Object Initialization
Learning Goals
- Learn about the
#initialize
method - Use
#initialize
to set attributes when an instance is created
Introduction
We've already seen new instances of classes being created with the #new
method. For example:
class Dog
attr_accessor :breed
end
snoopy = Dog.new #=> #<Dog:0x007f970a2edfd0>
The code above creates a new instance of the Dog
class and sets that object
equal to a variable, snoopy
. If we want to give our dog a breed, we have to
use the following code:
snoopy = Dog.new #=> #<Dog:0x007f970a2edfd0>
snoopy.breed #=> nil
snoopy.breed = "Beagle"
snoopy.breed #=> "Beagle"
However, most dogs are born with a breed, not assigned a breed afterwards. How
can we model the behavior of dogs being born with a breed in our Dog
class? If
only there was a way for us to assign an individual dog a breed automatically
upon creation, or instantiation.
Lucky for us, there is! It's called the #initialize
method.
#initialize
Method
The We already know that any Ruby class can produce new instances of itself, via the
<Class Name>.new
method, whether or not that class has an #initialize
method. However, if we want each instance of our class to be created with
certain attributes, we must define an #initialize
method. An #initialize
method is a method that is called automatically whenever #new
is used.
Let's define an #initialize
method that takes in an argument of a dog's breed
and sets a @breed
variable equal to that argument. In other words, let's
define our #initialize
method to contain the functionality of the #breed=
method, so that a dog instance will get a breed assigned to it right away when
it is created, without us having to explicitly use the #breed=
method.
class Dog
attr_reader :breed
def initialize(breed)
@breed = breed
end
end
Now, we can call #new
like this:
lassie = Dog.new("Collie")
lassie.breed #=> "Collie"
When #new
is called, it will automatically invoke the #initialize
method and
pass the argument (or arguments). You can also think of the initialize method as
a constructor method. A constructor method is invoked upon the creation of an
instance of a class and used to help define the instance of that class.
Note: In the code above, we are now using the attr_reader
macro instead of
the attr_accessor
macro because we are setting the value of the breed
attribute inside our #initialize
method. This means we are not able to change
the value of breed
after it's initially set. In some cases this is desireable,
but if we want to be able to change the value of an attribute later, we would
use the attr_accessor
macro in addition to #initialize
.
Lab
Fork and clone the lab and run the tests with learn test
.
Person#initialize
with a Name
1. Define a Person
class in lib/person.rb
that provides an #initialize
method
that accepts an argument for the person's name. That argument should be stored
within a @name
instance variable.
Dog#initialize
with Name and Breed defaulting to "Mutt"
2. Define a Dog
class in lib/dog.rb
that provides an #initialize
method that
accepts an argument for the dog's name. That argument should be stored within a
@name
instance variable.
Additionally, Dog#initialize
should accept a second optional argument for the
dog's breed stored in an instance variable @breed
. When none is provided, it
should default to "Mutt".