1. Workshop for Programming And Systems Management Teachers
Chapter 2
Introduction to Object-Oriented Programming
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Learning Goals
Understand at a conceptual level
What is object-oriented programming?
What is an object?
What is a class?
An object-oriented program is a simulation
Objects are responsible for their data and operations
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3. What does Object-Oriented mean?
The focus is on objects
Finding objects
Describing objects
Classifying objects (What type of thing?)
Describing how objects interact to accomplish a task
Determining the relationships between types of objects
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4. Object-Oriented Example
What happens when you go to a restaurant to get a meal?
How many objects are involved (people and other things)?
What things do the objects know about?
What things (operations) can they do?
When you go to a restaurant you usually will have someone greet you and ask how many are in your party. They will probably also ask if you want the smoking or non-smoking section. They will show you to your table and perhaps notify the waiter or waitress that you are there.
The waiter or waitress takes your order and gives it to the chef. The chef cooks the order and notifies the waiter or waitress that it is ready. The waiter or waitress picks up the food and brings it to you.
You eat the food and pay the bill.
Each object in this simulation has data and operations. The objects work together to complete the task.
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Objects and Classes
The objects in the restaurant are the people or things doing the work or being acted upon
The classes are how we classify the objects
What type of thing is the object?
What job classifications are there in a restaurant?
How many people do you need for each job classification?
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What is an Object?
Person, place, or thing
that knows something about itself
has attributes (data)
and can do something
has operations (behaviors)
Example object
It is a dog named Spot. He is a small dog.
He can wag his tail. He can bark.
Look around you and notice the object objects near you. What data do they have and what operations can they do?
Attributes are also called fields, properties, data, instance variables, and state.
Operations are also called behaviors.
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7. Objects are Responsible
Objects are responsible for their data
The data is private
Methods are used to modify the data
An object may refuse to do the requested action because the data would be left in an incorrect state
Like a negative account balance
Keeping your data private is also known as information hiding.
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8. Objects know how to do their jobs
Other objects don’t worry about how the object handles the request
They just assume it knows how
This makes it easy to change one object without affecting others
Meaning the programs can be assembled from existing objects
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What is a Class?
The class is the abstract basis for the object.
It describes the data that all objects of that class have
All dogs have a name, and a size
and the things they can do
All dogs can wag their tails and bark
This slide shows two objects of the dog class. They both have names and sizes and can bark and wag their tails. However, they have different names and sizes.
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Class as Blueprint
The class can be thought of as the blueprint, cookie cutter, or factory
It gives the objects created from it their shape.
You can make many objects from the same blueprint or cookie cutter.
A software class creates the objects of that class
An object is an instance of a class.
The blueprint is the class the created houses are the objects.
Objects know what class created them.
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11. Object and Class Example
These are objects (instances) of the car class
Cars have data
a manufacturer, model, year, etc
Cars have operations
can go forward, go backward, turn, stop
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12. Computation as Simulation
An object-oriented program is a simulation of the domain
Objects send each other messages to accomplish a task
A message is a request
Requests can be refused
Stop Playing No
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13. Software Objects are Models
The objects we create in software are models of the physical object
We can’t stick a person in our software
We can create a model of the person with the information we need to know for that person for our task
Person
identifier
name
address
phoneNumber
We don’t put the real car in our software either. Car objects would be models or representations of the real car.
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Gopher Bash Exercise
Go to
What objects do you see?
What type of things are they?
What data do they have and what can they do?
This is a simulation of the gopher bash game in arcades.
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15. Object-Oriented Principles
Abstract Data Types - Objects and Classes
Encapsulation of data and associated operations
The ability to create your own data types (classes)
Inheritance
The ability to factor common things into a parent class (generalize)
This ability to do something different (specialize)
Polymorphism
A variable can be of different types (classes) at run-time
Requires dynamic (run-time) binding
If a system claims to be object-oriented test that it meets these three principles.
Dynamic binding is also called late binding.
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Encapsulation
Objects encapsulate attributes (data) and operations (methods)
The data is modified by the methods
Other objects can ask an object to do something to its’ data by sending a message
data
data
Only the object should modify its’ data. This means the object controls the data and is responsible for it. If the data is wrong it is the objects fault for allowing it to be wrong.
methods
methods
message
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Inheritance
Attributes and operations are inherited from the parent class.
Dogs and cats are types of mammal and thus bear live young
The mammal class is a generalization
The dog and cat classes are specializations
Mammal
If a new animal was found and you were told it was a mammal, what could you tell me about it?
The parent class is also called the base class or superclass. The child class is also called the derived or subclass.
Cat
Dog
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18. Advantage of Inheritance
Handles commonality
Common attributes and operations are factored out and put in one place
Not copied to several locations
Easier to maintain, extend, and reuse
Handles differences
Children can inherit the parts that are common from the parent
They can add attributes and operations to handle how they differ from the parent
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19. Polymorphism – Many Forms
Dynamic binding
The method depends on the run-time type
Say we are creating an animal symphony
All animals can make a noise
The noise that is made depends on which animal
Animal
makeNoise()
We would keep a list of animals that have been picked and ask each animal to make a noise. The noise that is made would depend on the actual animal at run-time. We have no way of knowing at compile-time which animal a child will pick first. We just know that it will be an animal. But, at run-time the objects will know what class they belong to.
Cat
Dog
Pig
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Kinds of Polymorphism
Unrestricted
No types are declared
Any object can be of any type at run-time
All binding of operations is done at run-time
Restricted (inheritance-based)
Types are declared
Objects can be the declared type or a child of the declared type at run-time
May be able to do some compile-time binding
Scheme and Lisp use unrestricted polymorphism. Java, C++, and C# all use restricted or inheritance-based polymorphism.
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21. Advantages of Polymorphism
Used to create general algorithms that work on objects of different types
Collections that hold objects
Makes it easy to add new types
Just create the new class and implement the required operations
Animal
makeNoise()
Compare polymorphism with how this result is achieved in a procedural language. You would need an complex conditional based on the type of object. If you wanted to add a new type you would have to edit your code everyplace the conditional exists.
Cat
Dog
Pig
Bird
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Summary
Object-oriented programs are simulations
Object-oriented programs have objects that interact by sending each other messages
Objects have data (fields) and operations (methods)
The three principles of object-oriented programs are
Abstract data types (objects)
Inheritance
Polymorphism
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