java语言程序设计 基础篇 原书第八版 课件 PPT(第十七章).ppt

1 Chapter8ObjectsandClasses 2 Motivations Afterlearningtheprecedingchapters youarecapableofsolvingmanyprogrammingproblemsusingselections loops methods andarrays However theseJavafeaturesarenotsufficientfordevelopinggraphicaluserinterfacesandlargescalesoftwaresystems Supposeyouwanttodevelopagraphicaluserinterfaceasshownbelow Howdoyouprogramit 3 Objectives Todescribeobjectsandclasses 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 4 OOProgrammingConcepts Object orientedprogramming OOP involvesprogrammingusingobjects Anobjectrepresentsanentityintherealworldthatcanbedistinctlyidentified Forexample astudent adesk acircle abutton andevenaloancanallbeviewedasobjects Anobjecthasauniqueidentity state andbehaviors Thestateofanobjectconsistsofasetofdatafields alsoknownasproperties withtheircurrentvalues Thebehaviorofanobjectisdefinedbyasetofmethods 5 Objects Anobjecthasbothastateandbehavior Thestatedefinestheobject andthebehaviordefineswhattheobjectdoes 6 Classes Classesareconstructsthatdefineobjectsofthesametype AJavaclassusesvariablestodefinedatafieldsandmethodstodefinebehaviors Additionally aclassprovidesaspecialtypeofmethods knownasconstructors whichareinvokedtoconstructobjectsfromtheclass 7 Classes 8 UMLClassDiagram 9 Example DefiningClassesandCreatingObjects Objective Demonstratecreatingobjects accessingdata andusingmethods TestCircle1 Run 10 Example DefiningClassesandCreatingObjects Objective Demonstratecreatingobjects accessingdata andusingmethods TestTV Run TV 11 Constructors Circle Circle doublenewRadius radius newRadius Constructorsareaspecialkindofmethodsthatareinvokedtoconstructobjects 12 Constructors cont Aconstructorwithnoparametersisreferredtoasano argconstructor Constructorsmusthavethesamenameastheclassitself Constructorsdonothaveareturntype notevenvoid Constructorsareinvokedusingthenewoperatorwhenanobjectiscreated Constructorsplaytheroleofinitializingobjects 13 CreatingObjectsUsingConstructors newClassName Example newCircle newCircle 5 0 14 DefaultConstructor Aclassmaybedeclaredwithoutconstructors Inthiscase ano argconstructorwithanemptybodyisimplicitlydeclaredintheclass Thisconstructor calledadefaultconstructor isprovidedautomaticallyonlyifnoconstructorsareexplicitlydeclaredintheclass 15 DeclaringObjectReferenceVariables Toreferenceanobject assigntheobjecttoareferencevariable Todeclareareferencevariable usethesyntax ClassNameobjectRefVar Example CirclemyCircle 16 Declaring CreatingObjectsinaSingleStep ClassNameobjectRefVar newClassName Example CirclemyCircle newCircle Createanobject Assignobjectreference 17 AccessingObjects Referencingtheobject sdata objectRefVar datae g myCircle radiusInvokingtheobject smethod objectRefVar methodName arguments e g myCircle getArea 18 TraceCode CirclemyCircle newCircle 5 0 SCircleyourCircle newCircle yourCircle radius 100 DeclaremyCircle novalue myCircle animation 19 TraceCode cont CirclemyCircle newCircle 5 0 CircleyourCircle newCircle yourCircle radius 100 novalue myCircle Createacircle animation 20 TraceCode cont CirclemyCircle newCircle 5 0 CircleyourCircle newCircle yourCircle radius 100 referencevalue myCircle AssignobjectreferencetomyCircle animation 21 TraceCode cont CirclemyCircle newCircle 5 0 CircleyourCircle newCircle yourCircle radius 100 referencevalue myCircle novalue yourCircle DeclareyourCircle animation 22 TraceCode cont CirclemyCircle newCircle 5 0 CircleyourCircle newCircle yourCircle radius 100 referencevalue myCircle novalue yourCircle CreateanewCircleobject animation 23 TraceCode cont CirclemyCircle newCircle 5 0 CircleyourCircle newCircle yourCircle radius 100 referencevalue myCircle referencevalue yourCircle AssignobjectreferencetoyourCircle animation 24 TraceCode cont CirclemyCircle newCircle 5 0 CircleyourCircle newCircle yourCircle radius 100 referencevalue myCircle referencevalue yourCircle ChangeradiusinyourCircle animation 25 Caution RecallthatyouuseMath 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 26 ReferenceDataFields Thedatafieldscanbeofreferencetypes Forexample thefollowingStudentclasscontainsadatafieldnameoftheStringtype publicclassStudent Stringname namehasdefaultvaluenullintage agehasdefaultvalue0booleanisScienceMajor isScienceMajorhasdefaultvaluefalsechargender chasdefaultvalue u0000 27 ThenullValue Ifadatafieldofareferencetypedoesnotreferenceanyobject thedatafieldholdsaspecialliteralvalue null 28 DefaultValueforaDataField Thedefaultvalueofadatafieldisnullforareferencetype 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 29 Example publicclassTest publicstaticvoidmain String args intx xhasnodefaultvalueStringy yhasnodefaultvalueSystem out println xis x System out println yis y Compilationerror variablesnotinitialized Javaassignsnodefaultvaluetoalocalvariableinsideamethod 30 DifferencesbetweenVariablesofPrimitiveDataTypesandObjectTypes 31 CopyingVariablesofPrimitiveDataTypesandObjectTypes 32 GarbageCollection Asshowninthepreviousfigure aftertheassignmentstatementc1 c2 c1pointstothesameobjectreferencedbyc2 Theobjectpreviouslyreferencedbyc1isnolongerreferenced Thisobjectisknownasgarbage GarbageisautomaticallycollectedbyJVM 33 GarbageCollection cont TIP Ifyouknowthatanobjectisnolongerneeded youcanexplicitlyassignnulltoareferencevariablefortheobject TheJVMwillautomaticallycollectthespaceiftheobjectisnotreferencedbyanyvariable 34 TheDateClass Javaprovidesasystem independentencapsulationofdateandtimeinthejava util Dateclass YoucanusetheDateclasstocreateaninstanceforthecurrentdateandtimeanduseitstoStringmethodtoreturnthedateandtimeasastring 35 TheDateClassExample Forexample thefollowingcodejava util Datedate newjava util Date System out println date toString displaysastringlikeSunMar0913 50 19EST2003 36 TheRandomClass YouhaveusedMath random toobtainarandomdoublevaluebetween0 0and1 0 excluding1 0 Amoreusefulrandomnumbergeneratorisprovidedinthejava util Randomclass 37 TheRandomClassExample IftwoRandomobjectshavethesameseed 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 38 DisplayingGUIComponents Whenyoudevelopprogramstocreategraphicaluserinterfaces youwilluseJavaclassessuchasJFrame JButton JRadioButton JComboBox andJListtocreateframes buttons radiobuttons comboboxes lists andsoon HereisanexamplethatcreatestwowindowsusingtheJFrameclass TestFrame Run 39 TraceCode JFrameframe1 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 andassigninonestatement reference frame1 JFrametitle width height visible animation 40 TraceCode JFrameframe1 newJFrame frame1 setTitle Window1 frame1 setSize 200 150 frame1 setVisible true JFrameframe2 newJFrame frame2 setTitle Window2 frame2 setSize 200 150 frame2 setVisible true reference frame1 JFrametitle Window1 width height visible Settitleproperty animation 41 TraceCode JFrameframe1 newJFrame frame1 setTitle Window1 frame1 setSize 200 150 frame1 setVisible true JFrameframe2 newJFrame frame2 setTitle Window2 frame2 setSize 200 150 frame2 setVisible true reference frame1 JFrametitle Window1 width 200height 150visible Setsizeproperty animation 42 TraceCode JFrameframe1 newJFrame frame1 setTitle Window1 frame1 setSize 200 150 frame1 setVisible true JFrameframe2 newJFrame frame2 setTitle Window2 frame2 setSize 200 150 frame2 setVisible true reference frame1 JFrametitle Window1 width 200height 150visible true Setvisibleproperty animation 43 TraceCode JFrameframe1 newJFrame frame1 setTitle Window1 frame1 setSize 200 150 frame1 setVisible true JFrameframe2 newJFrame frame2 setTitle Window2 frame2 setSize 200 150 frame2 setVisible true reference frame1 JFrametitle Window1 width 200height 150visible true Declare create andassigninonestatement reference frame2 JFrametitle width height visible animation 44 TraceCode JFrameframe1 newJFrame frame1 setTitle Window1 frame1 setSize 200 150 frame1 setVisible true JFrameframe2 newJFrame frame2 setTitle Window2 frame2 setSize 200 150 frame2 setVisible true reference frame1 JFrametitle Window1 width 200height 150visible true reference frame2 JFrametitle Window2 width height visible Settitleproperty animation 45 TraceCode JFrameframe1 newJFrame frame1 setTitle Window1 frame1 setSize 200 150 frame1 setVisible true JFrameframe2 newJFrame frame2 setTitle Window2 frame2 setSize 200 150 frame2 setVisible true reference frame1 JFrametitle Window1 width 200height 150visible true reference frame2 JFrametitle Window2 width 200height 150visible Setsizeproperty animation 46 TraceCode JFrameframe1 newJFrame frame1 setTitle Window1 frame1 setSize 200 150 frame1 setVisible true JFrameframe2 newJFrame frame2 setTitle Window2 frame2 setSize 200 150 frame2 setVisible true reference frame1 JFrametitle Window1 width 200height 150visible true reference frame2 JFrametitle Window2 width 200height 150visible true Setvisibleproperty animation 47 AddingGUIComponentstoWindow Youcanaddgraphicaluserinterfacecomponents suchasbuttons labels textfields comboboxes lists andmenus tothewindow Thecomponentsaredefinedusingclasses Hereisanexampletocreatebuttons labels textfields checkboxes radiobuttons andcomboboxes GUIComponents Run 48 InstanceVariables andMethods Instancevariablesbelongtoaspecificinstance Instancemethodsareinvokedbyaninstanceoftheclass 49 StaticVariables Constants andMethods Staticvariablesaresharedbyalltheinstancesoftheclass Staticmethodsarenottiedtoaspecificobject Staticconstantsarefinalvariablessharedbyalltheinstancesoftheclass 50 StaticVariables Constants andMethods cont Todeclarestaticvariables constants andmethods usethestaticmodifier 51 StaticVariables Constants andMethods cont 52 ExampleofUsingInstanceandClassVariablesandMethod Objective Demonstratetherolesofinstanceandclassvariablesandtheiruses ThisexampleaddsaclassvariablenumberOfObjectstotrackthenumberofCircleobjectscreated TestCircle2 Run Circle2 53