Objectoriented Programming Concepts

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Object-oriented Programming Concepts
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What is an object ?

• Real-world objects
• All have state and behavior
• Bicycles have state current gear, two wheels,
  number of gears...
• Bicycles have behavior braking, accelerating,
  changing gears...
• Software objects
• Also have state and behavior
• Maintains its state in variables
• Implements its behavior in methods

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Definition

• An object is a software bundle of variables and
  related methods.
• Software objects can
• represent real-world objects, such as a bicycle
• represent abstract concepts, such as an "Event"
  object in a window system

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Encapsulation

• Anything that an object does not know or cannot
  do is excluded from the object.
• Encapsulation is used to hide unimportant
  implementation details from other objects.
• Implementation details of an object can be
  changed at any time without affecting other parts
  of the program.
• Definition Packaging an object's variables
  within the protective custody of its methods is
  called encapsulation.

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Benefits of Encapsulation

• Modularity
• Source code can be written and maintained
  independently
• An object can be passed easily around
• Information hiding
• An object has a public interface for other
  objects to communicate with.
• An object can maintain private information and
  methods that can be changed without affecting
  other objects.

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Examples

• When you want to change gears on your bicycle,
  you don't need to know how the gear mechanism
  works, you just need to know which lever to move.
  
• With software objects, you don't need to know how
  a method is implemented, you just need to know
  which method to invoke and its interface.

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Messages

• A single object is a component of a larger
  program that contains many other objects.
• Application functionality is achieved by
  interactions of these objects.
• Software objects interact and communicate with
  each other by sending messages to each other.

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Messaging

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Components of a message

• The object to whom the message is addressed.
  (Your Bicycle)
• The name of the method to perform. (changeGears)
• Any parameters needed by the method. (lower gear)

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What are classes?

• Your bicycle Your bicycle is just one of many
  bicycles in the world.
• Bicycle have some state and behavior in common.
• However, each bicycle's state is independent of
  and can be different from other bicycles.
• Your bicycle object is an instance of the class
  of objects known as bicycles.
• Bicycles of the same model are built using the
  same blueprint.

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Software objects

• Possible to have many objects of the same kind
  that share common characteristics.
• Objects of the same kind can be built using the
  same blueprint.
• Software "blueprints" for objects are called
  classes.
• Definition A class is a blueprint or prototype
  that defines the variables and methods common to
  all objects of a certain kind.

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Object instantiation

• Remember a blueprint of a bicycle is not really
  a bicycle.
• When you create an instance of a class, you
  create an object of that type.
• The system allocates memory for the instance
  variables declared by the class.
• You can then invoke the object's instance methods
  to make it do something.

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Reusing Classes

• Classes provide the benefit of reusability.
• Programmers use the same class, and thus the same
  code, over and over again to create many objects.

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Polymorphism

• The attribute that allows one interface to be
  used with a general class of actions.
• It is the compiler's job to select the specific
  action as it applies to each situation.
• Polymorphism helps reduce complexity by allowing
  the same interface to be used to specify a
  general class of actions.

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Example

• Three types of stacks, one for integer values,
  one for strings, and one for a specific
  structure.
• You can create three sets of functions called
  push() and pop() , one for each type of data.
• The general interface is that of pushing and
  popping data onto and from a stack.
• When you push data on or pop from the stack, it
  is the type of the data that will determine which
  specific version of push() or pop(), that will be
  called.

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Inheritance

• The process by which one object can acquire the
  properties of another object.
• Concept of classification
• Red Delicious apple ---gt apple ---gt fruit ---gt
  food
• An object (class) need only define those
  qualities that make it unique.
• Allow classes to be defined in terms of other
  classes.

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Example

• Mountain bikes, racing bikes, and tandems are all
  different kinds of bicycles.
• These are subclasses of the bicycle class.
• The bicycle class is the superclass of these
  subclasses.

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More on Inheritance

• Each subclass inherits instance variables from
  the superclass.
• Each subclass inherits methods from the
  superclass.
• Subclasses can add their own variables and
  methods.
• Subclasses can override inherited methods.
• The class hierarchy can be as deep as needed.
• Methods and variables are inherited down through
  the levels.
• The further down in the hierarchy a class
  appears, the more specialized its behavior.