Classes and Objects

Question 1

What do ‘states’ do?

Answer 1

States track attributes for individual objects.

Question 2

What does ‘behavior’ do?

Answer 2

Behaviors are what objects are capable of doing.

Question 3

How are instance variables scoped?

Answer 3

Instance variables are scoped at the object (or instance) level, and are how objects keep track of their states.

Question 4

Describe an instance method.

Answer 4

Instance methods define the behaviors in a class. Instance methods defined in a class are available to objects (or instances) of that class.

Question 5

What is the initialize method often called.

Answer 5

We refer to the initialize method as a constructor, because it gets triggered whenever we create a new object.

Question 6

What is an instance variable?

Answer 6

An instance variable is a variable that exists as long as the object instance exists, and it is one of the ways we tie data to objects. It does not “die” after the initialize method is run. It “lives on”, to be referenced, until the object instance is destroyed. Every object’s state is unique, and instance variables are how we keep track.

Question 7

What do instance methods do within a class?

Answer 7

Instance methods allow all objects of the same class to have the same behaviors, though they contain different states.

Question 8

What is attr_reader?

Answer 8

This is a getter method that takes a symbol as an argument. An example of how it replaces the code for a getter method is:

Getter method:
def name
@name
end

replaced by : attr_reader :name

Question 9

What is attr_writer?

Answer 9

This is a setter method that takes a symbol as an argument. An example of how it replaces the setter method is:

Setter method
def set_name=(name)
@name = name
end

replaced by : attr_writer :name

Question 10

What is attr_accessor?

Answer 10

This is a Setter and a Getter method which takes a symbol as an argument.

attr_accessor :name

Question 11

Why not just reference the syntax for instance variables (@instance_variable) when using a setter or getter method?

Answer 11

Technically, you could just reference the instance variable, but it’s generally a good idea to call the getter method instead.

ie) “#{name} is old.” = getter method
“#{@name} is old.” = instance variable

Question 12

How many methods does this line of code give us?

attr_accessor :name, :height, :weight

Answer 12

6 - (name, name=, height, height=, weight, weight=)

Question 13

How do you disambiguate setter methods and creating a local variable?

Answer 13

Prefixing ‘self’ to the setter method name (i.e.. self.name). This tells Ruby that we’re calling a setter method, not creating a local variable. To be consistent, we could also adopt this syntax for the getter methods as well, though it is not required.

Question 14

What are class methods?

Answer 14

Class methods are methods we can call directly on the class itself, without having to instantiate any objects.

Question 15

Why do we need a class methods?

Answer 15

Class methods are where we put functionality that does not pertain to individual objects.

def self.what_am_i                # Class method definition
  "I'm a GoodDog class!"
end

GoodDog.what_am_i # => I’m a GoodDog class!

Question 16

What are class variables?

Answer 16

Class variables create variables for an entire class, just as instance variables capture information related to specific instances of classes (i.e., objects). Class variables are created using two @ symbols like so: @@class_variable..

Question 17

What are constants?

Answer 17

Variables that you never want to change.

Question 18

When is to_s called?

Answer 18

‘puts’ and string interpolation automatically call to_s on the object. May need to override the default to_s for these. However, it is also called with ‘p’, but this should not be overridden as it is useful in debugging.

Question 19

What are 2 use cases for ‘self’?

Answer 19

1) Use self when calling setter methods from within the class. Using self allows Ruby to disambiguate between initializing a local variable and calling a setter method.
self. name = ‘bob’ - calls setter method on instance variable ‘name’
2) Use self for class method definitions.

def self.what_am_i                # Class method definition
  "I'm a GoodDog class!"
end

GoodDog.what_am_i # => I’m a GoodDog class!

Question 20

What does an instance method return from within the class when it calls self?

Answer 20

From within the class, when an instance method calls self, it is returning the calling object.

self, inside of an instance method, references the instance (object) that called the method - the calling object. Therefore, self.weight= is the same as sparky.weight=, in our example.

Question 21

What is self referring to when used a class method?

Answer 21

self, inside a class but outside an instance method, is actually referring to the class itself.

self, outside of an instance method, references the class and can be used to define class methods. Therefore, def self.name=(n) is the same as def GoodDog.name=(n), in our example.

Question 22

What is a class?

Answer 22

Ruby defines the attributes and behaviors of its objects in classes. You can think of classes as basic outlines of what an object should be made of and what it should be able to do.

Question 23

What are objects?

Answer 23

Objects are created from classes. Think of classes as molds and objects as the things you produce out of those molds.

Question 24

Why do we need to use self with setter methods?

Answer 24

To disambiguate from creating a local variable.