OOD – Identifying Classes

• Identifying the nouns (and noun phrases) in the application description

• The attributes correspond to descriptive words• The behavior corresponds to public methods

contained in their corresponding classes• Occasionally classes are not build to generate

objects but to collect static methods and constants.– Utility classes, e.g., Math, Color, Integer

• Create classes which represent a single concept

class Purse { public Purse(); public void addNickels(int

count); public addDimes(int count); public addQuarters(int count); public double getTotal();}

class Coin{ public Coin(double value, String

aName); public double getValue();}class Purse { public Purse(); public void add(Coin aCoin); public double getTotal();}Two concepts:

1. a purse that holds coins, and

2. the value of each coin.

Class Purse uses/depends on class Coin

Case Study: Invoices

• Write a set of classes to support the creation and printing of invoices

• Classes:– Invoice

– Product

– LineItem

– Address

public class Product { public Product(String aDescription, double aPrice) { description = aDescription; price = aPrice; } public String getDescription() { return description; } public double getPrice() { return price; } private String description; private double price;}public class LineItem{ public LineItem(Product aProduct, int aQuantity) { theProduct = aProduct; quantity = aQuantity; } public double getTotalPrice() { return theProduct.getPrice() * quantity; } public String format() { return String.format("%-30s%8.2f%5d%8.2f", theProduct.getDescription(), theProduct.getPrice()

, quantity, getTotalPrice()); }  private int quantity; private Product theProduct;} 

Product and LineItem classes

public class Address{ public Address(String aName, String aStreet, String aCity, String aState, String aZip) { name = aName; street = aStreet; city = aCity; state = aState; zip = aZip; }   public String format() { return name + "\n" + street + "\n" + city + ", " + state + " " + zip; } private String name; private String street; private String city; private String state; private String zip;} 

Address class

public class Invoice{ public Invoice(Address anAddress) { items = new ArrayList<LineItem>(); billingAddress = anAddress; } public void add(Product aProduct, int quantity) { LineItem anItem = new LineItem(aProduct, quantity); items.add(anItem); } public String format() { String r = " I N V O I C E\n\n" + billingAddress.format() + String.format("\n\n%-30s%8s%5s%8s\n", "Description", "Price", "Qty", "Total");  for (LineItem i : items) { r = r + i.format() + "\n"; }  r = r + String.format("\nAMOUNT DUE: $%8.2f", getAmountDue()); return r; }  public double getAmountDue() { double amountDue = 0; for (LineItem i : items) { amountDue = amountDue + i.getTotalPrice(); } return amountDue; } private Address billingAddress; private ArrayList<LineItem> items;}

Invoice class

public class InvoiceTester{ public static void main(String[] args) { Address samsAddress = new Address("Sam's Small Appliances", "100 Main Street", "Anytown", "CA", "98765"); Invoice samsInvoice = new Invoice(samsAddress); samsInvoice.add(new Product("Toaster", 29.95), 3); samsInvoice.add(new Product("Hair dryer", 24.95), 1); samsInvoice.add(new Product("Car vacuum", 19.99), 2); System.out.println(samsInvoice.format()); }}

InvoiceTester

UML

• Universal Modeling Language (UML)– a standard  language for specifying, visualizing,

constructing, and documenting the artifacts of software systems, as well as for business modeling and other non-software systems.

• Class diagram– a type of static structure diagram that describes the

structure of a system – class name, attributes, methods– relationship between classes

• Object diagram– shows the snapshot of a modeled system, at a specific point

in time – Not required

A Case Study

• An Application Centre for university admissions• A student can choose 3 different universities, and

let’s assume that this center will accept up to 100 different applications.

• This program has to allow the user to: – enter the student applications,

– accept students in some of the universities that they chose,

– display the status of the applications.

Class Candidates Based on Nouns

• Application Centre• University Admissions• Student• Universities• Applications• Status of Applications

Two classes: ApplicationCentre and StudentApplication

✓

✗ ✓ ✓ ✓

✗

Too simple to constitute separate classes

Should be a property

UML Class Diagram

Application Centre

- st[100]: StudentApplication

- name : String

+ addStudent(): boolean

+ getStudent(int): StudentApplication

StudentApplication

- name: String

- univ1, univ2, univ3: String

- acc1, acc2, acc3: boolean

+ setAcceptance(): void

+ toString(): String

*1

1. class StudentApplication{2. private String name;3. private String university0;4. private String university1;5. private String university2;6. private boolean accept0;7. private boolean accept1;8. private boolean accept2;9. public StudentApplication (String n, String u0, String u1, String u2){10. name = n;11. university0=u0;12. university1=u1;13. university2=u2;14. accept0=accept1=accept2=false;15. }16. public void setAcceptance(int which, boolean decision){17. switch(which){18. case 0: accept0=decision; break;19. case 1: accept1=decision; break;20. case 2: accept2=decision; break;21. }22. }23. public String toString(){24. String result = name + ":\n";25. result += university0;26. if (accept0) result += " - accepted\n";27. else result += " - rejected\n";28. result += university1;29. if (accept1) result += " - accepted\n";30. else result += " - rejected\n";31. result += university2;32. if (accept2) result += " - accepted\n";33. else result += " - rejected\n";34. return result;35. }36. }

1. class ApplicationCentre {2. private String name;3. private StudentApplication[] st;4. private int studentCount;5. private int size;6. 7. public ApplicationCentre(String s){8. name=s;9. size=100;10. st = new StudentApplication[size];11. studentCount=0;12. }13. public String getName() {14. return name;15. }16. public boolean addStudent(StudentApplication s){17. if (studentCount==size) return false;18. st[studentCount]=s;19. studentCount ++;20. return true;21. }22. public StudentApplication getStudent(int which){23. if ( which < 0 || which > studentCount-1){24. return null;25. }26. return st[which];27. }28.}

JOptionPane

• a class in javax.swing

Method Name Description

showConfirmDialog Asks a confirming question, like yes/no/cancel.

showInputDialog Prompt for some input.

showMessageDialog Tell the user about something that has happened.

showOptionDialog The Grand Unification of the above three.

1. import javax.swing.JOptionPane;2. public class Applications {3. public static void main( String args[] ) {4. ApplicationCentre appCentre= new ApplicationCentre (“OUAC");5. String stopper="quit";6. int nrStud=0;7. JOptionPane.showMessageDialog(null,"Enter student names, each followed by 3 choices8. of universities. To stop enter for name the word quit","Input", JOptionPane.PLAIN_MESSAGE);9. String n = JOptionPane.showInputDialog("Student's name ?");10. boolean flag = true;11. while ( !n.equals(stopper) && flag){12. String u1 = JOptionPane.showInputDialog("1st university ?");13. String u2 = JOptionPane.showInputDialog("2nd university ?");14. String u3 = JOptionPane.showInputDialog("3rd university ? ");15. StudentApplication s=new StudentApplication (n, u1, u2, u3);16. flag = appCentre.addStudent(s); 17. n = JOptionPane.showInputDialog("Student's name ?");18. nrStud++;19. }20.21. String line = JOptionPane.showInputDialog(" Indicate which applications are to be 22. accepted by entering the student index number. To stop enter for name the word quit");23. while (!line.equals(stopper)) {24. int stnr = Integer.parseInt(line);25. line = JOptionPane.showInputDialog("Enter university index 0..2");26. int anr = Integer.parseInt(line);27. appCentre.getStudent(stnr).setAcceptance(anr, true);28. line= JOptionPane.showInputDialog(" Indicate which applications are to be accepted by29. entering the student index number. To stop enter for name the word quit");30. }31. 32. JOptionPane.showMessageDialog(null, " Records for all applicants to "+ appCentre.getName() +33. "\n","Records", JOptionPane.PLAIN_MESSAGE);34. for (int i = 0; i < nrStud; i++)35. JOptionPane.showMessageDialog(null,appCentre.getStudent(i).toString() ,"Records", 36. JOptionPane.PLAIN_MESSAGE);37. }38.}

Arrays

• Related data items of same type• Group of contiguous memory locations

• Each memory location has same name

• Each memory location has same type

A 12-element array

-45

6

0

72

1543

-89

0

62

-3

1

6453

78

c[ 1 ]

c[ 2 ]

c[ 4 ]

c[ 3 ]

c[ 5 ]

c[ 6 ]

c[ 7 ]

c[ 8 ]

c[ 9 ]

c[ 10 ]

c[ 11 ]

c[ 0 ]Name of array (Notethat all elements ofthis array have thesame name, c)

Position number (indexof subscript) of the element within array c

Declaring and Allocating Arrays

• Declaring and allocating arrays– Arrays are objects that occupy memory

– Allocated dynamically with operator new int c[] = new int[ 12 ];

– Equivalent to int c[]; // declare array c = new int[ 12 ]; // allocate array

• We can allocate arrays of objects too

String b[] = new String[ 100 ];

• Obtaining the size of an array– c.length

Initializing Arrays

• Initialize array elements– Use initializer list

• Items enclosed in braces ({})

• Items in list separated by commas

int n[] = { 10, 20, 30, 40, 50 };– Creates a five-element array

– Subscripts of 0, 1, 2, 3, 4

– Do not need operator new

Arrays of Objects

• Allocating memory for an array does not automatically allocate the memory for each element! – Only references to the elements are created

– The objects need to be allocated separately

• ExampleAccount accounts[] = new Account[12];

accounts[0].getBalance(); // wrong, Account[0] is not initialized

Account accounts[] = new Account[12];

for (int i = 0; i < accounts.length; i++)

accounts[i] = new Account(); // initializing the array elements

accounts[0].getBalance(); // correct