MSc IT Programming Methodology (2)

Oblong EasyScanner BankAccount

Implementing Classes Learning objectives design classes using the notation of the Unified Modelling Language (UML); write the Java code for a specified class; explain the difference between public and private access to attributes and methods; explain the use of the static keyword; pass objects as parameters.

Structured programming…

// switch on light // get building // check if passed main boolean light; int mark; String roomNumber;

Object-oriented programming…

String roomNumber; int mark; // switch on light // get building // check if passed Room Student Projector boolean light;

Looking inside a class….

Student

data Student

name number marks data

name number marks Student data methods

name number marks Student data methods getName getNumber setMarks Student

Oblong

data

Oblong data length height

Oblong data length height methods

Oblong data length height methods getLength calculateArea Oblong

The notation of the Unified Modelling Language (UML)

Oblong length height Oblong( ) setLength( ) setHeight( ) getLength( ) getHeight( ) calculateArea( ) calculatePerimeter( ) Class Name data methods attributes

Oblong length : ? height : ? Oblong( ? ) setLength( ? ): ? setHeight( ? ): ? getLength( ? ): ? getHeight( ? ): ? calculateArea( ? ): ? calculatePerimeter( ? ): ?

Oblong length : double height : ? Oblong( ? ) setLength( ? ): ? setHeight( ? ): ? getLength( ? ): ? getHeight( ? ): ? calculateArea( ? ): ? calculatePerimeter( ? ): ?

Oblong length : double height : double Oblong( ? ) setLength( ? ): ? setHeight( ? ): ? getLength( ? ): ? getHeight( ? ): ? calculateArea( ? ): ? calculatePerimeter( ? ): ?

Oblong length : double height : double Oblong( double, double ) setLength( ? ): ? setHeight( ? ): ? getLength( ? ): ? getHeight( ? ): ? calculateArea( ? ): ? calculatePerimeter( ? ): ?

Oblong length : double height : double Oblong( double, double ) setLength( double ) setHeight( ? ): ? getLength( ? ): ? getHeight( ? ): ? calculateArea( ? ): ? calculatePerimeter( ? ): ?

Oblong length : double height : double Oblong( double, double ) setLength( double ) setHeight( double ) getLength( ? ): ? getHeight( ? ): ? calculateArea( ? ): ? calculatePerimeter( ? ): ?

Oblong length : double height : double Oblong( double, double ) setLength( double ) setHeight( double ) getLength( ): double getHeight( ? ): ? calculateArea( ? ): ? calculatePerimeter( ? ): ?

Oblong length : double height : double Oblong( double, double ) setLength( double ) setHeight( double ) getLength( ): double getHeight( ): double calculateArea( ? ): ? calculatePerimeter( ? ): ?

Oblong length : double height : double Oblong( double, double ) setLength( double ) setHeight( double ) getLength( ): double getHeight( ): double calculateArea( ): double calculatePerimeter( ? ): ?

Oblong length : double height : double Oblong( double, double ) setLength( double ) setHeight( double ) getLength( ): double getHeight( ): double calculateArea( ): double calculatePerimeter( ): double

Oblong length : double height : double Oblong( double, double ) setLength( double ) setHeight( double ) getLength( ): double getHeight( ): double calculateArea( ): double calculatePerimeter( ): double public class Oblong { // attributes // methods }

public class Oblong { } double length; double height; Attributes declared outside of any methods

public class Oblong { } double length; double height;

public class Oblong { } double length; double height; Attributes should be hidden inside the class

public class Oblong { } double length; double height; private

Implementing the Oblong methods…

public class Oblong { } double length; double height; private Oblong ob1 = new Oblong (12.5, 7.5);

public class Oblong { } double length; double height; private Oblong ob1 = new Oblong (12.5, 7.5); Oblong(double ?, double ?) { } // more methods here

public class Oblong { } double length; double height; private Oblong ob1 = new Oblong (12.5, 7.5); Oblong(double ?, double ?) { } // more methods here Methods should be visible outside of the class

public class Oblong { } double length; double height; private Oblong ob1 = new Oblong (12.5, 7.5); Oblong(double ?, double ?) { } // more methods here Methods should be visible outside of the class public

public class Oblong { } double length; double height; private Oblong ob1 = new Oblong (12.5, 7.5); Oblong(double ?, double ?) { } // more methods here public A constructor has no return type

public class Oblong { } double length; double height; private Oblong ob1 = new Oblong (12.5, 7.5); Oblong(double ?, double ?) { } // more methods here public

public class Oblong { } double length; double height; private Oblong ob1 = new Oblong (12.5, 7.5); Oblong(double lengthIn, double ?) { } // more methods here public

public class Oblong { } double length; double height; private Oblong ob1 = new Oblong (12.5, 7.5); Oblong(double lengthIn, double ?) { } // more methods here public

public class Oblong { } double length; double height; private Oblong ob1 = new Oblong (12.5, 7.5); Oblong(double lengthIn, double heightIn) { } // more methods here public

public class Oblong { } double length; double height; private Oblong ob1 = new Oblong (12.5, 7.5); Oblong(double lengthIn, double heightIn) { // set the values of length and height } // more methods here public

public class Oblong { } double length; double height; private Oblong ob1 = new Oblong (12.5, 7.5); Oblong(double lengthIn, double heightIn) { // set the values of length and height } // more methods here public

public class Oblong { } double length; double height; private Oblong ob1 = new Oblong (12.5, 7.5); Oblong(double lengthIn, double heightIn) { length = lengthIn; } // more methods here public

public class Oblong { } double length; double height; private Oblong ob1 = new Oblong (12.5, 7.5); Oblong(double lengthIn, double heightIn) { length = lengthIn; height = heightIn; } // more methods here public

What happens if you don’t write a constructor?

A default constructor is added for you!

public class Oblong { } double length; double height; private Oblong() { } public // more methods here

public class Oblong { } double length; double height; private Oblong() { } public // more methods here A default constructor has no code inside it.

public class Oblong { } double length; double height; private Oblong() { } public // more methods here Oblong ob1 = new Oblong ( );

public class Oblong { } double length; double height; private Oblong() { } public // more methods here Oblong ob1 = new Oblong (12.5, 7.5);

public class Oblong { } double length; double height; private Oblong ob1 = new Oblong (12.5, 7.5);

public class Oblong { } double length; double height; private public Oblong(double lengthIn, double heightIn) { } // more methods here length = lengthIn; height = heightIn; Oblong ob1 = new Oblong (12.5, 7.5);

public class Oblong { } double length; double height; private System.out.println( ob1.getLength() ); getLength() { } publicdouble return length; // more methods here

public class Oblong { } double length; double height; private System.out.println( ob1.getHeight() ); getHeight() { } publicdouble return height; // more methods here

public class Oblong { } double length; double height; private ob1.setLength( 14.75); setLength( double ?) { } public // more methods here

public class Oblong { } double length; double height; private ob1.setLength( 14.75); setLength( double ?) { } public // more methods here void

public class Oblong { } double length; double height; private ob1.setLength( 14.75); setLength( double ?) { } public // more methods here void

public class Oblong { } double length; double height; private ob1.setLength( 14.75); setLength( double lengthIn) { } public // more methods here void

public class Oblong { } double length; double height; private ob1.setLength( 14.75); setLength( double lengthIn) { // reset length } public // more methods here void

public class Oblong { } double length; double height; private ob1.setLength( 14.75); setLength( double lengthIn) { } publicvoid length = lengthIn; // more methods here

public class Oblong { } double length; double height; private ob1.setHeight( 5.9 ); // more methods here

public class Oblong { } double length; double height; private ob1.setHeight( 5.9 ); setHeight( double heightIn) { } publicvoid height = heightIn; // more methods here

public class Oblong { } double length; double height; private System.out.println( ob1.calculateArea() ); // more methods here

public class Oblong { } double length; double height; private System.out.println( ob1.calculateArea() ); // more methods here calculateArea() { } publicdouble return length * height;

public class Oblong { } double length; double height; private System.out.println( ob1.calculatePerimeter()); // more methods here

public class Oblong { } double length; double height; private System.out.println( ob1.calculatePerimeter()); // more methods here calculatePerimeter() { } publicdouble return 2 * (length + height);

The BankAccount class….

BankAccount accountNumber : String accountName : String balance : double BankAccount (String, String) getAccountNumber() : String getAccountName() : String getBalance() : double deposit(double) withdraw(double)

public class BankAccount { private String accountNumber; private String accountName; private double balance; }

public class BankAccount { private String accountNumber; private String accountName; private double balance; }

public class BankAccount { private String accountNumber; private String accountName; private double balance; } public BankAccount(String numberIn, String nameIn) { accountNumber = numberIn; accountName = nameIn; balance = 0; }

public class BankAccount { private String accountNumber; private String accountName; private double balance; } public String getAccountName() { return accountName; }

public class BankAccount { private String accountNumber; private String accountName; private double balance; } public String getAccountNumber() { return accountNumber; }

public class BankAccount { private String accountNumber; private String accountName; private double balance; } public String getABalance() { return balance; }

public class BankAccount { private String accountNumber; private String accountName; private double balance; } public void deposit(double amountIn) { balance = balance + amountIn; }

public class BankAccount { private String accountNumber; private String accountName; private double balance; } public void withdraw(double amountIn) { balance = balance – amountIn; }

public class BankAccountTester { public static void main(String[ ] args) { BankAccount account1 = new BankAccount(" ","Susan Richards"); account1.deposit(1000); System.out.println("Account number: " + account1.getAccountNumber()); System.out.println("Account name: " + account1.getAccountName()); System.out.println("Current balance: " + account1.getBalance()); } }

Amending the BankAccount class…

“ ” “Funmi Odulopo” £ “ ” “Mary Stephenson” £ “ ” “Dilraj Mann” £ acc1 acc2 acc3 1.25% 1.25% 1.25% acc1.getBalance(); acc1.getInterestRate(); acc2.addInterest ();

“ ” “Funmi Odulopo” £ “ ” “Mary Stephenson” £ “ ” “Dilraj Mann” £ acc1 acc2 acc3 1.25% acc1.getBalance(); acc1.getInterestRate(); acc2.addInterest ();

“ ” “Funmi Odulopo” £ “ ” “Mary Stephenson” £ “ ” “Dilraj Mann” £ acc1 acc2 acc3 1.25% acc1.getBalance(); acc1.getInterestRate(); acc2.addInterest (); acc2.getInterestRate();

“ ” “Funmi Odulopo” £ “ ” “Mary Stephenson” £ “ ” “Dilraj Mann” £ acc1 acc2 acc3 1.25% acc1.getBalance(); acc1.getInterestRate(); acc2.addInterest (); acc2.getInterestRate(); acc3.deposit( 500 );

“ ” “Funmi Odulopo” £ “ ” “Mary Stephenson” £ “ ” “Dilraj Mann” £ acc1 acc2 acc3 1.25% acc1.getBalance(); acc1.getInterestRate(); acc2.addInterest (); acc2.getInterestRate(); acc3.deposit( 500 );

“ ” “Funmi Odulopo” £ “ ” “Mary Stephenson” £ “ ” “Dilraj Mann” £ acc1 acc2 acc3 1.25% acc1.getBalance(); acc1.getInterestRate(); acc2.addInterest (); acc2.getInterestRate(); acc3.deposit( 500 ); acc3.setInterestRate(1.5);

“ ” “Funmi Odulopo” £ “ ” “Mary Stephenson” £ “ ” “Dilraj Mann” £ acc1 acc2 acc3 1.25% 1.5% acc1.getBalance(); acc1.getInterestRate(); acc2.addInterest (); acc2.getInterestRate(); acc3.deposit( 500 ); acc3.setInterestRate(1.5);

“ ” “Funmi Odulopo” £ “ ” “Mary Stephenson” £ “ ” “Dilraj Mann” £ acc1 acc2 acc3 acc1.getBalance(); acc1.getInterestRate(); acc2.addInterest (); acc2.getInterestRate(); acc3.deposit( 500 ); acc3.setInterestRate(1.5); 1.5%

“ ” “Funmi Odulopo” £ “ ” “Mary Stephenson” £ “ ” “Dilraj Mann” £ acc1 acc2 acc3 1.5% acc1.getBalance(); acc2.addInterest (); acc3.deposit( 500 );

“ ” “Funmi Odulopo” £ “ ” “Mary Stephenson” £ “ ” “Dilraj Mann” £ acc1 acc2 acc3 1.5% acc1.getBalance(); acc2.addInterest (); acc3.deposit( 500 ); BankAccount.setInterestRate(1.4);

“ ” “Funmi Odulopo” £ “ ” “Mary Stephenson” £ “ ” “Dilraj Mann” £ acc1 acc2 acc3 1.4% acc1.getBalance(); acc2.addInterest (); acc3.deposit( 500 ); BankAccount.setInterestRate(1.4);

“ ” “Funmi Odulopo” £ “ ” “Mary Stephenson” £ “ ” “Dilraj Mann” £ acc1 acc2 acc3 1.4% acc1.getBalance(); acc2.addInterest (); acc3.deposit( 500 ); BankAccount.setInterestRate(1.4); BankAccount.getInterestRate( );

“ ” “Funmi Odulopo” £ “ ” “Mary Stephenson” £ “ ” “Dilraj Mann” £ acc1 acc2 acc3 1.4% acc1.getBalance(); acc2.addInterest (); acc3.deposit( 500 ); BankAccount.setInterestRate(1.4); BankAccount.getInterestRate( ); This is a class attribute.

“ ” “Funmi Odulopo” £ “ ” “Mary Stephenson” £ “ ” “Dilraj Mann” £ acc1 acc2 acc3 1.4% acc1.getBalance(); acc2.addInterest (); acc3.deposit( 500 ); BankAccount.setInterestRate(1.4); BankAccount.getInterestRate( ); This is a class method.

How do we create class attributes and class methods?

Use the static keyword!

private static double interestRate; public static void setInterestRate(double rateIn) { interestRate = rateIn; } public static double getInterestRate() { return interestRate; } public class BankAccount2 { // attributes and methods as before plus addInterest }

The addInterest method ….

public class BankAccount2 { private String accountNumber; private String accountName; private double balance; } private static double interestRate; public void addInterest() { balance = balance + (balance * interestRate)/100; }

The BankAccount2 constructor ?

public class BankAccount2 { private String accountNumber; private String accountName; private double balance; } private static double interestRate; public BankAccount2(String numberIn, String nameIn) { accountNumber = numberIn; accountName = nameIn; balance = 0; }

What about the interestRate attribute?

Java does not give an initial value to local variables but does initialize attributes!

Initializing attributes int double char Objects 0 boolean false null

public class BankAccount2 { private String accountNumber; private String accountName; private double balance; private static double interestRate ; // methods here } = 0;

The EasyScanner class…

Remember how we used EasyScanner? int x; System.out.println(“Enter number”); X = EasyScanner.nextInt() ; This must be a class method!

import java.util.*; public class EasyScanner { public static int nextInt() { Scanner sc = new Scanner(System.in); int i = sc.nextInt(); return i; } // more methods here }

Sending objects as parameters….

public class ParameterTest { public static void main(String[ ] args) { BankAccount acc = new BankAccount("1", "Samsun Okoyo"); test(acc); System.out.println("Account Number: " + acc.getAccountNumber()); System.out.println("Account Name: " + acc.getAccountName()); System.out.println("Balance: " + acc.getBalance()); } private static void test(BankAccount accIn) { accIn.deposit(2500); } }

Account Number: 1 Account Name: Samsun Okoyo Balance:

Effect on computer memory of sending an object as a parameter…

public static void main (String[] args) { BankAccount acc = new BankAccount(…) ; } Computer MemoryJava Instructions acc a BankAccount object private static void test(BankAccount accIn) { } accIn test(acc); accIn.deposit(2500);

Practical Work

Student studentNumber : String studentName : String markForMaths : int markForEnglish : int markForScience : int Student(String, String) getNumber() : String getName() : String enterMarks(int, int, int) getMathsMark() : int getEnglishMark() : int getScienceMark() : int calculateAverageMark() : double public class Student { // attributes // methods } Implement this class in your practical

Student studentNumber : String studentName : String markForMaths : int markForEnglish : int markForScience : int Student(String, String) getNumber() : String getName() : String enterMarks(int, int, int) getMathsMark() : int getEnglishMark() : int getScienceMark() : int calculateAverageMark() : double public class Student { // attributes // methods } Implement this class in your practical

Student studentNumber : String studentName : String markForMaths : int markForEnglish : int markForScience : int Student(String, String) getNumber() : String getName() : String enterMarks(int, int, int) getMathsMark() : int getEnglishMark() : int getScienceMark() : int calculateAverageMark() : double public class Student { private int markForMaths; // more attributes here // methods } Implement this class in your practical

Student studentNumber : String studentName : String markForMaths : int markForEnglish : int markForScience : int Student(String, String) getNumber() : String getName() : String enterMarks(int, int, int) getMathsMark() : int getEnglishMark() : int getScienceMark() : int calculateAverageMark() : double public class Student { private int markForMaths; // more attributes here // methods } Implement this class in your practical

Student studentNumber : String studentName : String markForMaths : int markForEnglish : int markForScience : int Student(String, String) getNumber() : String getName() : String enterMarks(int, int, int) getMathsMark() : int getEnglishMark() : int getScienceMark() : int calculateAverageMark() : double Implement this class in your practical public class Student { private int markForMaths; // more attributes here public int getMathsMark( ) { return markForMaths; } // more methods here }

Student studentNumber : String studentName : String markForMaths : int markForEnglish : int markForScience : int Student(String, String) getNumber() : String getName() : String enterMarks(int, int, int) getMathsMark() : int getEnglishMark() : int getScienceMark() : int calculateAverageMark() : double Then write a tester to check your class

Student studentNumber : String studentName : String markForMaths : int markForEnglish : int markForScience : int Student(String, String) getNumber() : String getName() : String enterMarks(int, int, int) getMathsMark() : int getEnglishMark() : int getScienceMark() : int calculateAverageMark() : double Then write a tester to check your class public class StudentTester { public static void main(String[ ] args) { } }

Student studentNumber : String studentName : String markForMaths : int markForEnglish : int markForScience : int Student(String, String) getNumber() : String getName() : String enterMarks(int, int, int) getMathsMark() : int getEnglishMark() : int getScienceMark() : int calculateAverageMark() : double Then write a tester to check your class public class StudentTester { public static void main(String[ ] args) { /* code to create at least two Student objects and call their methods */ } }