacesandlargescalesoftwaresystems.Supposeyouwanttodevelopagraphicaluseri

2、nterfaceasshownbelow.Howdoyouprogramit?2ObjectivesFTodescribeobjectsandclasses,anduseclassestomodelobjects(8.2).FTouseUMLgraphicalnotationstodescribeclassesandobjects(8.2).FTodemonstratedefiningclassesandcreatingobjects(8.3).FTocreateobjectsusingconstructors(8.4).FToaccessobjectsviaobjectreferenceva

3、riables(8.5).FTodefineareferencevariableusingareferencetype(8.5.1).FToaccessanobjectsdataandmethodsusingtheobjectmemberaccessoperator(.)(8.5.2).FTodefinedatafieldsofreferencetypesandassigndefaultvaluesforanobjectsdatafields(8.5.3).FTodistinguishbetweenobjectreferencevariablesandprimitivedatatypevari

4、ables(8.5.4).FTouseclassesDate,Random,andJFrameintheJavalibrary(8.6).FTodistinguishbetweeninstanceandstaticvariablesandmethods(8.7).FTodefineprivatedatafieldswithappropriategetandsetmethods(8.8).FToencapsulatedatafieldstomakeclasseseasytomaintain(8.9).FTodevelopmethodswithobjectargumentsanddifferent

5、iatebetweenprimitive-typeargumentsandobject-typearguments(8.10).FTostoreandprocessobjectsinarrays(8.11).3OOProgrammingConceptsObject-orientedprogramming(OOP)involvesprogrammingusingobjects.Anobjectrepresentsanentityintherealworldthatcanbedistinctlyidentified.Forexample,astudent,adesk,acircle,abutton

6、,andevenaloancanallbeviewedasobjects.Anobjecthasauniqueidentity,state,andbehaviors.Thestateofanobjectconsistsofasetofdatafields(alsoknownasproperties)withtheircurrentvalues.Thebehaviorofanobjectisdefinedbyasetofmethods.4ObjectsAnobjecthasbothastateandbehavior.Thestatedefinestheobject,andthebehaviord

7、efineswhattheobjectdoes.5ClassesClassesareconstructsthatdefineobjectsofthesametype.AJavaclassusesvariablestodefinedatafieldsandmethodstodefinebehaviors.Additionally,aclassprovidesaspecialtypeofmethods,knownasconstructors,whichareinvokedtoconstructobjectsfromtheclass.6Classes7UMLClassDiagram8Example:

8、Defining Classes and Creating ObjectsFObjective:Demonstratecreatingobjects,accessingdata,andusingmethods.TestCircle1Run9Example:Defining Classes and Creating ObjectsFObjective:Demonstratecreatingobjects,accessingdata,andusingmethods.TestTVRunTV10ConstructorsCircle()Circle(double newRadius)radius=new

9、Radius;Constructorsareaspecialkindofmethodsthatareinvokedtoconstructobjects.11Constructors,cont.Aconstructorwithnoparametersisreferredtoasano-arg constructor.Constructorsmusthavethesamenameastheclassitself.Constructorsdonothaveareturntypenotevenvoid.Constructorsareinvokedusingthenewoperatorwhenanobj

10、ectiscreated.Constructorsplaytheroleofinitializingobjects.12CreatingObjectsUsingConstructorsnew ClassName();Example:new Circle();new Circle(5.0);13DefaultConstructorAclassmaybedeclaredwithoutconstructors.Inthiscase,ano-argconstructorwithanemptybodyisimplicitlydeclaredintheclass.Thisconstructor,calle

11、da default constructor,isprovidedautomaticallyonly if no constructors are explicitly declared in the class.14DeclaringObjectReferenceVariablesToreferenceanobject,assigntheobjecttoareferencevariable.Todeclareareferencevariable,usethesyntax:ClassName objectRefVar;Example:Circle myCircle;15Declaring/Cr

12、eatingObjectsinaSingleStepClassName objectRefVar=new ClassName();Example:Circle myCircle=new Circle();CreateanobjectAssignobjectreference16AccessingObjectsFReferencingtheobjectsdata:objectRefVar.data e.g.,myCircle.radiusFInvokingtheobjectsmethod:objectRefVar.methodName(arguments)e.g.,myCircle.getAre

13、a()17TraceCodeCirclemyCircle=newCircle(5.0);SCircleyourCircle=newCircle();yourCircle.radius=100;DeclaremyCirclenovaluemyCircleanimation18TraceCode,cont.CirclemyCircle=newCircle(5.0);CircleyourCircle=newCircle();yourCircle.radius=100;novaluemyCircleCreateacircleanimation19TraceCode,cont.CirclemyCircl

14、e=newCircle(5.0);CircleyourCircle=newCircle();yourCircle.radius=100;referencevaluemyCircleAssignobjectreferencetomyCircleanimation20TraceCode,cont.CirclemyCircle=newCircle(5.0);CircleyourCircle=newCircle();yourCircle.radius=100;referencevaluemyCirclenovalueyourCircleDeclareyourCircleanimation21Trace

15、Code,cont.CirclemyCircle=newCircle(5.0);CircleyourCircle=newCircle();yourCircle.radius=100;referencevaluemyCirclenovalueyourCircleCreateanewCircleobjectanimation22TraceCode,cont.CirclemyCircle=newCircle(5.0);CircleyourCircle=newCircle();yourCircle.radius=100;referencevaluemyCirclereferencevalueyourC

16、ircleAssignobjectreferencetoyourCircleanimation23TraceCode,cont.CirclemyCircle=newCircle(5.0);CircleyourCircle=newCircle();yourCircle.radius=100;referencevaluemyCirclereferencevalueyourCircleChangeradiusinyourCircleanimation24CautionRecallthatyouuseMath.methodName(arguments)(e.g.,Math.pow(3,2.5)toin

17、vokeamethodintheMathclass.CanyouinvokegetArea()usingCircle1.getArea()?Theanswerisno.Allthemethodsusedbeforethischapterarestaticmethods,whicharedefinedusingthestatickeyword.However,getArea()isnon-static.ItmustbeinvokedfromanobjectusingobjectRefVar.methodName(arguments)(e.g.,myCircle.getArea().Moreexp

18、lanationswillbegiveninthesectionon“StaticVariables,Constants,andMethods.”25ReferenceDataFieldsThedatafieldscanbeofreferencetypes.Forexample,thefollowingStudentclasscontainsadatafieldnameoftheStringtype.public class Student String name;/name has default value null int age;/age has default value 0 boo

19、lean isScienceMajor;/isScienceMajor has default value false char gender;/c has default value u000026ThenullValueIfadatafieldofareferencetypedoesnotreferenceanyobject,thedatafieldholdsaspecialliteralvalue,null.27DefaultValueforaDataFieldThedefaultvalueofadatafieldisnullforareferencetype,0foranumerict

20、ype,falseforabooleantype,andu0000forachartype.However,Javaassignsnodefaultvaluetoalocalvariableinsideamethod.public class Test public static void main(String args)Student student=new Student();System.out.println(name?+student.name);System.out.println(age?+student.age);System.out.println(isScienceMaj

21、or?+student.isScienceMajor);System.out.println(gender?+student.gender);28Examplepublic class Test public static void main(String args)int x;/x has no default value String y;/y has no default value System.out.println(x is +x);System.out.println(y is +y);Compilationerror:variablesnotinitializedJavaass

22、ignsnodefaultvaluetoalocalvariableinsideamethod.29DifferencesbetweenVariablesofPrimitiveDataTypesandObjectTypes30CopyingVariablesofPrimitiveDataTypesandObjectTypes31GarbageCollectionAsshowninthepreviousfigure,aftertheassignmentstatementc1=c2,c1pointstothesameobjectreferencedbyc2.Theobjectpreviouslyr

23、eferencedbyc1isnolongerreferenced.Thisobjectisknownasgarbage.GarbageisautomaticallycollectedbyJVM.32GarbageCollection,contTIP:Ifyouknowthatanobjectisnolongerneeded,youcanexplicitlyassignnulltoareferencevariablefortheobject.TheJVMwillautomaticallycollectthespaceiftheobjectisnotreferencedbyanyvariable

24、.33TheDateClassJavaprovidesasystem-independentencapsulationofdateandtimeinthejava.util.Dateclass.YoucanusetheDateclasstocreateaninstanceforthecurrentdateandtimeanduseitstoStringmethodtoreturnthedateandtimeasastring.34TheDateClassExampleForexample,thefollowingcodejava.util.Date date=new java.util.Dat

25、e();System.out.println(date.toString();displaysastringlikeSun Mar 09 13:50:19 EST 2003.35TheRandomClassYouhaveusedMath.random()toobtainarandomdoublevaluebetween0.0and1.0(excluding1.0).Amoreusefulrandomnumbergeneratorisprovidedinthejava.util.Randomclass.36TheRandomClassExampleIftwoRandomobjectshaveth

26、esameseed,theywillgenerateidenticalsequencesofnumbers.Forexample,thefollowingcodecreatestwoRandomobjectswiththesameseed3.Random random1=new Random(3);System.out.print(From random1:);for(int i=0;i 10;i+)System.out.print(random1.nextInt(1000)+);Random random2=new Random(3);System.out.print(nFrom rando

27、m2:);for(int i=0;i 10;i+)System.out.print(random2.nextInt(1000)+);Fromrandom1:734660210581128202549564459961Fromrandom2:73466021058112820254956445996137DisplayingGUIComponentsWhenyoudevelopprogramstocreategraphicaluserinterfaces,youwilluseJavaclassessuchasJFrame,JButton,JRadioButton,JComboBox,andJLi

28、sttocreateframes,buttons,radiobuttons,comboboxes,lists,andsoon.HereisanexamplethatcreatestwowindowsusingtheJFrameclass.TestFrameRun38TraceCodeJFrameframe1=newJFrame();frame1.setTitle(Window1);frame1.setSize(200,150);frame1.setVisible(true);JFrameframe2=newJFrame();frame2.setTitle(Window2);frame2.set

29、Size(200,150);frame2.setVisible(true);Declare,create,andassigninonestatementreferenceframe1:JFrametitle:width:height:visible:animation39TraceCodeJFrameframe1=newJFrame();frame1.setTitle(Window1);frame1.setSize(200,150);frame1.setVisible(true);JFrameframe2=newJFrame();frame2.setTitle(Window2);frame2.

30、setSize(200,150);frame2.setVisible(true);referenceframe1:JFrametitle:Window1width:height:visible:Settitlepropertyanimation40TraceCodeJFrameframe1=newJFrame();frame1.setTitle(Window1);frame1.setSize(200,150);frame1.setVisible(true);JFrameframe2=newJFrame();frame2.setTitle(Window2);frame2.setSize(200,

31、150);frame2.setVisible(true);referenceframe1:JFrametitle:Window1width:200height:150visible:Setsizepropertyanimation41TraceCodeJFrameframe1=newJFrame();frame1.setTitle(Window1);frame1.setSize(200,150);frame1.setVisible(true);JFrameframe2=newJFrame();frame2.setTitle(Window2);frame2.setSize(200,150);fr

32、ame2.setVisible(true);referenceframe1:JFrametitle:Window1width:200height:150visible:trueSetvisiblepropertyanimation42TraceCodeJFrameframe1=newJFrame();frame1.setTitle(Window1);frame1.setSize(200,150);frame1.setVisible(true);JFrameframe2=newJFrame();frame2.setTitle(Window2);frame2.setSize(200,150);fr

33、ame2.setVisible(true);referenceframe1:JFrametitle:Window1width:200height:150visible:trueDeclare,create,andassigninonestatementreferenceframe2:JFrametitle:width:height:visible:animation43TraceCodeJFrameframe1=newJFrame();frame1.setTitle(Window1);frame1.setSize(200,150);frame1.setVisible(true);JFramef

34、rame2=newJFrame();frame2.setTitle(Window2);frame2.setSize(200,150);frame2.setVisible(true);referenceframe1:JFrametitle:Window1width:200height:150visible:truereferenceframe2:JFrametitle:Window2width:height:visible:Settitlepropertyanimation44TraceCodeJFrameframe1=newJFrame();frame1.setTitle(Window1);f

35、rame1.setSize(200,150);frame1.setVisible(true);JFrameframe2=newJFrame();frame2.setTitle(Window2);frame2.setSize(200,150);frame2.setVisible(true);referenceframe1:JFrametitle:Window1width:200height:150visible:truereferenceframe2:JFrametitle:Window2width:200height:150visible:Setsizepropertyanimation45T

36、raceCodeJFrameframe1=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:Window1width:200height:150visible:truereferenceframe2:JFrametitle:Wi

37、ndow2width:200height:150visible:trueSetvisiblepropertyanimation46AddingGUIComponentstoWindowYoucanaddgraphicaluserinterfacecomponents,suchasbuttons,labels,textfields,comboboxes,lists,andmenus,tothewindow.Thecomponentsaredefinedusingclasses.Hereisanexampletocreatebuttons,labels,textfields,checkboxes,

38、radiobuttons,andcomboboxes.GUIComponentsRun47InstanceVariables,andMethodsInstancevariablesbelongtoaspecificinstance.Instancemethodsareinvokedbyaninstanceoftheclass.48StaticVariables,Constants,andMethodsStaticvariablesaresharedbyalltheinstancesoftheclass.Staticmethodsarenottiedtoaspecificobject.Stati

39、cconstantsarefinalvariablessharedbyalltheinstancesoftheclass.49StaticVariables,Constants,andMethods,cont.Todeclarestaticvariables,constants,andmethods,usethestaticmodifier.50StaticVariables,Constants,andMethods,cont.51ExampleofUsingInstanceandClassVariablesandMethodObjective:Demonstratetherolesofins

40、tanceandclassvariablesandtheiruses.ThisexampleaddsaclassvariablenumberOfObjectstotrackthenumberofCircleobjectscreated.TestCircle2RunCircle252VisibilityModifiersandAccessor/MutatorMethodsBydefault,theclass,variable,ormethodcanbeaccessedbyanyclassinthesamepackage.FpublicTheclass,data,ormethodisvisible

41、toanyclassinanypackage.FprivateThedataormethodscanbeaccessedonlybythedeclaringclass.Thegetandsetmethodsareusedtoreadandmodifyprivateproperties.53Theprivatemodifierrestrictsaccesstowithinaclass,thedefaultmodifierrestrictsaccesstowithinapackage,andthepublicmodifierenablesunrestrictedaccess.54NOTEAnobj

42、ectcannotaccessitsprivatemembers,asshownin(b).ItisOK,however,iftheobjectisdeclaredinitsownclass,asshownin(a).55WhyDataFieldsShouldBeprivate?Toprotectdata.Tomakeclasseasytomaintain.56ExampleofDataFieldEncapsulationCircle3RunTestCircle357PassingObjectstoMethodsFPassingbyvalueforprimitivetypevalue(thev

43、alueispassedtotheparameter)FPassingbyvalueforreferencetypevalue(thevalueisthereferencetotheobject)TestPassObjectRun58PassingObjectstoMethods,cont.59ArrayofObjects Circle circleArray=new Circle10;Anarrayofobjectsisactuallyanarray of reference variables.SoinvokingcircleArray1.getArea()involvestwolevelsofreferencingasshowninthenextfigure.circleArrayreferencestotheentirearray.circleArray1referencestoaCircleobject.60ArrayofObjects,cont.Circle circleArray=new Circle10;61ArrayofObjects,cont.SummarizingtheareasofthecirclesTotalAreaRun62