OBJECT-ORIENTED CONCEPTS

1
OBJECT-ORIENTED CONCEPTS
2
What is an object?

• An object is a software entity that mirrors the
  real world in some way.
• A software object in OOP has data (the
  attributes) and code that responds to
  instructions. Think of the code and data as
  being welded together --the code completely
  wrapping the data in a shell to protect it from
  the hostile outside world." Franz

3
What is an object?(Continued)

• An object has state, behavior, and identity
  Booch 91
• An object is an encapsulation of state (data
  values) and behavior (operations). Thus an
  object is in many ways similar to a module, or an
  abstract data type." Budd

4
Procedural languages vs Object-oriented
languages?

• Procedural languages (Pascal, C, Fortran, etc.)
  operate on data a step at a time. A program
  written in one of these languages has two
  components
• data (variables and records)
• code (instructions that act on the data and do
  the work).
• There is a rigid separation of code from data.

5
PRO-Lang vs OO-Lang (Continued)

• An object-oriented language breaks a program into
  hundreds of tiny entities that have small bits of
  code and data fused tightly together. Where
  subroutines are a convenience in a procedural
  language, the separateness of these entities is
  essential to an object-oriented program.

6
OBJECT-ORIENTATION - A NEW PARADIGM

• Object-oriented programming is a new programming
  paradigm (a new way of viewing the world).
• The underlying ideas of the object-oriented
  technique are the same basic methods humans use
  to solve problems in the world (e.g.,
  abstraction).

7
OO A NEW PARADIGM (Continued)

• Computer novices and children grasp the ideas of
  OOP easier than computer professionals. The
  professionals require a paradigm shift which is
  very difficult (due to preconceived notions)
  whereas the children don't.

8
TERMS

• Object A software entity that mirrors the real
  world in some way. Everything in an
  object-oriented programming language is an
  object.
• Class A class is a template from which objects
  are created (i.e., a blueprint for the objects
  it defines). Every object made from a class is
  the same as every other.

9
TERMS(Continued)

• Instance Instances are the individual objects
  created by a class. Every object is an instance
  of a class.
• Related Term
• Instantiation Creating a new (specific) object
  from a general class is called instantiation.

10
TERMS(Continued)

• Method The behavior of a class, and all its
  instances, is described by a method.
• Methods are like procedures
• They execute step-by-step,
• Can do branching and looping,
• Take arguments,
• Have local variables,
• Return values (objects).

11
TERMS(Continued)

• Methods are unlike procedures
• they may only access data of the class for which
  they are defined, an instance's data can only be
  accessed through its class' method.
• Note A class' methods completely encapsulate
  the data of objects of the class for which they
  are defined.

12
TERMS(Continued)

• Message A request to a specific instance,
  called the receiver, to invoke a specific method.
  Along with the name of the method, (the
  selector) additional information (arguments,
  i.e., objects) can be passed as part of the
  message.

13
TERMS(Continued)

• When the object receives the message, it invokes
  the method for the selector. This can
• (1) change the object's instance variables,
• (2) send another object back as a direct
  response to the message,
• (3) do something in the real world of the
  computer, such as display information, print a
  message, or read a block of data from a file.
• Much of the code within methods consists of
  passing messages to other objects.

14
TERMS(Continued)

• Abstraction A problem solving tool that allows
  one to think of a problem at a higher level
  without worrying about the details.
• Abstraction refers to organizing information
  based on three abstraction mechanisms
  classification, generalization, and aggregation.
  

15
ABSTRACTION MECHANISMS(Continued)

• Classification forms new objects by suppressing
  the details of individual instances and
  emphasizing properties of the whole. New object
  types (classes) are formed by collecting
  instances.
• Classification set theoretic operation
  'membership'
• gt describes the 'instance-of'
  relationship

16
ABSTRACTION MECHANISMS(Continued)

• EX Classification would allow one to collect
  the instances 'Jane, Bob, Sue' into a new
  higher-order type called 'secretary.'

17
ABSTRACTION MECHANISMS(Continued)

• Generalization merges existing types to form new
  types. Individual differences between subtypes
  are ignored and common traits are emphasized.
• Generalization set theoretic operation
  'union'
• gt describes the 'is-a relationship

18
ABSTRACTION MECHANISMS(Continued)

• EX Existing employee types 'secretary', and
  'teacher' can be generalized to form the new type
  'employee'.

19
ABSTRACTION MECHANISMS(Continued)

• Generalization implies that every instance of the
  subtype is an instance of the type. Every
  instance of 'teacher' is also an instance of
  'employee'

20
ABSTRACTION MECHANISMS(Continued)

• Aggregation forms an object as a relationship
  among other objects. It suppresses the details of
  the components and emphasizes the details of the
  relationship as a whole.
• Aggregation set theoretic operation
  'cartesian product'
• gt describes the 'is-part-of relationship

21
ABSTRACTION MECHANISMS(Continued)

• EX Consider object types 'stove',
  'refrigerator', 'sink', 'cabinet'. A 'kitchen'
  object can be abstracted from the given objects.
  The individual objects (e.g., 'stove') are
  components of the aggregate ('kitchen').

22
ABSTRACTION MECHANISMS(Continued)

• An instance of a component is said to be an
  attribute of an instance of the aggregate.

23
ABSTRACTION MECHANISMS(Continued)

• Classification, generalization, and aggregation
  are the basic ways we have of structuring
  information. When they are repeatedly applied to
  objects, hierarchies of new objects are formed.

24
TERMS(Continued)

• Information hiding as defined by Parnas Par72
  suggests decomposing systems into components that
  hide or encapsulate design decisions about
  abstractions.
• Data abstraction refers to defining data types in
  terms of the operations that apply to objects of
  the type, with the constraint that the values of
  such objects can be modified and observed only by
  the use of the operations Oxf86.

25
TERMS(Continued)

• Data abstraction is thinking in terms of what you
  can do to a collection of data independently of
  how you do it.
• Related term
• Abstract data type (ADT) is a collection of data
  structured in a particular fashion together with
  a set of operations that manipulate the data.

26
TERMS(Continued)

• Inheritance The principle of receiving
  properties or characteristics from an ancestor
  and is used to define new data types as
  extensions or restrictions of existing types.
• Polymorphism and dynamic binding Program
  entities should be permitted to refer to objects
  of more than one class, and operations should be
  permitted to have different realizations in
  different classes.

27
TERMS(Continued)

• Polymorphism refers to the situation whereby a
  given program entity may refer at run-time to
  instances of different classes.
• Dynamic binding refers to the situation whereby
  the run-time system will automatically select the
  version of an operation adapted to the
  corresponding instance.

28
TERMS(Continued)

• Multiple and repeated inheritance It should be
  possible to declare a class as heir to more than
  one class, and more than once to the same class.