Ruby 101: Method Definitions

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Objects manage state, and they also exhibit behavior. In this article, we'll be learning how to add behavior to classes that we write in Ruby.

By "behavior" we mean, what the class, or an instance of the class, can "do." While variables allow an object to store data, Ruby objects can define methods that execute code. This is an essential concept in object-oriented programming. Objects don't just encapsulate data, they also perform actions and can have responsibilities assigned to them.

1. The Basics

Sometimes the best way to learn is to dive right in. Take a look at this Ruby code.

1 2	class Telephone end

This Ruby code defines our Telephone class. A class is a blueprint for how to make objects of that class. We can create a new Telephone object by calling the new method on our class:

1	my_phone = Telephone.new

my_phone is now an instance of a Telephone. What can our instance variable do that a plain Object can't do? Well... nothing yet. Let's teach a Telephone how to dial a phone number:

class Telephone

  def dial(number)
    puts "Dialing #{number}..."
  end
  
end

my_phone = Telephone.new
my_phone.dial '1-800-555-1212'

Here on line 3, we've defined an instance method by using the def keyword. Save this code into a file named phone.rb, and run it:

c:\> ruby phone.rb
Dialing 1-800-555-1212...

We can define as many instance methods as we'd like. Let's teach our object to hang up the phone.

class Telephone

  def dial(number)
    puts "Dialing #{number}..."
  end

  def hang_up
    puts "Hung up."
  end
end

my_phone = Telephone.new
my_phone.dial '1-800-555-1212'
my_phone.hang_up

We'll get this:

c:\> ruby phone.rb
Dialing 1-800-555-1212...
Hung up.

There is one special instance method you should know about, called the initializer. We don't have to define this method, but if we do, Ruby will call it for us automatically every time a new instance is created. Here's an example:

class Telephone

  def initialize
    puts "New telephone created!"
  end
  
  def dial(number)
    puts "Dialing #{number}..."
  end

  def hang_up
    puts "Hung up."
  end
end

my_phone = Telephone.new
my_phone.dial '1-800-555-1212'
my_phone.hang_up

Let's run it again:

c:\> ruby phone.rb
New telephone created!
Dialing 1-800-555-1212...
Hung up.

Initializers can take arguments, just like our dial method did. We pass the actual argument values when we call new to create instances of our object. If we change our initializer to this:

def initialize(home_area_code)
  puts "New telephone created!"
  @area_code = home_area_code
  puts "Our home area code is #{@area_code}."
end

Now we are required to provide the phone's home area code when we construct a Telephone instance:

my_phone = Telephone.new('312')

And now we'll get output like this:

c:\> ruby phone.rb
New telephone created!
Our home area code is 312.
Dialing 1-800-555-1212...
Hung up.

Class Methods vs. Instance Methods

Java and C# static methods are the same as class methods in Ruby. (Switching from .NET to Rails? Buy the book).

So far we've learned how to create instance methods. They're called that because we have to first create an instance of our class before we can call them.

Sometimes we have functionality that seems appropriate to all telephones, regardless of _which_ telephone we might use. We can create class methods - methods that apply to the whole class, and not any specific instance.

Since all of our telephones have one thing in common - their manufacturer - we can create a class method to report the name of the manufacturer:

class Telephone

  def self.manufacturer
    return "Purple Telco"
  end
  
  # Rest of class definition goes here
  # ...
  
end

puts Telephone.manufacturer

Notice how we prefix the self keyword in front of the method name to indicate that it's a class method, instead of an instance method. Let's run it:

c:\> ruby phone.rb
Purple Telco

Notice how we didn't need to create any instances first. manufacturer is a class method: all we needed was the name of the class.

Congratulations! You now know the basics of instance and class methods in Ruby.