Using Objects and Classes

1
Using Objects and Classes

• Object creation
• Object reference
• Class definition
• constructors
• methods
• variables

2
Object Creation

• When we express a computation using a diagram, we
  never have to create any objects, we just draw
  them and then send messages to them.
• In a written program, we must provide some
  instructions to create objects before we can send
  any messages to them.

3
Object Creation

• Every object must be created before it can be
  used.
• An object is created by a constructor
  specified in the corresponding class.
• A class may have more than one constructor.
• In many languages, the name of the constructor is
  the name of the class.

4
Example
import java.io. import java.util. / to
demo the use of constructors / public class
date_demo public static void main( String
args ) Date today new Date()
Date otherday new Date(20000000 )
System.out.println(" Today is "
today.toString() ) System.out.println(" The
other day is " otherday.toString() )
5
Example
gtjava date_demo Today is Sun Sep 19 190240 MDT
1999 The other day is Wed Dec 31 223320 MST
1969
6
Constructor for Date( from jdk )

• Date()
• Allocates a Date object and initializes it so
  that it represents the time at which it was
  allocated measured to the nearest millisecond. .
• Date(long)
• Allocates a Date object and initializes it to
  represent the specified number of milliseconds
  since January 1, 1970, 000000 GMT.
• Date(String)
• Allocates a Date object and initializes it so
  that it represents the date and time indicated by
  the string s, which is interpreted as if by the

7
Object Reference

• In a diagram, we can send a message to any object
  that we have drawn simply by pointing a message
  arrow at it.
• In a written program, we need to have some
  notation for referring to objects so we can send
  messages to them.
• If we create an object and dont have a reference
  to it, we can never use it.

8
Object References

• An object reference is a language expression that
  refers to an object.
• literal reference
• Constant reference
• Variable reference

9
Literal Object References

• The simplest kind of object reference is a
  literal object reference.
• A literal object reference refers to the same
  object at all times.
• You can think of a literal object reference as a
  nameplate attached to an object.

10
Example
import java.io. / to demo the literal
reference note that "this is to ... " is an
literal string object. / public class literal
public static void main( String args )
System.out.println("this is to demo literal
reference".toUpperCase() )
gtjava literal THIS IS TO DEMO LITERAL REFERENCE
11
Variable Reference

• A variable object reference is an object
  reference that may refer to different objects at
  different times.
• A variable can be re-bound to a different object
  or value.
• A variable is restricted to objects (or value) of
  a particular type called its declared type.
• More than one variable can be bound to the same
  object (or value) at the same time.

12
Example
import java.io. / to demo the variable
reference / public class variable public
static void main( String args ) String
aString "A string of chars" String bString
aString System.out.println(aString)
System.out.println(bString.toUpperCase() )
aString "another string of chars"
System.out.println(aString)
gtjava variable A string of chars A STRING OF
CHARS another string of chars
13
Constants

• A constant is like a variable, but once bound to
  an object (or value) it cannot be rebound.
• It is different than a literal since
• it is not restricted to those types that have
  literals
• its language notation is like a variable

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Constants - Example
Constants cannot be re-bound
constant object reference
birthDate
friend
friend
Oct 14 1979
birthDate
Fred
Barney
Dec 15 2010
constant value reference
taxRate
taxRate
now
29
later
26
15
General Object References

• Recall that an object reference is any language
  expression that refers to an object and a value
  reference is any language expression that refers
  to a value.
• There are many other kinds of references in
  addition to literals, variables and constants.
• For example, since a message expression returns
  an object or value, the message expression itself
  is also a reference.

16
Class Definitions

• Each class must have
• at least one constructor
• any number of methods
• any number of various variables

17
Constructor

• A constructor is used to create an object of the
  given class.
• A class has at least one and upto any number of
  constructors
• All constructors have the same name, but differ
  in the types of the argument objects.

18
Example
/ constructor demo / public class sports_car
int max_speed int price public
sports_car () public sports_car (int m
) max_speed m public
sports_car ( int m, int p ) max_speed
m price p
19
Method

• A class has any number of methods
• A method is a block of code that specifies a
  behavior of a message
• A method is specified by
• name
• parameter list
• return value
• modifiers
• body

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Example
Name
Modifier
public void run ( ) System.out.println("keep
going, keep going, ... ") public String
bigCase( String s ) return( s.toUppperCase
() )
Parameter
Return tupe
Body
21
Variables

• Class variables ( static variables )
• variables declared for the class
• Instance variables ( object variables )
• variables declared in the class definition but
  not within any method
• variables for the object
• Local variables
• declared within a method
• Paremeters
• declared in the parameter list of a method

22
import java.io. / to demo class definitions
/ public class sports_car int max_speed
int price public sports_car ()
public sports_car (int m ) max_speed m
public sports_car ( int m, int p )
max_speed m price p
public void run ( String slogan )
System.out.println( slogan ) public
static void main( String args ) String
slogan "Keep going, keep going, ..., "
sports_car my_baby new sports_car( 300, 120000
) my_baby.run( slogan )
23
Scope and Lifetime of Variables

• Every variable has two characteristics.
• The scope or visibility is the region of program
  code that can use the variable.
• The lifetime is the time that the variable exists
  (can be used).

24
Scope of static/instance variables

• The exporting class is the class in which it is
  declared.
• The using class is the class in which it is used
  to reference an object or value.
• private
• valid only within the exporting class
• public
• valid in the exporting class and all the using
  classes

25
Local Variables

• A local variable or temporary variable or method
  variable is declared in a block of code called a
  method.
• Its lifetime is the time that the method is
  running.
• Its scope is the method it is declared in.
• Local constants are also allowed.

26
Message Parameters

• A message parameter is declared at the start of a
  block of code called a method.
• A message parameter is bound to an argument
  object or value when the method is called and
  cannot be re-bound.
• Its lifetime is the time that the method is
  running.
• Its scope is the method it is declared in.