More about classes

1
More about classes
syllabus

• Defining classes revisited
• Constructors
• Defining methods and passing parameters
• Visibility modifiers and encapsulation revisited

basic programming concepts
object oriented programming
topics in computer science
2
Objects and Pointers to objects

• A class defines the characteristics associated
  with an object when an object is created, it is
  allocated a block of memory sufficient to hold
  all its state variables
• Variables of type object reference hold the
  memory address of the actual location of the data
• ChessPiece bishop1
• bishop1 new ChessPiece(bishop,b,1)
• The object reference variable and the object
  itself are separate entities why?
• when a variable of type object is defined, the
  size of the memory block it will require is not
  known on the other hand, a pointer is a
  primitive type
• we can treat all object references the same way

3
Designing a class the class abstraction
Clock(int hours, int minutes, int seconds)
void secondElapsed()
int getSeconds()
int getMinutes()
int getHours()
...

• // A clock representation class clock instances
• // represent a point of time during the day
• public class Clock
• // The hours, minutes, and second read
• private int hours, minutes, seconds
• // ...

4
Method Declarations Revisited

• A method declaration begins with a method header

String seeTime (int hours, int minutes, int
seconds)
method name
parameter list
The parameter list specifies the type and name of
each parameter The name of a parameter in the
method declaration is called a formal argument
return type
5
Method Declarations

• The method header is followed by the method body

String seeTime (int hours, int minutes, int
seconds)
String result hours minutes
seconds return result
result is local data It is created each time the
method is called, and is destroyed when it
finishes executing
The return expression must be consistent with the
return type
6
Constructors

• Objects must be initialized before they can be
  used
• We must specify the initial state of the object
  before we can use it
• This is very similar to primitive data types
  when we declare a new variable of type int, for
  example, we must give it an initial value
• We specify the way an object is initialized using
  a constructor, which is a special method invoked
  every time we create a new object

7
Constructors

• // A clock representation class clock instances
• // represent a point of time during the day
• public class Clock
• // The hours, minutes, and second read
• private int hours, minutes, seconds
• // Constructs a new clock, sets the
• // clock to the time 000000
• public Clock()
• hours 0
• minutes 0
• seconds 0

8
Constructors

• The statement new Clock() does the following

9
Constructors

• The statement new Clock() does the following
• 1) Allocates the memory for a new clock object

clock hours minutes seconds
10
Constructors

• The statement new Clock() does the following
• 1) Allocates the memory for a new clock object
• 2) Initializes its state by calling the
  constructor

clock hours 0 minutes 0 seconds 0
11
Constructors Revisited

• When writing a constructor, remember that
• it has the same name as the class
• it does not return a value
• it has no return type, not even void
• it often sets the initial values of instance
  variables
• The programmer does not have to define a
  constructor for a class

12
Methods

• To make the clock object useful, we must provide
  methods that define its behavior
• Example

// Advance the clock by one hour public
void hourElapsed() hours (hours 1)
24 // Return the hour read public
int getHours() return hours
13
Modifiers of methods

• The modifier public denotes that the methods
  hourElapsed() and getHour() are part of the
  interface of the class (the services that the
  class exposes to outside world clients)
• The keyword void denotes that the method
  hourElapsed() has no return value

14
Return Types

• The return type of a method indicates the type of
  value that the method sends back to the calling
  client
• The return-type of getHours() is int when a
  client asks for the hours read of a clock it gets
  the answer as an int value
• A method that does not return a value (such as
  hourElapsed()) has a void return type
• The return statement specifies the value that
  should be returned, which must conform with the
  return type of the method

15
Method Context

• The getHours() and hourElapsed() methods are
  instance methods, which means they act on a
  particular instance of the class
• They cannot be invoked out of the blue, but
  must act on a particular object
• Clock c new Clock()
• getHours() // error of which clock?
• c.getHours() // will return 0
• An instance method is executed in the context of
  the object it acts upon

16

• public class ClockTest
• public static void main(String args)
• Clock swatch new Clock()
• Clock seiko new Clock()
• System.out.println(swatch.getHours()) // 0
• System.out.println(seiko.getHours()) // 0
• swatch.hourElapsed()
• System.out.println(swatch.getHours()) // 1
• System.out.println(seiko.getHours()) // 0

17
The this Reference

• When appearing inside an instance method, the
  this keyword denotes a reference to the object
  that the method is acting upon
• The following are equivalent
• public int getHours()
• return hours
• public int getHours()
• return this.hours

18
Method Parameters

• A method can be defined to accept zero or more
  parameters each parameter in the parameter list
  is defined by its type and name
• The parameters in the method definition are
  called formal parameters the values passed to a
  method when it is invoked are called actual
  parameters
• The name of the method together with the list of
  its formal parameters is called the signature of
  the method
• public void setTime(int hours, int minutes, int
  seconds)

19

• public class Clock
• private int hours, minutes, seconds
• // Sets the clock to the specified time.
• // If one of the parameters is not in the
  allowed
• // range, the call does not have any effect on
  the clock.
• // _at_param hours the hours to be set (0-23)
• // _at_param minutes the minutes to be set (0-59)
• // _at_param seconds the seconds to be set (0-59)
• public void setTime(int hours, int minutes, int
  seconds)
• if ((seconds 0) (seconds
• (minutes 0) (minutes
• (hours 0) (hours
• this.hours hours
• this.minutes minutes
• this.seconds seconds

20
Example a Student Object
public class student private long ID
private String maslul // Constructor
public student(long ID, String maslul)
this.ID ID this.maslul maslul
// method change maslul public void
changeMaslul(String maslul)
this.maslul maslul
Student Rachel Rachel new Student(123456789,
math) Rachel.changeMaslul(computer science)
21
Passing parameters

• Each time a method is called, the actual
  arguments in the invocation are copied into the
  formal arguments

time obj.seeTime (20, 30, 40)
return 203040
22
Example a Bank Account Object
BankAccount public BankAccount(long
accountNumber, String owner) public void
deposit(float amount) public void withdraw(float
amount) public void transfer (float amount,
BankAccount targetAccount)
23
Writing Classes

• An aggregate object is an object that contains
  references to other objects
• A BankAccount object is an aggregate object
  because it contains a reference to a String
  object (that holds the owner's name)
• An aggregate object represents a has-a
  relationship
• A bank account has a owner

24
// A bank account public class BankAccount
private long accountNumber private float
balance // The balance in dollars
private String owner // Constructs a new
empty account public BankAccount(long
accountNumber, String name)
this.accountNumber accountNumber this.owner
name this.balance 0 //
Deposites a given amount into the account
public void deposit(float amount) // ...
perhaps perform some security checks
balance balance amount
25
// Withdraws a given amount from the account
public void withdraw(float amount) //
... perhaps perform some security checks
balance balance - amount //
Transfers a given amount into another bank
account public void transfer(float amount,
BankAccount targetAcc) // ... perhaps
perform some security checks
this.withdraw(amount) targetAcc.deposit(am
ount) --------------------------------------
--------------------- BankAccount tomAccount
new BankAccount(1398723,Tom) BankAccount
shirAccount new BankAccount(1978394,Shir) tom
Account.deposit(500) // Toms balance
500 tomAccount.transfer(700,shirAccount) //
Toms balance -200, Shirs balance 700
26
Encapsulation not Among Instances of Same Class

• Sometimes object instances of the same class need
  to access each others guts (e.g., for state
  copying - if we want to create an identical
  instance of an object we have)
• Example from within a BankAccount object, any
  private member of a different BankAccount object
  can be accessed
• public void transfer(float amount, BankAccount
  targetAcc)
• // ... perhaps perform some security checks
• this.withdraw(amount)
• targetAcc.balance amount // not
  targetAcc.deposit(amount)

27
Passing Objects to Methods

• Parameters in a Java method are passed by value
  a copy of the actual parameter (the value passed
  in) is stored into the formal parameter (in the
  method header)
• Passing parameters is essentially an
  assignment
• Both primitive types and object references can be
  passed as parameters
• When an object is passed to a method, the actual
  parameter and the formal parameter become aliases