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Appendix G: Object-Oriented Programming with Java 6. How

webmaster — Sun, 05 Jul 2009 15:09:02 +0000

Appendix G: Object-Oriented Programming with Java 6. How is a redefined method introduced into a Java subclass? 7. The Java keyword abstract has two uses. Give examples of each and explain their usage. 8. Answer the following, giving explanations. (a) Can an interface extend another interface? (b) Can an interface extend more than one interface? (c) Can an interface subclass an abstract class? (d) Can an abstract class extend another abstract class? (e) Can a concrete class extend an interface? (f) Can a concrete class implement an interface? (g) Can a concrete class implement two interfaces and extend an abstract class? (h) Can a concrete class extend two abstract classes? 9. Develop a Person class with name and age properties. Now develop two specializations from class Person. Prepare the class Student as a subclass of Person with a matriculation number attribute. Revise the Employee class from this chapter with Person as its superclass. 10. Develop the class Employee with name and employee number attributes. Provide suitable methods for this class. Introduce into the class the abstract operation computeMonthlyPay that is redefined in the subclasses SalariedEmployee and HourlyEmployee. A salaried employee has a salary, which is paid out monthly. An hourly employee has a fixed pay rate and a number of hours worked per month. Implement the method computeMonthlyPay for these two subclasses. 11. Develop a class Account having a balance and account number attributes. Introduce the methods debit and credit that implement deposit and withdrawal transactions on a bank account. Form two specializations for the classes DepositAccount and CurrentAccount. A deposit account accrues interest and cannot be overdrawn while a current account may be overdrawn up to some overdraft limit. 12. Develop the concrete class Circle as a subclass of ClosedFigure. Test its behaviour using program G.4. 13. Develop the class Quadrilateral to represent any four-sided figure, then specialize this into the subclasses Rectangle and Square. Choose a suitable representation for the Quadrilateral and implement the operations getPerimeter and getArea. Redefine both these for the two subclasses. 14. Look up the Swing class Jframe in the Java documentation and present its super- class hierarchy. 15. Look up the Swing classes WindowListener and WindowAdapter in the Java documentation files. What is the class hierarchy of these two classes? Why is class WindowListener an interface? How is class WindowAdapter a concrete class?

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Object-Oriented Design with UML and Java public Rectangle(double

webmaster — Sun, 05 Jul 2009 05:07:52 +0000

Object-Oriented Design with UML and Java public Rectangle(double upperLeftX, double upperLeftY, double width, double height) { this(new Point(upperLeftX, upperLeftY), width, height); } public double getPerimeter() { return 2 * (theWidth + theHeight); } public double getArea() { return theWidth * theHeight; } // —– Attributes —————-private Point theUpperLeft; private double theWidth; private double theHeight; } The sample application code is given in program G.4. Here, we create instances of the Rectangle and Triangle classes and interrogate each for their perimeter and area values. Program G.4 public class Main { public static void main(String[ ] args) { Rectangle rect = new Rectangle(0.0, 4.0, 4.0, 4.0); // square 4 x 4 Triangle tri = new Triangle(0.0, 0.0, 3.0, 0.0, 3.0, 4.0); // 3-4-5 right triangle ConsoleIO.out.println( Rectangle: + rect.getPerimeter() + , +rect.getArea()); ConsoleIO.out.println( Triangle: + tri.getPerimeter() + , +tri.getArea()); } } G.4 Exercises 1. Carefully distinguish between the terms interface, abstract class and concrete class. 2. Carefully distinguish between the Java keywords extends and implements, ensuring that you are clear where they are used. 3. Carefully distinguish between the terms superclass and subclass. How may a subclass differ from its superclass? How much commonality is there? Describe what is meant by substitutability when discussing superclass and subclass. 4. What do you understand by the terms redefinition and the polymorphic effect? Explain their value to OO development. 5. What feature does a subclass constructor method use to initialize inherited attributes?

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Appendix G: Object-Oriented Programming with Java Java supports

webmaster — Sat, 04 Jul 2009 18:08:24 +0000

Appendix G: Object-Oriented Programming with Java Java supports multiple inheritance through interfaces. A class can be an extension of at most one abstract class or concrete class and implement any number of interfaces. The isA relation is still present, but now an instance of the subclass can be substituted for any of the superclasses, including the interfaces. Consider any two-dimensional closed figure such as a rectangle or a triangle for which we wish to obtain the length of its perimeter or its area. The interface ClosedFigure introduces the protocols for obtaining these values, which every concrete subclass must implement. Further, we can develop code in terms of this interface without consideration for the actual type of closed figure with which we are interacting. That way, our code is made much more adaptable. interface ClosedFigure Operations +getArea +getPerimeter Rectangle Triangle Figure G.6 Specialization hierarchy Figure G.6 shows the class hierarchy we shall develop. The code for the interface ClosedFigure is: public interface ClosedFigure { public abstract double getPerimeter(); public abstract double getArea(); } The code for the two concrete subclasses must include implementations for these two deferred methods. Below is the code for the Rectangle class using a variant of the Point class (using doubles rather than ints). The Triangle class would be developed similarly. public class Rectangle implements ClosedFigure { public Rectangle(Point upperLeft, double width, double height) { theUpperLeft = upperLeft; theWidth = width; theHeight = height; }

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Object-Oriented Design with UML and Java co.hire(e1, m3);

webmaster — Sat, 04 Jul 2009 07:33:25 +0000

Object-Oriented Design with UML and Java co.hire(e1, m3); co.hire(e2, m3); co.printEmployeeDetails(); } } The UML collaboration diagram of figure G.5 shows the configuration of objects and the message flows between them. The application, represented by the actor, sends the message printEmployeeDetails to the Company object with identity co. This Company object then sends the message printDetails in turn to the three employee objects. When the message printDetails is received by the manager object it executes the redefined version for this method. The method printDetails from class SalesManager prints the managers details then gets the names of the two employees that are members of that manager s team. e1 : Sales Employee e2 : Sales Employee co : Company m3 : SalesManager 2: printDetails 4: printDetails 3: printDetails 1: printEmployee Details 2.2: getName 2.1: getName Figure G.5 Collaboration diagram G.3 Interfaces An interface is a particular kind of abstract class in which all methods are deferred and for which there are no (non static) attributes. In effect, an interface is a pure abstract class. A Java interface is implicitly public as are all its methods. Further, these methods are implicitly qualified as abstract. It is permissible in Java to make these qualifications explicit, rendering additional documentation for the reader. A Java interface is permitted to contain class attributes but these must be static and final. Java interfaces are sometimes used to collect a number of related constants. Other classes can then refer to them using the class name qualifier or by subclassing to provide direct referencing. Since an interface has no state we say that any specialized class implements an interface. An abstract class or a concrete class can implement an interface. A subclass that is itself another interface is said to extend the interface superclass.

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Appendix G: Object-Oriented Programming with Java public void

webmaster — Fri, 03 Jul 2009 20:29:07 +0000

Appendix G: Object-Oriented Programming with Java public void hire(SalesEmployee sales, SalesManager man) { Iterator iter = theEmployees.iterator(); while(iter.hasNext() == true) { Employee emp = (Employee)iter.next(); if(emp instanceof SalesManager && emp == man) { SalesManager manager = (SalesManager)emp; manager.addSalesMember(sales); } } theEmployees.add(sales); } public void printEmployeeDetails() { ConsoleIO.out.println( Company: +theName); Iterator iter = theEmployees.iterator(); while(iter.hasNext() == true) { Employee emp = (Employee)iter.next(); emp.printDetails(); } } // —– Attributes —————-private String theName; private ArrayList theEmployees = new ArrayList(10); } Finally, the application is shown in Program G.3. The program run produces the output shown below. The manager Jessie Kennedy presents both her details as well as listing the names of her sales team. The other employees simply list their details. Company: Napier Manager: Jessie Kennedy, Professor, 1200, Budget: 8000 Sales member: Ken Barclay Sales member: John Savage Sales employee: Ken Barclay, Lecturer, 1000, Sales target: 2000 Sales employee: John Savage, Lecturer, 1200, Sales target: 2000 Program G.3 public class Main { public static void main(String[ ] args) { SalesEmployee e1 = new SalesEmployee( Ken Barclay , Lecturer , 1000, 5000); SalesEmployee e2 = new SalesEmployee( John Savage , Senior Lecturer , 1200, 2000); SalesManager m3 = new SalesManager( Jessie Kennedy , Professor , 1200, 10000); Company co = new Company( Napier ); co.hire(m3);

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Object-Oriented Design with UML and Java Concrete subclass

webmaster — Fri, 03 Jul 2009 09:45:55 +0000

Object-Oriented Design with UML and Java Concrete subclass SalesManager and SalesEmployee must provide an implementation for printDetails. For class SalesManager we first print the details for the manager, then we cycle through the sales team members printing their name. Note also the method addSalesMember for adding a new SalesEmployee object to the team overseen by the manager. public class SalesManager extends Employee { public SalesManager(String aName, String aJobTitle, int aSalary, int aBudget) { super(aName, aJobTitle, aSalary); theBudget = aBudget; } public void addSalesMember(SalesEmployee emp) { theSalesTeam.add(emp); } public void printDetails() { ConsoleIO.out.print( SalesManager: + theName); ConsoleIO.out.print( , +theJobTitle); ConsoleIO.out.print( , +theSalary); ConsoleIO.out.println( , +theBudget); Iterator iter = theSalesTeam.iterator(); while(iter.hasNext() == true) { SalesEmployee emp = (SalesEmployee)iter.next(); ConsoleIO.out.println( tSales team member: +emp.getName()); } } // —– Attributes —————-private int theBudget; private ArrayList theSalesTeam = new ArrayList(10); } Class Company is modified from the previous version. Two hire methods are available. The first simply includes a new manager as part of the workforce. The second (overloaded) hire method includes the sales employee in the company workforce, but additionally includes her into the sales team managed by a particular team leader. To implement this we must find an existing employee that is the given manager. The code for this is: if(emp instanceof SalesManager && emp == man) { … } where the first part of the conditional checks the run-time type of the employee. public class Company { public Company(String aName) { theName = aName; } public void hire(SalesManager man) { theEmployees.add(man); }

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Appendix G: Object-Oriented Programming with Java All staff

webmaster — Thu, 02 Jul 2009 21:51:35 +0000

Appendix G: Object-Oriented Programming with Java All staff have a name, job title and salary. A SalesEmployee has a sales target set against them. A SalesManager has a budget and is a manager for any number of the sales employees. -theEmployees 0..* 0..* -theSalesTeam Company SalesManager SalesEmployee abstract Employee Operations +printDetails Figure G.4 Abstract Employee class Class Employee is abstract with a deferred method printDetails. Both concrete subclasses SalesEmployee and SalesManager must define this method. For class SalesEmployee this method gives an employee summary listing their name, job title, salary and sales target. For class SalesManager the printDetails method again lists the manager s details but additionally gives the names of the sales staff in her team. Here is class Employee: public abstract class Employee { public Employee(String aName, String aJobTitle, int aSalary) { theName = aName; theJobTitle = aJobTitle; theSalary = aSalary; } public String getName() { return theName; } public abstract void printDetails(); // DEFERRED // —– Attributes —————- protected String theName; protected String theJobTitle; protected int theSalary; } Note that the class is declared as abstract. No instances of an abstract class can be created. Observe also how the deferred operation printDetails is introduced. No method body is given, simply the operation signature giving the name, return type and formal parameters, if any. The operation is qualified as abstract.

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Object-Oriented Design with UML and Java The collection

webmaster — Thu, 02 Jul 2009 11:44:58 +0000

Object-Oriented Design with UML and Java The collection is, of course, a mix of Employee and Manager objects. When we select an item from the collection we cast it to some kind of Employee object. The polymorphic effect of the operation printDetails is determined dynamically at run-time according to the type of the recipient object. This is exhibited by the program s output: Company: Napier Employee: Ken Barclay, Lecturer, 1000 Manager: John Savage, Senior Lecturer, 1200, 2000 Manager: Jessie Kennedy, Reader, 1200, 3000 According to the actual object s type the correct printDetails method is executed. Program G.2 public class Main { public static void main(String[ ] args) { Employee e1 = new Employee( Ken Barclay , Lecturer , 1000); Manager m2 = new Manager( John Savage , Senior Lecturer , 1200, 2000); Manager m3 = new Manager( Jessie Kennedy , Reader , 1200, 3000); Company co = new Company( Napier ); co.hire(e1); co.hire(m2); co.hire(m3); co.printEmployeeDetails(); } } Substitution and polymorphism combine to make our designs extensible. The Employee/Manager class hierarchy of figure G.3 may be both broadened and deepened without incurring changes elsewhere. For example, we could introduce further kinds of employees or more specialized managers. No change would be necessary to the class Company. Through substitution, any of these specialized employees may be hired by the company. The company can produce a listing of all its employees through the polymorphism of the operation printDetails. G.2 Abstract classes An abstract class has one or more abstract (or deferred) methods. An abstract method does not include a method body, only a signature. The abstract methods may be introduced in the class itself or inherited from another abstract class. These abstract methods introduce a protocol that must be respected by all subclasses. Thus, a concrete subclass of an abstract superclass must present an implementation for all inherited abstract methods. Abstract classes are illustrated with the example of figure G.4. A Company has any number of employees working for it. The employees are either sales staff or sales managers.

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Appendix G: Object-Oriented Programming with Java Employee Manager

webmaster — Wed, 01 Jul 2009 23:59:10 +0000

Appendix G: Object-Oriented Programming with Java Employee Manager Company -theEmployees 0..* Figure G.3 Company/Employee class diagram The one-to-many relationship between a Company object and its Employee objects is realized by an ArrayList object as part of the Company class. The method hire in the Company class is supplied with a handle to some Employee object that is added to the ArrayList container. In program G.2 the actual parameter objects given to method hire are instances of both the Employee class and the Manager class, illustrating the use of substitution. import java.util.*; import textio.*; public class Company { public Company(String aName) { theName = aName; } public void hire(Employee emp) { theEmployees.add(emp); } public void printEmployeeDetails() { ConsoleIO.out.println( Company: +theName); Iterator iter = theEmployees.iterator(); while(iter.hasNext() == true) { Employee emp = (Employee)iter.next(); emp.printDetails(); } } // —– Attributes —————-private String theName; private ArrayList theEmployees = new ArrayList(10); } The method printEmployeeDetails from class Company cycles through the members of the collection and sends each the message printDetails. Here, we utilize the Iterator class from the Java Collections Framework classes. The various containers provide an Iterator object through the message iterator. The principal methods from this class are hasNext (which determines if there are more elements in the collection yet to visit) and next (which returns a handle to the next available item from the collection).

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Object-Oriented Design with UML and Java Observe how

webmaster — Wed, 01 Jul 2009 12:10:08 +0000

Object-Oriented Design with UML and Java Observe how the method printDetails is reintroduced in the subclass Manager. This is an example of method redefinition. The subclass is a more specialized version of the superclass with a budgetary responsibility, and requires a different presentation from this method. Note also how this method directly references the protected attributes theName, theJobTitle and theSalary from the superclass (but see also our discussions in chapter 5). We have said that a subclass inherits all the features of the superclass. Strictly, a subclass inherits all but the superclass constructor. This is not unreasonable since a subclass will frequently have additional attributes that will require appropriate treatment under, for example, initialization. The subclass constructor will be responsible for initializing both its own and its inherited attributes. When creating an instance of the class Employee that class constructor is employed to perform the correct initialization. In a similar manner the subclass Manager achieves proper initialization of its inherited attributes through the superclass constructor. The code is: public Manager(String aName, String aJobTitle, int aSalary, int aBudget) { super(aName, aJobTitle, aSalary); theBudget = aBudget; } The first statement is responsible for invoking the superclass constructor. The remaining statement initializes the additional subclass attribute in the usual way. Program G.1 illustrates the two classes in use. An instance of the class Employee and an instance of the class Manager are first created. Both are then sent the message printDetails, producing the output: Employee: Ken Barclay, Lecturer, 1000 Manager: John Savage, Senior Lecturer, 1200, 2000 Since class Manager has redefined this method then the new effect is exhibited by the second line of the output. Program G.1 public class Main { public static void main(String[ ] args) { Employee e1 = new Employee( Ken Barclay , Lecturer , 1000); Manager m2 = new Manager( John Savage , Senior Lecturer , 1200, 2000); e1.printDetails(); m2.printDetails(); } } This next example illustrates the principle of substitution and the polymorphic effect. The class diagram is shown in figure G.3. An organization has any number of people working for it. They are classified as plain employees or managers as described in the preceding example.

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