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OBJECT ORIENTATION
Lecture 5
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Lecture Overview
n What is an object?
n How do objects communicate?
n How is an object’s interface defined
n What are classes?
n What is the purpose of classes?
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An object can be:
n a thing in the 'real world’
n a model of reality
n a tangible or visible thing
n a thing to which action or thought can be directed (intangible)
n passive - doing nothing until activated e.g. like a switch
n active - continually monitoring until conditions change e.g. like a thermostat
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World of Objects
n An Object is a thing one can
interact with.
n Interaction with an object can be
done by sending it messages.
n Object will react on receiving
message(s).
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Behavior, State and Identity
n The behavior of the object depends
upon the current internal state of the object.
n The internal state may change in
reaction to receiving the message.
n In order to send messages, an object is
usually addressed by its name which is the identity of the object.
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Object Defined
n Object is a thing that has behavior, state and identity
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Attributes
n Mutable : Their values can change n Immutable : Their values cannot change n Value of the attributes define the state of an object, NOT the attributes themselves n In OOL, the set of attributes cannot change during the life of the object , though the values of those attributes can change
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Behavior
n The way an object reacts as a result of message sending
n An object can receive messages and act on them
n The set of messages that an object understands is normally fixed
n The resulting response to the message(s) (i.e. behavior or service) depends upon the current values of its attributes
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Identity
n Identity: Name of the object.
n The values of attributes may change, but the object remains the same. The same object continues to exist.
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Messages
n Objects accept messages advertised in their interfaces
n Objects send messages to other objects
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Messages (2)
n Messages may contain arguments (parameters) to clarify the request e.g. n myCat catch: aMouse n myCat moveTo: saucerOfMilk at: highSpeed n myCustomer.changePhoneNo (01705844032) n myCustomer.changeAddress (“PO Nilore”, “Islamabad”, “Pakistan”, “45650)
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Messages (3)
n When a message is sent to an object, the sender will not know what code will be executed by the receiver because the information is encapsulated.
n That’s why we say that sender is making “assumptions”.
n Encapsulation allows the objects to offer services independent of implementation.
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Classes (Instantiating)
n Process of creating a new object of a class is called Instantiating the class, and the resulting object an instance of the class.
n That’s why the attributes (variables) belonging to an object are called instance variables.
n The values of attributes are set during the object creation process
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The Object Factory
n As class has a direct role in the
creation of a new object
n Object Template + Creation
Process
n Therefore a class is often seen as
The Object Factory
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Classes as Objects
n Most languages allow classes to act like objects – having attributes, behavior and identity
n Example: n Customer class may have an attribute numberOfInstances, which is incremented whenever a new customer is created n Customer class may send itself a message (to) getNumberOfCustomers()
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The Purpose of Classes
n Classes serve two purposes:
n A convenient way of describing a collection of consistently behaving objects n Classes provide the convenience of storing only one copy of code representing the object’s behavior (though conceptually every object encapsulates its own behavior)
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Summary
n Objects have state, behavior and identity
n Objects interact with each other by sending messages
n Interface defines a collection of messages that an object understands
n Classes describe a collection of objects
n A Class ensures a consistent object interface
