ObjectOrientation

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Object-Orientation
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Outline

• A non-language overview of object orientation

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Object Oriented Programming

• Object Orientation A problem solving technique
  to develop software systems
• Objects
• the basic unit of object orientation
• an entity with attributes (values), behavior
  (methods), and identity (a way to find it)
• may thing can be modeled as software
• temperature sensor, employee, color, button,
  string, socket for networking, number, Connect-4
  Strategy

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Objects

• Objects
• Have attributes set of stored values
• Have responsibilities provided services
• Provide access to the state such as whether a
  sensor is on or how many elements are in a list
• Provide behavior such as reporting a temperature
  is too high or returning a reference to an object
  in a collection

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OO Programs

• Object oriented programs
• have a collection of different types of objects,
  each with their own set of responsibilities
• consist of many objects of the same type (5
  buttons, 1 million transactions)
• A collection of objects of the same type are
  instances of the same class

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Classes

• Classes
• A description of the attributes and behaviors of
  a set of objects
• an object is an instance of a class all
  instances of a class have the same behavior (but
  different values)
• Consider a system to monitor sensors in a factory
• Will probably have a class named Sensor
• The class defines location, ID, and services such
  as notifying of certain conditions as they occur

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Object-oriented systems

• Characteristics of object-oriented systems
• Abstraction
• Classes encapsulate state and behavior
• Communication done via messages
• Objects have a lifetime
• Polymorphism via interfaces
• Polymorphism via inheritance
• Class hierarchies

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Abstraction

• Abstraction
• A model of a real world object (e.g. a list) or a
  concept (e.g. a CardPlayerStrategy)
• A simplified model of complex processing (sort
  the list, make a card playing decision)
• A way to deal with the complexities of life
• Car gas and brake pedals
• I need air to breathe
• Insert card, enter PIN, get money

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Encapsulation

• Encapsulation means
• The state and behavior of objects are hidden from
  other parts of the system
• Behaviors are invoked with well-defined messages
• Encapsulation is made available via the class
  programming language construct
• Behavior via methods
• Attributes via instance variables

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Communication via messages

• Objects communicate with each other via messages
• Messages consist of
• the receiver (the object reference to which the
  message is sent)
• a message name
• arguments (optional)

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Messages

• Messages
• can be sent as a normal part of the program
• may originate from the operating system interface
  (mouse movement, button click)
• may originate from the language runtime system
  (garbage collector reclaims memory)
• Flow of messages is not always linear
• may have users interacting with program to
  determine what the program will do next

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Object lifetime

• Some objects live only when the program runs
• Other objects live only during a message objects
  constructed by a method
• Other object persist between program runs
• Serialize and write to disk

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Object lifetime

• Objects are created at runtime via a constructor
  (instantiation)
• the constructor initializes state, allocates
  memory from the OS, and returns a reference to
  the object
• Reference a way to find the object and its type
• Objects can also be
• Sent over the network
• Read from and written to a disk

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Object Lifetime

• Objects have state and behavior
• Objects have their own identity
• can be distinguished from other objects
• Java uses references to keep track of objects
• A reference
• has a way to locate the object
• knows the object's type
• it's more than an address, but it's not an address

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Objects can Disappear

• Objects are destroyed when no longer needed
• When a method that constructs a local object
  finishes, the reference disappears
• C code can explicitly destroy objects
• Java uses a garbage collector to reclaim memory
  used by objects when there are 0 references to it

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Summary

• This has been an overview of object orientation
• Object Oriented Programming and Design is also
  about these upcoming topics
• Polymorphism
• Inheritance
• Design Heuristics
• Design Patterns
• Writing good code