實現(xiàn)關(guān)聯(lián)關(guān)系實用教案

1、CustomernameaddressphoneNoDockdockIDlocationelectricitywateraddSliptoDockLeaseamountstartDateendDatecalculateFee()SlipslipIDwidthslipLengthBoatstateRegistrationNoboatLengthmanufacturerYear10.10.10.11.*10.10.1類圖CoveredSlipheightdoor第2頁/共77頁第1頁/共77頁第一頁，共77頁。Bradshaw類圖顯示以下關(guān)聯(lián)關(guān)系：類圖顯示以下關(guān)聯(lián)關(guān)系： 客戶客戶(k h)擁有船只

2、；擁有船只； 船只被客戶船只被客戶(k h)擁有；擁有； 船只被指定給船臺；船只被指定給船臺； 船臺包含船只；船臺包含船只； 碼頭包含許多船臺；碼頭包含許多船臺； 船臺被連接到碼頭；船臺被連接到碼頭； 船臺被租給客戶船臺被租給客戶(k h) （Lease是一個關(guān)聯(lián)類）是一個關(guān)聯(lián)類） 客戶客戶(k h)租用船臺租用船臺 第3頁/共77頁第2頁/共77頁第二頁，共77頁。 船只必須被客戶擁有（BoatBoat和CustomerCustomer之間是強制(qingzh)(qingzh)關(guān)系） 客戶不一定擁有船只（ Customer Customer和BoatBoat之間是可選關(guān)系） 使用JavaJa

3、va實現(xiàn)關(guān)聯(lián)關(guān)系的方法： 在BoatBoat類中添加屬性：CustomerCustomer類的引用變量； 在Customer Customer 類中添加屬性： Boat Boat類的引用變量；第4頁/共77頁第3頁/共77頁第三頁，共77頁。修改修改(xigi)Customer(xigi)Customer類：類：public class Customerpublic class Customer / attribute definitions/ attribute definitions private String name;private String name; private Stri

4、ng address;private String address; private String phoneNo;private String phoneNo; / reference variable for Boat instance/ reference variable for Boat instanceprivate Boat boat;private Boat boat;第5頁/共77頁第4頁/共77頁第四頁，共77頁。/ constructor with parameterspublic Customer(String aName, String anAddress, Stri

5、ng aPhoneNo)/ invoke accessors to populate attributessetName(aName);setAddress(anAddress);setPhoneNo(aPhoneNo);/ initially no BoatsetBoat(null); 第6頁/共77頁第5頁/共77頁第五頁，共77頁。/ get accessors public String getName() return name;public String getAddress() return address;public String getPhoneNo() return ph

6、oneNo;public Boat getBoat() return boat;第7頁/共77頁第6頁/共77頁第六頁，共77頁。/ set accessorspublic void setName(String newName) name = newName;public void setAddress(String newAddress) address = newAddress;public void setPhoneNo(String newPhoneNo) phoneNo = newPhoneNo;public void setBoat(Boat aBoat) boat = aBoa

7、t; 第8頁/共77頁第7頁/共77頁第七頁，共77頁。public class TesterOneApublic static void main(String args) / create a Customer instance Customer firstCustomer = new Customer(Eleanor, Atlanta, 123-4567); / create a Boat instance Boat firstBoat = new Boat(MO34561, 28, Tartan, 2002); / set the Boat for the customer first

8、Customer.setBoat(firstBoat); Boat aBoat = firstCustomer.getBoat(); System.out.println(Customer boat information is + aBoat.getStateRegistrationNo() + + aBoat.getManufacturer() + + aBoat.getLength() + + aBoat.getYear();第9頁/共77頁第8頁/共77頁第八頁，共77頁。System.out.println(Again, customer boat information is +

9、firstCustomer.getBoat().getStateRegistrationNo() + + firstCustomer.getBoat().getManufacturer() + + firstCustomer.getBoat().getLength() + + firstCustomer.getBoat().getYear(); 第10頁/共77頁第9頁/共77頁第九頁，共77頁。Customer boat information is MO34561 Tartan 28.0 2002Again, customer boat information is MO34561 Tar

10、tan 28.0 2002Press any key to continue.第11頁/共77頁第10頁/共77頁第十頁，共77頁。修改修改Boat類實現(xiàn)另一個方向上的關(guān)系類實現(xiàn)另一個方向上的關(guān)系: 船只船只(chunzh)被客戶擁有被客戶擁有.public class Boat/ attributesprivate String stateRegistrationNo; private double length;private String manufacturer;private int year;/ reference variable points to a Customer ins

11、tanceprivate Customer customer;第12頁/共77頁第11頁/共77頁第十一頁，共77頁。/ constructorpublic Boat(String aStateRegistrationNo, double aLength,String aManufacturer, int aYear)setStateRegistrationNo(aStateRegistrationNo);setLength(aLength);setManufacturer(aManufacturer);setYear(aYear);/ initially no Customer for th

12、is boatsetCustomer(null); 第13頁/共77頁第12頁/共77頁第十二頁，共77頁。/ 建立兩個方向建立兩個方向(fngxing)上的關(guān)系上的關(guān)系public void assignBoatToCustomer(Customer aCustomer) setCustomer(aCustomer);/ point Boat to the Customer /instancecustomer.setBoat(this); / point Customer to this Boat第14頁/共77頁第13頁/共77頁第十三頁，共77頁。/ set accessor metho

13、dspublic void setStateRegistrationNo(String aStateRegistrationNo) stateRegistrationNo = aStateRegistrationNo; public void setLength(double aLength) length = aLength;public void setManufacturer(String aManufacturer) manufacturer = aManufacturer; public void setYear(int aYear)year = aYear; public void

14、 setCustomer(Customer aCustomer)customer = aCustomer; 第15頁/共77頁第14頁/共77頁第十四頁，共77頁。/ get accessor methodspublic String getStateRegistrationNo() return stateRegistrationNo; public double getLength() return length; public String getManufacturer() return manufacturer; public int getYear() return year; p

15、ublic Customer getCustomer() return customer; 第16頁/共77頁第15頁/共77頁第十五頁，共77頁。public class TesterOneBpublic static void main(String args) / create a Customer instanceCustomer firstCustomer = new Customer(Eleanor, Atlanta, 123-4567); / create a Boat instanceBoat firstBoat = new Boat(MO34561, 28, Tartan,

16、2002);/ assign the Boat to the CustomerfirstBoat.assignBoatToCustomer(firstCustomer);第17頁/共77頁第16頁/共77頁第十六頁，共77頁。/ verify Boat to Customer association:/ ask boat for its Customer reference / then use it with Customer accessorSystem.out.println(Boat owner information is + firstBoat.getCustomer().getN

17、ame() + + firstBoat.getCustomer().getAddress() + + firstBoat.getCustomer().getPhoneNo();/ verify Customer to Boat association:/ ask Customer for its Boat reference/ then use it with Boat accessorSystem.out.println(Customer boat information is + firstCustomer.getBoat().getStateRegistrationNo() + + fi

18、rstCustomer.getBoat().getManufacturer() + + firstCustomer.getBoat().getLength() + + firstCustomer.getBoat().getYear(); 第18頁/共77頁第17頁/共77頁第十七頁，共77頁。Boat owner information is Eleanor Atlanta 123-4567Customer boat information is MO34561 Tartan 28.0 2002Press any key to continue.第19頁/共77頁第18頁/共77頁第十八頁，共

19、77頁。將Boat和Customer之間的關(guān)系變?yōu)閺娭?qingzh)的,而不是可選的. 在Boat的構(gòu)造函數(shù)中增加參數(shù)：Customer引用Boat構(gòu)造函數(shù)的頭為: / constructor public Boat(String aStateRegistrationNo, double aLength, String aManufacturer, int aYear, Customer aCustomer) 構(gòu)造函數(shù)中的代碼為所有的屬性設(shè)置值,然后調(diào)用assignBoatToCustomer方法,該方法會設(shè)置Boat的Customer屬性并請求Costomer設(shè)置其boat屬性.第20頁/

20、共77頁第19頁/共77頁第十九頁，共77頁。/ constructorpublic Boat(String aStateRegistrationNo, double aLength, String aManufacturer, int aYear, Customer aCustomer)setStateRegistrationNo(aStateRegistrationNo);setLength(aLength);setManufacturer(aManufacturer);setYear(aYear);/ initially no Customer for this boat/ setCus

21、tomer(null);assignBoatToCustomer(aCustomer);第21頁/共77頁第20頁/共77頁第二十頁，共77頁。增加tellAboutSelf方法：由于Boat和Customer之間的關(guān)聯(lián)是強制的, tellAboutSelf方法可以返回與船只有關(guān)(yugun)的信息,及與擁有該船只的客戶有關(guān)(yugun)的信息.public String tellAboutSelf()String boatDetails = I am Boat + state reg number + getStateRegistrationNo() + length + getLength

22、() + Manufacturer + getManufacturer() + Year + getYear();String customerDetails = n and Owner is + customer.getName() + living in + customer.getAddress() + with phone + customer.getPhoneNo();return boatDetails + customerDetails;第22頁/共77頁第21頁/共77頁第二十一頁，共77頁。public class TesterTwopublic static void ma

23、in(String args) / create several customers Customer firstCustomer = new Customer(Eleanor, Atlanta, 123-4567); Customer secondCustomer = new Customer(JoAnn, St Louis, 987-6543); / create boats passing customer references Boat firstBoat = new Boat(MO34561, 28, Tartan, 2002, firstCustomer); Boat second

24、Boat = new Boat(MO98765, 32, Catalina, 2001, secondCustomer); / use Boat tellAboutSelf method to get back details System.out.println(firstBoat.tellAboutSelf(); System.out.println(secondBoat.tellAboutSelf(); 第23頁/共77頁第22頁/共77頁第二十二頁，共77頁。I am Boatstate reg numberMO34561length28.0ManufacturerTartanYear

25、2002 and Owner is Eleanorliving in Atlantawith phone123-4567I am Boatstate reg numberMO98765length32.0ManufacturerCatalinaYear2001 and Owner is JoAnnliving in St Louiswith phone987-6543Press any key to continue.第24頁/共77頁第23頁/共77頁第二十三頁，共77頁。 Slip與Dock在兩個方向上都有關(guān)聯(lián)關(guān)系：船臺被連接到碼頭,碼頭包含許多船臺。 實現(xiàn)一對多的關(guān)聯(lián)關(guān)系要求Dock實例

26、有一個以上船臺的引用變量，可以在Dock類中使用Vector保存(bocn)多個Slip類的引用變量。第25頁/共77頁第24頁/共77頁第二十四頁，共77頁。DockDock類定義類定義(dngy)(dngy)屬性屬性: : ID ID號號 位置位置 (location) (location) 有電有電 (electricity) (electricity) 有水有水 (water) (water) slips slips第26頁/共77頁第25頁/共77頁第二十五頁，共77頁。import java.util.*;public class Dock/ attributesprivate i

27、nt id;private String location;private boolean electricity;private boolean water;/ implement slip association with Vector classprivate Vector slips;第27頁/共77頁第26頁/共77頁第二十六頁，共77頁。/ constructorpublic Dock(int anId, String aLocation, boolean anElectricity, boolean aWater)setId(anId); setLocation(aLocatio

28、n); setElectricity(anElectricity); setWater(aWater); slips = new Vector(10); / start with Vector for 10 slips/ custom method addSlipToDockpublic void addSlipToDock(Slip aSlip)slips.addElement(aSlip); / connect dock to slip (1.*) aSlip.setDock(this); / connect slip to dock (1.1)第28頁/共77頁第27頁/共77頁第二十七

29、頁，共77頁。/ set accessor methodspublic void setId(int anId) id = anId;public void setLocation(String aLocation) location = aLocation;public void setElectricity(boolean anElectricity) electricity = anElectricity;public void setWater(boolean aWater) water = aWater;第29頁/共77頁第28頁/共77頁第二十八頁，共77頁。/ custom me

30、thod to return vector of slips public Vector getSlips() return slips;/ get accessor methodspublic int getId() return id;public String getLocation() return location;public boolean getElectricity() return electricity;public boolean getWater() return water;第30頁/共77頁第29頁/共77頁第二十九頁，共77頁。將將SlipSlip類與類與Doc

31、kDock相關(guān)聯(lián)相關(guān)聯(lián)修改修改SlipSlip類定義：類定義：增加增加DockDock引用變量引用變量(binling)(binling)作為屬性；作為屬性；修改構(gòu)造函數(shù)修改構(gòu)造函數(shù): : 增加增加DockDock引用參數(shù)，這樣，實例化引用參數(shù)，這樣，實例化SlipSlip時，就必須與時，就必須與DockDock相關(guān)聯(lián)；構(gòu)造函數(shù)中的語相關(guān)聯(lián)；構(gòu)造函數(shù)中的語句還會請求碼頭向碼頭添加船臺，以便在兩個方句還會請求碼頭向碼頭添加船臺，以便在兩個方向建立關(guān)聯(lián)關(guān)系。向建立關(guān)聯(lián)關(guān)系。第31頁/共77頁第30頁/共77頁第三十頁，共77頁。/ Slip with Boat reference and acce

32、ssors/ and Dock reference variable and accessorspublic class Slip/ attributesprivate int no;private int width;private double slipLength;private Boat boat;private Dock dock;第32頁/共77頁第31頁/共77頁第三十一頁，共77頁。/ constructor with 3 parameters plus dock referencepublic Slip(int aNo, int aWidth, double aSlipLen

33、gth, Dock aDock) / invoke accessors to populate attributessetNo(aNo);setWidth(aWidth);setSlipLength(aSlipLength);/ assign slip to an existing docksetDock(aDock);/ tell dock to associate with this slipdock.addSlipToDock(this);/ initially no boat in slipsetBoat(null); 第33頁/共77頁第32頁/共77頁第三十二頁，共77頁。/ se

34、t accessor methods public void setNo(int aNo) no = aNo; public void setWidth(int aWidth) width = aWidth; public void setSlipLength(double aSlipLength) slipLength = aSlipLength;public void setBoat(Boat aBoat) boat = aBoat;public void setDock(Dock aDock) dock = aDock; 第34頁/共77頁第33頁/共77頁第三十三頁，共77頁。/ ge

35、t accessor methodspublic int getNo() return no;public int getWidth() return width;public double getSlipLength() return slipLength;public Boat getBoat() return boat; public Dock getDock() return dock; 第35頁/共77頁第34頁/共77頁第三十四頁，共77頁。測試測試“碼頭包含船臺碼頭包含船臺”關(guān)聯(lián)關(guān)系關(guān)聯(lián)關(guān)系聲明一個聲明一個Dock及及3個個Slip引用變量引用變量(binling)，并進行，并進

36、行實例化。實例化。將將3個個Slip能夠與該能夠與該Dock關(guān)聯(lián)，且該關(guān)聯(lián)，且該Dock能夠與每個能夠與每個Slip關(guān)聯(lián)。關(guān)聯(lián)。第36頁/共77頁第35頁/共77頁第三十五頁，共77頁。/ Dock has Slips (ignores Boat and Customer for now)import java.util.*;public class TesterThreeApublic static void main(String args) / declare Dock and Slip reference variables Dock firstDock;Slip firstSlip;

37、Slip secondSlip;Slip thirdSlip;第37頁/共77頁第36頁/共77頁第三十六頁，共77頁。/ create a Dock instancefirstDock = new Dock(1, Main Cove, true, false);/ create three Slip instances for the DockfirstSlip = new Slip(1, 10, 20, firstDock);secondSlip = new Slip(2, 12, 25, firstDock);thirdSlip = new Slip(3, 14, 25, firstDo

38、ck);第38頁/共77頁第37頁/共77頁第三十七頁，共77頁。/ verify Dock to Slip association (1 to many):/ first get the Vector of slips from the DockVector slips = firstDock.getSlips();/ next use Vector size method to get number of slipsSystem.out.println(Dock 1 has + slips.size() + slips);/ iterate through Vector to get in

39、formation on each slipfor(int i = 0; i slips.size(); i+)/ get slip reference variable from slips Vector of DockSlip aSlip = (Slip) slips.elementAt(i);/ verify slip information System.out.println( Slip number + aSlip.getNo() + has width of + aSlip.getWidth()+ has length of + aSlip.getSlipLength();第39

40、頁/共77頁第38頁/共77頁第三十八頁，共77頁。/ verify slip to dock association (1:1)System.out.println(First slip is on Dock + firstSlip.getDock().getId()+ with location + firstSlip.getDock().getLocation()+ with electricity + firstSlip.getDock().getElectricity()+ and water + firstSlip.getDock().getWater(); 第40頁/共77頁第3

41、9頁/共77頁第三十九頁，共77頁。Dock 1 has 3 slips Slip number 1 has width of 10 has length of 20.0 Slip number 2 has width of 12 has length of 25.0 Slip number 3 has width of 14 has length of 25.0First slip is on Dock 1 with location Main Cove with electricity true and waterfalsePress any key to continue.第41頁/共7

42、7頁第40頁/共77頁第四十頁，共77頁。向示例中添加向示例中添加BoatBoat和和CustomerCustomer類類對對BoatBoat類進行類進行(jnxng)(jnxng)修改修改: : 增加增加SlipSlip引用屬性和存取器方法，以便與引用屬性和存取器方法，以便與SlipSlip關(guān)聯(lián)關(guān)聯(lián); ; 最最初不必將初不必將BoatBoat指定給指定給SlipSlip， 這樣構(gòu)造函數(shù)就可以將這樣構(gòu)造函數(shù)就可以將SlipSlip引用屬性設(shè)置為引用屬性設(shè)置為nullnull了。了。增加增加assignBoatToSlipassignBoatToSlip方法。方法。第42頁/共77頁第41頁/共

43、77頁第四十一頁，共77頁。public class Boat/ attributesprivate String stateRegistrationNo; private double length;private String manufacturer;private int year;/ reference variable points to a Customer instanceprivate Customer customer;/ 增加(zngji)對Slip的引用private Slip slip;第43頁/共77頁第42頁/共77頁第四十二頁，共77頁。/ constructo

44、r (adding Customer reference)public Boat(String aStateRegistrationNo, double aLength,String aManufacturer, int aYear, Customer aCustomer) setStateRegistrationNo(aStateRegistrationNo);setLength(aLength);setManufacturer(aManufacturer);setYear(aYear);/ 建立(jinl) boat 和 customer 的關(guān)聯(lián)assignBoatToCustomer(a

45、Customer);setSlip(null); / boat not in slip yet 第44頁/共77頁第43頁/共77頁第四十三頁，共77頁。/ 將 Boat指定(zhdng)給 Customerpublic void assignBoatToCustomer(Customer aCustomer) setCustomer(aCustomer);customer.setBoat(this); /將 Boat指定(zhdng)到 Slippublic void assignBoatToSlip(Slip aSlip)setSlip(aSlip);slip.setBoat(this);

46、第45頁/共77頁第44頁/共77頁第四十四頁，共77頁。/ tellAboutSelf method returning Boat and Customer informationpublic String tellAboutSelf()String boatDetails = I am a Boat + state reg number + getStateRegistrationNo() + length + getLength() + Manufacturer + getManufacturer() + Year + getYear(); String customerDetails

47、= n and Owner is + customer.getName() + living in + customer.getAddress() + with phone + customer.getPhoneNo();return boatDetails + customerDetails; 第46頁/共77頁第45頁/共77頁第四十五頁，共77頁。/ set accessor methodspublic void setStateRegistrationNo(String aStateRegistrationNo) stateRegistrationNo = aStateRegistra

48、tionNo; public void setLength(double aLength) length = aLength;public void setManufacturer(String aManufacturer) manufacturer = aManufacturer; public void setYear(int aYear) year = aYear; public void setCustomer(Customer aCustomer) customer = aCustomer; public void setSlip(Slip aSlip) slip = aSlip;

49、第47頁/共77頁第46頁/共77頁第四十六頁，共77頁。/ get accessor methodspublic String getStateRegistrationNo() return stateRegistrationNo; public double getLength() return length; public String getManufacturer() return manufacturer; public int getYear() return year; public Customer getCustomer() return customer; public

50、Slip getSlip() return slip; 第48頁/共77頁第47頁/共77頁第四十七頁，共77頁。根據(jù)目前根據(jù)目前(mqin)的關(guān)聯(lián)關(guān)系，如果有的關(guān)聯(lián)關(guān)系，如果有Customer引用，引用，即可導航以查找客戶的船只、它所在的船臺及其所在即可導航以查找客戶的船只、它所在的船臺及其所在的碼頭；的碼頭；同樣，如果有同樣，如果有Dock引用，即可查找每個船臺并導航到引用，即可查找每個船臺并導航到船臺的船只和擁有船只的客戶。船臺的船只和擁有船只的客戶。TestThreeB提供對這些關(guān)聯(lián)關(guān)系的綜合測試。提供對這些關(guān)聯(lián)關(guān)系的綜合測試。第49頁/共77頁第48頁/共77頁第四十八頁，共77頁。

51、/ TesterThreeB/ Dock has Slips with Boat and Customerimport java.util.*;public class TesterThreeBpublic static void main(String args) / declare reference variablesDock firstDock;Slip firstSlip;Slip secondSlip;Customer firstCustomer;Customer secondCustomer;Boat firstBoat;Boat secondBoat;第50頁/共77頁第49頁

52、/共77頁第四十九頁，共77頁。/ create a Dock instancefirstDock = new Dock(1, Main Cove, true, false);/ create two Slip instances for the DockfirstSlip = new Slip(1, 10, 20, firstDock);secondSlip = new Slip(2, 12, 25, firstDock);/ create two Customer instances firstCustomer = new Customer(Eleanor, Atlanta, 123-45

53、67); secondCustomer = new Customer(JoAnn, St Louis, 987-6543); 第51頁/共77頁第50頁/共77頁第五十頁，共77頁。/ create Boats passing Customer references firstBoat = new Boat(MO34561, 28, Tartan, 2002, firstCustomer);secondBoat = new Boat(MO98765, 32, Catalina, 2001, secondCustomer); / assign the Boats to the Slipsfirs

54、tBoat.assignBoatToSlip(firstSlip);secondBoat.assignBoatToSlip(secondSlip);/ verify Customer to Boat to Slip to DockSystem.out.println(Information for customer + firstCustomer.getName()+ n Boat is + firstCustomer.getBoat().getManufacturer()+ n Slip is + firstCustomer.getBoat().getSlip().getNo()+ n Do

55、ck is + firstCustomer.getBoat().getSlip().getDock().getId();第52頁/共77頁第51頁/共77頁第五十一頁，共77頁。/ verify Dock to Slip to Boat association (1 to many):/ first get the Vector of slips from the DockVector slips = firstDock.getSlips();/ next use Vector size method to get number of slipsSystem.out.println(Dock

56、1 has + slips.size() + slips);/ iterate through Vector to get information on each Slipfor(int i = 0; i slips.size(); i+)Slip aSlip = (Slip) slips.elementAt(i);/ verify Slip to Boat to Customer System.out.println( Slip number + aSlip.getNo() + has width of + aSlip.getWidth()+ has length of + aSlip.ge

57、tSlipLength()+ n containing + aSlip.getBoat().tellAboutSelf(); 第53頁/共77頁第52頁/共77頁第五十二頁，共77頁。Information for customer Eleanor Boat is Tartan Slip is 1 Dock is 1Dock 1 has 2 slips Slip number 1 has width of 10 has length of 20.0 containing I am a Boat state reg number MO34561 length 28.0 Manufacturer

58、Tartan Year 2002 and Owner is Eleanor living in Atlanta with phone 123-4567 Slip number 2 has width of 12 has length of 25.0 containing I am a Boat state reg number MO98765 length 32.0 Manufacturer Catalina Year 2001 and Owner is JoAnn living in St Louis with phone 987-6543Press any key to continue.

59、第54頁/共77頁第53頁/共77頁第五十三頁，共77頁。 LeaseLease類層次結(jié)構(gòu)是一個關(guān)聯(lián)類，表示租約是客戶(k h)(k h)和船臺之間的關(guān)聯(lián)關(guān)系，具有起始日期、終止日期、租金等屬性。 CustomerCustomer和SlipSlip之間是一對一的關(guān)聯(lián)關(guān)系，將LeaseLease連接到此關(guān)系的虛線表示每個客戶(k (k h)h)和船臺之間有一份租約第55頁/共77頁第54頁/共77頁第五十四頁，共77頁。CustomernameaddressphoneNoLeaseamountstartDateendDatecalculateFee()SlipslipIDwidthslipLen

60、gth11類圖AnnualLeasepayMonthlybalanceDuecalculateFee()DailyLeasenumberOfDayscalculateFee()第56頁/共77頁第55頁/共77頁第五十五頁，共77頁。對對LeaseLease超類進行修改：超類進行修改：使其包括使其包括SlipSlip引用引用(ynyng)(ynyng)屬性，及屬性，及CustomerCustomer引用引用(ynyng)(ynyng)屬性。屬性。增加相應(yīng)的存取器方法。增加相應(yīng)的存取器方法。第57頁/共77頁第56頁/共77頁第五十六頁，共77頁。修改(xigi)后的Lease類定義：/ Abs