Classes and objects

Learning objectives

By the end of this lecture you should be able to:

• explain the meaning of the term object-oriented;

• explain the concept of encapsulation;

• explain the terms class and object;

• create objects in Java;

• call the methods of an object;

• Use a number of methods of the String class;

• create and use arrays of objects.

Dealing with complexity

Structured programming

boolean light;

String roomNumber

int mark;

// switch on light

// get building

// check if passed

main

Object-oriented programming

boolean light;

String roomNumber

int mark;

// switch on light

// get building

// check if passed

RoomStudentProjector

Classes

A class is the blueprint from which objects are generated;

Student

data

methods

Class

Objects

Student s1;

Student s2;

Student s3;

Student s4;

A class can also be thought of as a type!

?

When we use a class as a data type, we need information about its methods only.

Creating Objects

Robot

?

moveRight(int)

moveLeft(int)

moveUp(int)

moveDown(int)

Robot( ) int, int

Robot r1;

r1 = new Robot(5, 50);

Robot r2;

r2 = new Robot(10, 2);

r1.moveRight(20);

r2.moveDown(25);

A constructor is a special method that generates an object from a class.

Effect on computer memory

Robot r1 ;

Computer Memory Java Instructions

r1

This is the space for the new Robot

object

r1 = new Robot( 5, 50);

null

Pre-defined Java classes

We have already been using a few pre-defined classes in our programs:

Scanner

String

Revisiting the Scanner and String classes

To create a Scanner object we call the Scanner constructor:

Scanner sc = new Scanner (System.in);

The String class is different from all other classes in Java.

We can create String objects without a call to a constructor:

String s = “Wednesday”; new String (“Wednesday”);

Revisiting the Scanner and String classes

We also used methods of the Scanner and String classes.

int x = sc.nextInt();

String s = sc.next();

char reply = sc.next().charAt(0);

charAt

Accepts an integer and returns the character at that position in the string. Note that indexing starts from zero, not 1!

An item of type

int

An item of type

char

length Returns the length of the string .

None An item of type

int

substring Accepts two integers (eg m and n) and returns a copy of a chunk of the string, from position m to position n-1.

Two items of type

int

A String

object

concat Accepts a string and returns a new string consisting of the that string joined to the end of the original string.

A String

object

A String

object

Method Description Inputs Output

toUpperCase

Returns a copy of the original string, all upper case.

None A String object

toLowerCase Returns a copy of the original string, all lower case.

None A String object

compareTo

Accepts a string and compares it to the object's string. It returns zero if the strings are identical, a negative number if the object's string comes first in the alphabet, and a positive number if it comes later.

A String object

An item of type

int.

equals Accepts an object and compares this to this to the string. It returns true if these are identical, otherwise returns false.

An object of any class

A boolean

value.

Method Description Inputs Output

import java.util.*;

public class StringTest

{

public static void main(String[] args)

{

Scanner sc = new Scanner(System.in);

String str = new String();

System.out.print("Enter a string: ");

str = sc.next();

System.out.println("The length of the string is " + str.length() );

System.out.println("The character at position 3 is " + str.charAt(2) );

System.out.println("Characters 2 to 4 are " + str.substring(1,4) );

System.out.println( str.concat(" was the string entered") );

System.out.println("This is upper case: " + str.toUpperCase() );

System.out.println("This is lower case: " + str.toLowerCase() );

}

}

Enter a string: Europe

The length of the string is 6

The character at position 3 is r

Characters 2 to 4 are uro

Europe was the string entered

This is upper case: EUROPE

This is lower case: europe

RUN

import java.util.*;

public class StringTest

{

public static void main(String[] args)

{

Scanner sc = new Scanner(System.in);

String str = new String();

System.out.print("Enter a string: ");

str = sc.next();

System.out.println("The length of the string is " + str.length());

System.out.println("The character at position 3 is " + str.charAt(2));

System.out.println("Characters 2 to 4 are " + str.substring(1,4));

System.out.println(str.concat(" was the string entered"));

System.out.println("This is upper case: " + str.toUpperCase());

System.out.println("This is lower case: " + str.toLowerCase());

}

}

Comparing strings

When comparing two objects, such as Strings, we should

do so by using a method called equals;

We should not use the equality operator (==);

if(firstString == secondString)

{

// more code here

}

String firstString = “Hello”;

String secondString = “Goodbye”;

if(firstString.equals(secondString))

import java.util.*;

public class StringComp

{

public static void main(String[] args)

{

Scanner sc = new Scanner(System.in);

String string1, string2;

int comparison;

System.out.print("Enter a String: ");

string1 = sc.next();

System.out.print("Enter another String: ");

string2 = sc.next();

comparison = string1.compareTo(string2);

// check comparison

}

}

if (comparison < 0)

{

System.out.println(string1 + " comes before " + string2

+ " in the alphabet");

}

else if (comparison > 0)

{

System.out.println(string2 + " comes before " + string1

+ " in the alphabet");

}

else

{

System.out.println("The strings are identical");

}

RUN

Enter a String:

Enter another String:

goodbye comes before hello in the alphabet

hello

goodbye

Using some classes written for you

We have written some classes for you to use:

Oblong

EasyScanner

BankAccount

Next week we will show you how we wrote them.

Methods of the Oblong class

The constructor.

Takes two items of data, both of type double, representing the length and height of the oblong respectively.

Oblong

Oblong myOblong

12.5

20

= new Oblong(12.5, 20);

Methods of the Oblong class

Takes an item type double and re-sets the length of the oblong.

Returns no value.

setLength

myOblong.setlength(17.5);

12.5

20

17.5

Methods of the Oblong class

Takes an item type double and re-sets the height of the oblong.

Returns no value.

setHeight

myOblong.setHeight(12);

20

17.5

12

Methods of the Oblong class

Takes no parameters and returns the value of the height of the oblong

getHeight

System.out.println( );

17.5

12

myOblong.getHeight( )

Methods of the Oblong class

Takes no parameters and returns the value of the length of the oblong.

getLength

System.out.println( );

17.5

12

myOblong.getLength( )

Methods of the Oblong class

Takes no parameters and returns the area of the oblong.

calculateArea

System.out.println( );

17.5

12

myOblong.calculateArea( )

Methods of the Oblong class

Takes no parameters and returns the perimiter of the oblong.

calculatePerimeter

System.out.println( );

17.5

12

myOblong.calculatePerimeter( )

The EasyScanner class

Issues with using the Scanner class for keyboard input

• it is necessary to create a new Scanner object in every method that uses the Scanner class;

• there is no simple method such as nextChar for getting a single character like there is for the int and double types;

• the next method doesn't allow us to enter strings containing spaces.

The input methods of the EasyScanner class

Java type EasyScanner method

int nextInt()

double nextDouble()

char nextChar()

String nextString()

Examples of using EasyScanner

char c;

System.out.print("Enter a character: ");

c = EasyScanner.nextChar();

String s;

System.out.print("Enter a string: ");

c = EasyScanner.nextString();

The BankAccount class

Contains methods to process bank accounts.

BankAccount

99786754Susan Richards

500.0

46376205 Sumana Khan

150.0

The methods of the BankAccount classMethod Description Inputs Output

BankAccount A constructor. It accepts two strings and assigns them to the account number and account name respectively. It also sets the account balance to zero.

Two String objects

Not applicable

getAccountNumber Returns the account number. None An item of type String

getAccountName Returns the account name. None An item of type String

getBalance Returns the balance. None An item of type double

deposit Accepts an item of type double and adds it to the balance

An item of type double

None

withdraw Accepts an item of type double and subtracts it from the balance

An item of type double

None

public class BankAccountTester

{

public static void main(String[ ] args)

{

BankAccount account1

= new BankAccount("99786754","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());

}

}

Account number: 99786754

Account name: Susan Richards

Current balance: 1000.0

Arrays of objects

If we have to process many BankAccount objects, we can create an array of BankAccounts.

int[] someArray = new int[3];

An array of integers

An array of BankAccount objects

BankAccount[] accountList = new BankAccount[3];

The effect on memory of creating an array of objects

null

null

null

accountList

accountList

= new BankAccount [3];

reference to bank account

reference to bank account

reference to bank account

Java instructionComputer memory

BankAccount[] accountList;

null

Adding bank accounts to the array

accountList[0] = new BankAccount("99786754","Susan Richards");

accountList[1] = new BankAccount("44567109","Delroy Jacobs");

accountList[2] = new BankAccount("46376205","Sumana Khan");

accountList[1]

the BankAccountobject with account

number "46376205" and name "Sumana Khan"

accountList[2]

accountList[0]

the BankAccountobject with account

number "99786754" and name "Susan Richards"

the BankAccountobject with account

number "44567109" and name "Delroy Jacobs"

accountList

Accessing objects in an array

accountList[0].deposit(1000);

returns a BankAccount object

calls a BankAccount method

accountList[0].withdraw(500);

accountList[2].deposit(150);

public class BankAccountTester2

{

public static void main(String[] args)

{

BankAccount[] accountList = new BankAccount[3];

accountList[0] = new BankAccount("99786754","Susan Richards");

accountList[1] = new BankAccount("44567109","Delroy Jacobs");

accountList[2] = new BankAccount("46376205","Sumana Khan");

accountList[0].deposit(1000);

accountList[2].deposit(150);

accountList[0].withdraw(500);

for(BankAccount item : accountList)

{

System.out.println("Account number: " + item.getAccountNumber());

System.out.println("Account name: " + item.getAccountName());

System.out.println("Current balance: " + item.getBalance());

System.out.println();

}

}

}

Account number: 99786754

Account name: Susan Richards

Current balance: 500.0

Account number: 44567109

Account name: Delroy Jacobs

Current balance: 0.0

Account number: 46376205

Account name: Sumana Khan

Current balance: 150.0