Modeling in Problem Solving

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Modeling in Problem Solving

• A real-world problem consists of several
  collections of entities interacting with one
  another and with their surroundings.
• When solving a real-world problem, a simplified
  representation of the problem is used to study
  the problem and construct a solution.
• This representation is called a model of the
  problem.

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Modeling in Problem Solving

• In developing object-oriented applications, one
  of the main goals is to construct abstract
  representations in software of some aspect of the
  real world.
• An abstract representation is a simplified
  description with only the relevant or essential
  characteristics of part of a real system.

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Models with Objects

• A model is composed of objects, each one
  representing a real-world entity. The model
  includes descriptions about these objects and
  their interactions.
• Objects are the central focus of the
  object-oriented approach to problem solving.
• A model is an abstract description of some part
  of the problem domain. The task of designing and
  constructing a model is called modeling.

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Objects and Modeling

• Objects are given the responsibility of carrying
  out specific tasks of the solution.
• Objects are models of the real-world entities
  identified in the real-world environment of the
  problem.
• Objects with similar characteristics are grouped
  into collections, also known as classes.

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Object-Oriented Modeling
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Object Modeling

• Within the boundaries of the problem, three basic
  issues in modeling are
• Identifying the objects to include in the model
• Describing these objects
• Grouping similar objects into collections

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Describing Objects

• Real-world entities or objects are the
  fundamental components of a real world system.
• Recall that objects are abstract representations
  of real-world entities

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Types of Objects

• Physical objects, which are tangible objects,
  such as persons, animals, cars, balls, traffic
  lights, and so on
• Nontangible objects, which are not directly
  visible, such as contracts, accounts, and so on
• Conceptual objects, which do not clearly exist
  but are used to represent part of the components
  or part of the behavior of the problem. For
  example, the environment that surrounds the
  problem and that affects the entities of the
  problem

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Objects as Dynamic Entities

• An objects is a dynamic concept because
• Objects exhibit independent behavior
• Objects interact with one another.

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Objects Interact

• Objects communicate by sending messages to each
  other
• Objects collaborate for a common goal.

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General Characteristics of Objects

• State, represented by the set of properties (or
  attributes) and their associated values
• Behavior, represented by the operations, also
  known as methods, of the object
• Identity, which is a property that can help
  identify an object

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Objects

• Objects are dynamic entities
• They exhibit behavior
• They change state
• They interact with other objects

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Objects and Classes

• Every object belongs to a class.
• Classes are used to define the properties and
  behavior of similar objects.

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A Simple Example of an Object

• An object of class Ball. Its identity identifies
  an object of class Ball.
• The attributes of this object are color, size,
  and move_status.
• The figure shows the UML diagram for two Ball
  objects and illustrates their structure.

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Object Diagram

• An object diagram is basically a rectangle
  divided into three sections.
• The top section indicates the class of the object
• The middle section includes the list of the
  attributes and their current values
• The bottom section includes the list of
  operations in the object.

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Two Object Diagrams
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Another Example of an Object

• An object of class Person is shown in the next
  figure.
• This object has only two attributes, name and age
• The object has two operations, play and stop.
  The values of these attributes are also shown.

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Behavior of Class Person

• The behavior of this object is simple the object
  can
• start playing (with operation play)
• stop playing (with operation stop).

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Object of class Person
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Object Services

• An object provides one or more services
• Each service is defined by one of the operations
  in the object
• Other objects can request a service

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Object Interaction

• Objects interact by sending messages to each
  other.
• A message represents a request for service, which
  is provided by the object receiving the message.

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Client and Supplier Objects

• The sender object is known as the client of a
  service
• The receiver object is known as the supplier of
  the service.

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Message Sending

• The purpose of sending a message is the request
  for some operation of the receiver object to be
  carried out in other words, the request is a
  call to one of the operations of the supplier
  object.
• Objects carry out operations in response to
  messages.
• A message is always sent to a specific object.
  This is also known as method invocation.

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Message Description

• A message contains four parts
• The object owner of the function being called
• The operation to be invoked or started, which is
  the service requested and must be an accessible
  operation
• The input data required by the operation to
  perform, which is known as the arguments
• The output data, which is the reply to the
  message and is the actual result of the request

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Describing Object Interaction

• To describe the general interaction between two
  or more objects (the sending of messages between
  objects), a UML diagram known as a Collaboration
  Diagram is used to describe the interaction.
• The next figure describes an example using this
  type of UML diagram.

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Example of Object Interaction

• To describe the interaction among an object of
  class Person with the two objects of class Ball,
  a simple collaboration diagram is drawn.
• The next figure shows a collaboration diagram
  with these three objects.
• The Person object invokes the move operation of
  the Ball object by sending a message to the first
  Ball object, and as a result of this message,
  that object (of class Ball) performs its move
  operation.

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UML Collaboration Diagram
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Information Hiding

• Based on the idea of abstraction
• Objects present an
• Internal view.
• External view.

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Internal View

• A list of all the operations in the object
• This includes the operations in the external view
  and the internal operations only started by the
  object itself
• The internal operations cannot be started by
  other objects.

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External View

• This view is a list of all the services in an
  object that can be requested by other objects
• This is basically a list of the operations that
  can be started from outside the object
• This list is defined in the class

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Encapsulation

• Integration of the data (attributes) of an object
  with the behavior definitions (methods) in a
  single unit.
• A protection mechanism
• It involves information hiding

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Classes

• Defines a collection of similar objects
• A class is described by the attributes and
  operations of the objects that belong to the
  class.

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Classes (2)

• Classes are static entities, they only exists in
  the program before execution time.
• Classes are the principal decomposition units of
  programs.

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Programs

• There are two views for a program
• Static view -- a program is an assembly of
  classes (static view)
• Dynamic view -- a program in execution consists
  of a group of objects interacting among
  themselves.

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Programs and Classes

• An object-oriented program consists of one or
  more classes
• One of these classes has the main control in
  executing the program

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Category of OO Programs

• In Java and KJP there are three kinds of
  programs
• Console applications
• Graphics applications
• Applets

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Program with Four Classes
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Visibility of a Class

• Visibility is defined by the external view of the
  objects of the class
• All the features in the external view are said to
  be public

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Visibility of a Class (2)

• The features of an object that are accessible
  from other objects are public
• The private features of an object are not
  accessible from other objects

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General Structure of a Class
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General Structure of a Function