Lecture Points on Classes

classes_lec.md

Intro to Classes

Methods, Variables, and All That

Learning Objectives

• Understand difference between obbjects and classes
• Understand how objects are referenced
• Understand getters and setters
• Understand attr_writer, attr_reader, attr_accessor
• Understand instance variables and instance methods
• Understand class variables and class methods
• Recall the self keyword
• Understand Protected, Private, and Public methods
• Understand method chaining in a class

======================

OBJECTIVE
Identify principles of abstracting data into classes in ruby and to use instances of them to design a program.

We talked about methods earlier today and what we learned was a procedural abstraction. What we need to do is define a basic interface for our data that allows us to interact with it.

Class Definition of a person

Let's create our first class.

person.rb

class Person
	
end

This defines a class definition of a Person. The class keyword denotes the begining of a class Definition.

To create a new instance of our class we write the following:

> Person.new()

A particular instance of a class is a called an object. In general, languages that use objects as a primary means of data abstraction are said to be Object Oriented Programming (OOP) languages.

An Object By Any Other Name Is Just As Sweet

What is an object in ruby? Basically everything that isn't a keyword.

However, this can cause you some headaches if you're not careful.

Imagine we had the following

> arr_1 = [1,2,3]
> arr_2 = arr_1
> arr_1 << 4
=> [1,2,3,4]
> arr_2
=> [1,2,3,4]

Wow, the second array completely changed. That's because arr_2 was a reference.

Initialize and instance variables

In our class definition we can make use of an initialize method, which is run when a new instance of the class is created.

class Person
	def initialize
		puts "A new person was created"
	end
end

We can also make use of instance variables that are defined for each particular object and are available throughout other methods in the object. These variables are prefixed by an @ symbol, i.e. @my_var.

class Person 

	def initialize(name)
		@name = name
	end
	
	def greet
		puts "Hello! My name is #{@name}."
	end
end		

Now, when we create a new Person we are required to specify the name of the person.

> person = Person.new("John")
> person.greet
=> Hello! My name is John.

Getters and Setters

Having created an instance variable in our object, we might want to read that outside our object. However, we have to define a method that will act as an interface for reading for this variable called a Getter Method.

class Person
	
	def intialize(name)
		@name = name
	end
	
	def name
		@name
	end
	
	...
end

Now we can read or get the name outside the object.

> person = Person.new("Jane")
> person.name
=> "Jane"

Similarly, we may need to change or set an instance variable from outside the object, so we create a method called a setter method.

class Person

	def initialize(name)
		@name = name
	end
	
	def name
		@name
	end
	
	def name=(other)
		@name = other
	end
	...
end

We can now get and set the name of a person using the methods we created for @name.

> person  = Person.new("Samantha")
> person.name
=> "Samantha"
> person.name = "Sam"
> person.name
=> "Sam"

attr_reader, attr_writer, attr_accessor

In ruby, the practice of creating a getter method is so common their is a shorthand that can be used at the top of a class definition called attr_reader.

class Person
	
	attr_reader :name
	
	def initailize(name)
		@name = name
	end
	
	def name=(other)
		@name = other
	end
	...
end

Similarly, we can do the same with the setter method using attr_writer

class Person
	attr_reader :name
	attr_writer :name
	
	def initialize(name)
		@name = name
	end
	
	...
end

Moreover, we have a shorthand for telling our class to create both a getter and a setter method called attr_accessor.

class Person

	attr_accessor :name
	
	def initialize(name)
		@name = name
	end
	...
end

self, class methods, and class_variables (@@)

Let's first create a variable associated with our class using the @@var_name designates a class variable. Then we can access it with a class method using self.method_name.

class Person
	
	@@population = 0
	
	def initialize(name)
		@name = name
		@@population += 1
	end
	
	def self.population
		@@population
	end
end

If we create a few new people we see the following

> Person.new("John")
> Person.new("Jane")
> Person.population
=> 2

In most cases, inside an instance method, self refers to the Object, but when used in the context of a method name it refers to the class itself`.

You could also say

class Person 
	...
	def Person.population
		@@population
	end
end

but this has the unfortunate problem of rename each method when you rename the class.

Also, note that self can be used in instance methods to refer to particular object in use, i.e. self.var_name instead of @var_name.

Private and Protected Methods

If we create a class Person with a name attribute and use attr_accessor to create the getters and setters as follows

class Person
	attr_accessor :name
	
	def initialize(name)
		@name = name
	end
end

then anyone can read and access Person#name.

> person1 = Person.new("John")
> person1.name
=> "John"

We can change this using the private keyword and passing it a symbol, which will privatize the corresponding methods.

class Person
	attr_accessor :name
	
	def initialize(name)
		@name = name
	end
	
	private :name, :name=
end

We can also add to the list of private methods by defining new methods below the private keyword

class Person
	attr_accessor :name
	
	def initialize(name)
		@name = name
	end
	
	private 
		def make_call
			puts "Calling friends"
		end
end

or use a mixture of both

class Person 
	attr_accessor :name
	
	def initialize(name)
		@name = name
	end
	
	private :name=
	
	private
		def make_call
			puts "calling friends"
		end
end

Protected methods are similar expect that they are accessible from when an object of the same type.

class Person
	
	...
	def make_safe_talk(person)
		person.safe_talk
	end
	
	
	protected
		def safe_talk
			"whispers are heard!"
		end
end

so in terminal

> per1 = Person.new('john')
> per2 = Person.new('jane')
> per1.make_safe_talk(per2)
=> "whispers are heard!"

Chainable methods

What if I wanted to create a class that had chainable methods calling many methods in one line.

class Person
	
	def initialize(name)
		@name = name
	end
	
	def greet
		puts "Hello I am #{@name}."
		puts "What is your name?"
		@other = gets.chomp
		puts "Nice to meet you, #{@other}."
	end
	
	def thank
		puts "Thank you for coming."
	end
	
	def farewell
		puts "Farewell, #{@other}"
	end

end

Trying to do

> person1 = Person.new("john")
> person1.greet.thank.farewell
=> nil has no method `thank`

to achieve this we have to return a reference to the object after each method

class Person
	
	def initialize(name)
		@name = name
	end
	
	def greet
		puts "Hello I am #{@name}."
		puts "What is your name?"
		@other = gets.chomp
		puts "Nice to meet you, #{@other}."
		self
	end
	
	def thank
		puts "Thank you for coming."
		self
	end
	
	def farewell
		puts "Farewell, #{@other}"
		self
	end

end