Inheritance in Ruby
Objectives
- Implement inheritance
- Describe what has been inherited from one class to another
- Compare and contrast inheritance in Ruby with Prototypical inheritance in Javascript
- Utilize inheritance to reduce repetition
- Utilize inheritance to model the world
- Install
byebug
usinggem
. byebug is an improved REPL for Ruby that allows for better debugging and lets you work interactively with pre-written scripts through the use ofbyebug
Quick Review
- What is a method? What is a class?
- What is an instance method?
- Why do we use a class?
Inheritance
Inheritance is used to indicate that one class will get most or all of its features from a parent class. Inheritance is useful for a couple reasons.
- DRY - Don't Repeat Yourself & reuse code functionality
- Faster implementation time
Example
class Rectangle
def initialize(l, w)
@length = l
@width = w
end
def get_area
@length * @width
end
def print_shape
puts "This is a rectangle"
end
end
class Square < Rectangle
def initialize(s)
super(s, s)
end
def print_shape
puts "This is a rectangle and a square"
end
end
Note from the example above...
- In order to inherit from a class, we use the
<
keyword and specify the class we want to inherit from - In order to correctly inherit from
Square
, we need to call the parent class'sinitialize
method. We can do so by using thesuper
method.- If we forget to do this, class and instance variables in the parent will not be initialized
- When you invoke the
super
method with arguments, Ruby sends a message to the parent of the current object, asking it to invoke a method of the same name as the method invoking super.
- Once we inherit from another class, we can access methods and properties from the parent. In the case of this
Square
class, we also overrode the functionality of theprint_shape
method.
Binding to an interactive console with byebug
You can use byebug to halt the execution of your program and start an interactive console. This is a great debugging tool.
Create a few rectangles and a square at the end of your file. Write byebug
to stop the program after the shapes
are created.
r1 = Rectangle.new(3, 4)
r2 = Rectangle.new(4, 7)
s1 = Square.new(3)
# halt the program just after the shapes are created
byebug
Now you can type expressions into the console and have them evaluated. Your console will look like this. Typing r1
and r2
shows the type of objects that exist in those variables. Typing r1.get_area
executes the function and prints the
result.
Having access to a console like this is a great way to explore ruby with new pieces of code to make sure they work. Type things into the interactive console to figure out how you could calculate the area of a circle: Pi r squared.
A Guide through Animals
class Animal
def initialize(kind)
@kind = kind
@state = "awake"
end
def eat(food)
if @state == "awake"
puts "NOM-nom!!"
puts "(#{@kind} has eaten #{food})"
else
puts "SLEEPING"
end
self
end
def sleep
@state = "sleeping"
self
end
def wake
@state = "awake"
self
end
end
class Person < Animal
def initialize(age, gender, name)
super("Human")
@age = age
@gender = gender
@name = name
end
end
Exercise
Create a Mammal class, Cat class, and Dog class. Have Cat and Dog inherit from Mammal. Include some attributes for each class and a method for mammal.
Make cats fall asleep when they're fed tuna. Make dogs fall asleep when they're fed a bone.