In this blog post, we will see what the Class is and how to use the Class in Ruby.
The variable and data are all objects in Ruby. The Class is a blueprint of the object. In Ruby, You can create a class and you can combine the multiple classes if you need.
The languages based on objects such as Ruby are called OOP(Object Oriented Project). The biggest feature of OOP is that a class can inherit, and the inherited class can use the parent functions. Also, you can re-write the parent functions in the inherited class.(Override).
Writing the contents of the Class is called
Define the Class. We can use the
class statement to define the class.
class Book ... end
We can use the
new method to create an object from the class.
book = Book.new
We can use many kinds of Variables in Class. Let’s see what the variables we can use in Ruby.
The Instance variables are a global variable in the Class. The variable name is started with
class Book def printTitle puts @title end ... end
You can refer to the Instance variable via the method in the object created by the class. You can’t refer to the variable directly outside of the object.
class Book def initialize @title = 'Ruby' end def printTitle puts @title end end book = Book.new book.printTitle # Ruby
The Class variables are a common variable of the class. The class variables are shared in all objects created by the same class, and the variable name is started with
class Book @@publisher = 'dev-yakuza' def printPublisher puts @@publisher end end book = Book.new book.publisher # dev-yakuza
You can init the Class variables when you defined the class. You can refer to the Class variables via the method in the class, and you can’t refer to them outside of the class.
You can define the Constants in the class, and the variable name is started with the capital alphabet.
class Book Language = 'EN' end
You can refer to the Constants directly outside of the class.
The Constants are similar with the Class variables, but the class variables can be reassigned.
class Counter @@count = 0 def plus @@count += 1 end def printCount puts @@count end end counter = Counter.new counter.plus counter.printCount # 1
There are the Instance method and Class method in the class. Let’s see what they are.
The Instance method is a general method. You can call the Instance method via the object.
class Greeting def initialize @name = 'World' end def hello puts 'Hello ' + @name end end greeting = Greeting.new greeting.hello # Hello World
The Class method is used for the process independent with the object. You can call the method via the class
class Greeting @@name = 'World' def Greeting.hello puts 'Hello ' + @@name end end Greeting.hello # Hello World
The Class method is called by the class, so you can use the Instance variables because they are created when the instance is created from the class.
You can use the
initialize method to initialize the object of the class.
class Book def initialize @title = 'Ruby' end def printTitle puts @title end end
The initialize method is automatically called when the object is created by the
class Book def initialize (author) @author = author end def printAuthor puts @author end end book = Book.new('dev-yakuza') book.printAuthor # dev-yakuza
Like above, you can pass the parameters via the
new method to the initialize method.
You can access the Instance variables outside of the object by using the
Accessor instead of using the method. You can define the Accessor with
class Book def initialize @title = 'Ruby' end attr_accessor :title end book = Book.new puts book.title # Ruby book.title = 'Rails' puts book.title # Rails
In OOP, The class can be inherited the other classes.
Inheritance of the class
The inheritance of the class means that you can define a new class that extends or limits features from the existing classes. The
Super class is a target class of the inheritance and the
Sub class is a class which is inherited the super class.
class [Sub Class] < [Super Class] ... end
You can use the Inheritance of the class like below.
class Fruits def fruits puts "It's a Fruits" end end class Apple < Fruits def apple puts "It's an Apple" end end apple = Apple.new apple.fruits # It's a Fruits apple.apple # It's an Apple
As above, The
Apple class doesn’t have the
fruits method but is inherited from the Super class, we can use the
fruits method. Also, You can define a class from the inherited class from the Super class.
class Fruits def fruits puts "It's a Fruits" end end class Apple < Fruits def apple puts "It's an Apple" end end class Pie < Apple def pie puts "It's a Pie" end end pie = Pie.new pie.fruits # It's a Fruits pie.apple # It's an Apple pie.pie # It's a Pie
When you define the class from the Sub class, the class can’t inherit more than two classes.
Redefining the method of the Super class in the Sub class is called the
Override of the method. The
Override means that
class Fruits def name puts "It's a Fruits" end end class Apple < Fruits def name puts "It's an Apple" end end apple = Apple.new apple.name # It's an Apple
You can restrict a method’s access by using the
Access Modifier in the class. There are
protected of the Access Modifier. If you don’t set the Access Modifier for the method, all of them are under
class Book def protectedTest puts "this is a protected function" end protected :protectedTest def publicTest puts "this is a publicTest" end end book = Book.new book.publicTest # this is a publicTest book.protectedTest # protected method `protectedTest' called for #<Book:0x00007f8c6d88ebd0> (NoMethodError)
The Access Modifiers have characteristics like below.
|public||Called with no limitation.|
|private||Can’t be called.|
|protected||Can’t be called. However, it can be called from the object the method belongs.|
class Book def privateTest puts "this is a private function" end private :privateTest def protectedTest puts "this is a protected function" end protected :protectedTest def publicTest puts "this is a publicTest" protectedTest privateTest self.protectedTest self.privateTest end end book = Book.new book.publicTest # this is a publicTest # this is a protected function # this is a private function # this is a protected function # private method `privateTest' called for #<Book:0x00007fa2d9066a98> (NoMethodError)
Singular class definition
Singular class definition means adding the method or instance variable to the existing object.
class Book def initialize @title = 'Ruby' @author = 'dev-yakuza' end def printTitle puts @title end end book = Book.new class << book def printAll puts @title + ' / ' + @author end end book.printAll # Ruby / dev-yak def book.printAuthor puts 'Author: ' + @author end book.printAuthor # Author: dev-yakuza
Class split definition
In Ruby, you can split the definition of the class.
class Book def initialize @title = 'Ruby' @author = 'dev-yakuza' end def printTitle puts @title end end class Book def printAuthor puts @author end end book = Book.new book.printTitle # Ruby book.printAuthor # dev-yakuza
The method that defined later is added, but if there is the same method, the previous method is deleted, and added a new method.
class Book def initialize @title = 'Ruby' @author = 'dev-yakuza' end def printTitle puts @title end end class Book def printTitle puts 'title: ' + @title end end book = Book.new book.printTitle # title: Ruby
We’ve seen what the class is and what the inherit is in Ruby. Ruby is the OOP language so the class is used frequently. So let’s remember the Class usages in this blog post.
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