Self-knowledge is best learned, not by contemplation,
but by action.  Strive to do your duty and you will soon
discover of what stuff you are made.

					Johann Goethe

1. What is an Object?

   Merriam-Webster's dictionary:

   1. Something material that may be perceived by the senses
   2. something mental or physical toward which thought, feeling or action is directed

   1. refers to something physical.

   In the Student Registration System ( SRS ), examples of physical objects are:

   students who attend classes
   professors who teach them
   classrooms in which class meetings take place
   furniture in these classrooms
   buildings in which classrooms are located
   textbooks students use

   2. refers to conceptual objects

   courses that students attend
   departments that faculty works for
   degrees that students receive

   formal definition of a software object:

   A ( software ) object is a software construct that bundles together data ( state ) and functions ( behavior ) which, taken together, represent an abstraction of a 'real-world' ( physical or conceptual ) object.

2. Data/State/Attributes

   Data:

   Examples:

   Data of a student

   student's name
   students's address
   age
   gender
   courses taken
   advisor

   Data of a course:

   course number
   course name
   prerequites of course
   credit units
   professors teaching it

   In object nomenclature, the data elements used to describe an object are referred to as the object's attributes.

   An object's attribute values, when taken collectively, are said to define the state of the object.

   Example: examine the state of a student to see if she could graduate.

3. Behavior/Operations/Methods

   An object's behaviors, also known as operations are both the things an object does to access its data ( attributes ), and the things that an object does to modify/maintain its data ( attributes ).

   A student's behavior might include:

   Enrolling in a course
   Dropping a course
   Selecting a facutly advisor
   Telling you her GPA when asked

   A course's behavior might include:

   Permitting a student to register
   Determining whether or not a given student is already registered
   Telling you what its prerequisite courses are
   Telling you how many credit hours it is worth

   When we actually get around to programming an object in a language like Java, we refer to the programming language representation of an operation as a method, whereas, the term 'operation' is typically used to refer to a behavior conceptually. But, often, these two terms are used interchangeably.

4. Classes

   A class is an abstraction describing the common features of all members in a group of similar objects.

   A class defines:

   The data structures ( names and types of attributes ) needed to define an object belonging to that class.
   The operations ( methods ) to be performed by such objects.
   For example, the Student class might be defined to have the following nine attributes:

   Attribute Name		(Java) Data Type
   name		String
   studentId		String
   birthdate		Date
   address		String
   major		String
   gpa		Float
   advisor		???
   courseLoad		???
   transcript		???

   In terms of operations ( methods ), the Student class might define five methods:

   registerForCourse
   dropCourse
   chooseMajor
   changeAdvisor
   printTranscript

   The term feature is used to refer interchangeably to either an attribute or a method of a class.

5. Instantiation

   Instantiation is the process by which an object is created/constructed based upon a class definition.

   Difference between class and object:

   class -- specifications, defines attributes, like an empty template

   object -- filled in template, consumes resources

6. Encapsulation

   Encapsulation -- the mechanism that bundles together the state information ( attributes ) and behavior ( methods ) of an object into a single unit.

   Attributes: name ssn address <etc> Methods: registerForCourse dropCourse chooseMajor <etc>

7. Objects vs. Database Records

   Object-record analogy

   class ~ a table in a relational database
   each record, or row in a table ~ a different object instance
   each column in a table ~ a different attribute of the associated class

   studentid	name	major	gpa
   123456	Arnold Smith	P.E.	2.3

   Flaws of correlation between objects and records:

   Objects exhibit behavior, whereas records typically do not.

   Objects have unique identities; can have identical attributes.
   records do not, need primary key to distinguish them

8. Classes as Abstract Data Types

   non-OO language

   int x;
   x is a symbolic link that represents an integer value
   can be operated on by int data type operators

   OO language

   Student s
   s is a symbolic name referring to a Student object/instance
   s can be operated on by the methods defined in Student class

   Just as int is referred to as a simple, or built-in data type in a language like C or Java, we can refer to a user-defined class Student as an Abstract Data Type ( ADT ), which is a user-defined data type that specifies structure as well as behavior.

   Problem Statment

   ADT

   Implemented Program ( depends on language )

9. Closer Look at Instantiating Objects of Java
   C++ :

   	Student y;	//instantiation occurs, object created, default constructor used
   			// memory allocated, y is name of the object
   	

   Java :

   	Student y;	//instantiation has not occurred, 
   			//noobject created,
   			//y is a reference variable that has the potential
   			//to be a reference ( handel ) to a Student objec
   			//( you may also imagine that y is a pointer )
   
   
   	y = new Student();	//instantiation occurs
       

   y →	Student Object

   ------------------------------------------------------------------------------------

     Student x = new Student();	//Object created
   
     Student y = x;		//y references the same object as x does
     

   y → x →

   Student Object

   ------------------------------------------------------------------------------------

     Student x = new Student();	//Object created
   
     Student y = x;		//y references the same object as x does
   
     Student z = new Student();	//Another object created
   
     

   y → x → Student Object #1

   z → Student Object #2

   ------------------------------------------------------------------------------------

     y = z;		//y references the same object as z does
     

   x → Student Object #1

   y → z → Student Object #2

   ------------------------------------------------------------------------------------

     x = z;		//x references the same object as z does
     

   Student Object #1

   x → y → z → Student Object #2

10. Objects as Attributes

    Attribute Name		Data Type
    name		String
    studentId		String
    birthdate		Date
    address		String
    major		String
    gpa		Float
    advisor		Professor
    courseLoad		???
    transcript		???

    The Professor class, in turn, might be defined to have attributes as follows:

    Attribute Name		Data Type
    name		String
    studentId		String
    birthdate		Date
    address		String
    department		String
    studentAdvisee		Student
    teaching Assignments		???

11. Composite Classes

    Composite class -- a class in which one or more of the attributes are themselves references to other objects

    has-a relation

    

    We may encounter a situation where an object 'A' needs to refer to an object 'B', object 'B' needs to refer back to 'A' and both objects need to be able to respond to requests independently of each other.

    In this case, we cannot physically embed one object wholly inside the other. ( Why? )

    

    Resolved by using handles ( pointers ) ( C++ : forward declaration )
    We are not storing whole objects as attributes inside other objects, but rather references to objects. The two objects exist separately in memory

12. OO Characteristics
    ( Programmer creation of ) Abstract Data Types ( ADTs )
    Inheritance
    Polymorphism