COP 4331

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COP 4331 OODP

• Lecture 7
• Object Concepts

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What is an Object

• Programming language definition An instance of
  a class
• Design perspective is different from programming
  language perspective
• Object A thing you can interact with.
• Properties of Objects
• State
• Behavior
• Identity

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Object Their Properties

• Thing a conceptual component of a system
• May be physical or abstract
• State the data that an object encapsulates
  determines its state
• Data values are called attributes
• May be changeable (mutable) or immutable
• Behavior may vary depending on state of an object

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Object Their Properties.2

• Behavior the way an object acts or reacts
• Messages sent to objects trigger behavior
• Behavior often depends on state
• Message may change state
• Identity
• Each object can be distinguished from other
  objects in some way.

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Object Their Properties.2

• Objects must be animate
• They must have behavior
• Nouns in a problem statement are frequently
  associated with objects
• But they also describe attributes
• Note In object-oriented programming we sometimes
  introduce objects that are essentially only data
  holders

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Example Object

• Object myClock
• Attributes Time
• Methods
• resetTimeTo(theTime)
• reportTime()

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O-O Analysis

• Analysis to understand a system as a collection
  of interacting objects
• Identifying Objects
• Identifying attributes
• Identifying messages to which objects respond
• Identifying relationships between objects

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Object-Oriented Design

• Design a solution to a problem based on
  object-oriented analysis
• Describe a particular solution
• To a particular problem
• Show how relationships are applied
• Specify object interfaces
• Messages to which they respond
• Objects may have public and private interfaces

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Classes

• Class describes a collection of objects that have
  the same
• Role in the system
• Set of attributes
• Set of messages
• Interface
• Can be viewed as an object factory
• Does the work of creating objects
• An Object is an instance of a Class

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Object-OrientedSoftware Engineering

• Capture requirements easily and accurately
• Provide more natural model of interacting systems
• Support modularity, encapsulation, and
  abstraction
• Makes implementation in O-O programming language
  relatively easy

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Objects as Components

• Classes are, more or less, self-contained
  components
• Facilitates modification and reuse

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Inheritance

• Supports additional abstraction through
• Generalization from subclass to superclass
• Specialization from superclass to subclass
• (Subclass is also called derived class)

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Polymorphism

• Polymorphism allows the same name to be multiply
  defined
• The definition to use is determined from the
  context
• Most significant form in O-O design is when the
  meaning of a message is determined by what object
  receives it