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

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

Encapsulation

• What is 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.

• 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.

• Benefits:

  • 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.

• Sometimes:
  • For implementation or efficiency reasons, an object may wish to expose some of its variables or hide some of its methods.

What are 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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• 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)

What are classes?

• Your bicycle
  • Your bicycle is just one of many bicycles in the world.

  • Bicycle have some state and behaviour 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.

• 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.

Classes provide the benefit of reusability.

• Programmers use the same class, and thus the same code, over and over again to create many objects.

Polymorphism

• What is 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.

• Example,
  • Three types of stacks, one for interger 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.

What is inheritance?

• The process by which one object can acquire the properties of another object.

• Concept of classification

• Red Delicious apple ---> apple ---> fruit ---> food

• An object (class) need only define those qualities that make it unique.

• Allow classes to be defined in terms of other classes.

• 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.

• 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.