In this lesson, we'll cover more advanced aspects of OOP in Ruby, including information hiding, modules, and mixins.

Object-Oriented Programming II

You might be wondering why we need to hide information in Ruby. Isn't it okay for every part of your Ruby program to know what every other part is doing?

Possibly, if you're the only one writing and using your software. But when other people are working on or using your programs, they may attempt to muck around with the way different parts of your program do their jobs. You don't want that!

For this reason, Ruby allows you to explicitly make some methods `public` and others `private`. Public methods allow for an **interface** with the rest of the program; they say, "Hey! Ask me if you need to know something about my class or its instances."

Private methods, on the other hand, are for your classes to do their own work undisturbed. They don't want anyone asking them anything, so they make themselves unreachable!

Need-to-Know Basis

We saw in [the lesson on classes](https://www.codecademy.com/courses/learn-ruby/lessons/object-oriented-programming-i/exercises/why-classes) that Ruby needs methods in order to access attributes. For instance, if we want to access a `@name` instance variable, we had to write something like
```rb
def name
  @name
end
```
Well, no longer! We can use `attr_reader` to access a variable and `attr_writer` to change it. If we write
```rb
class Person
  attr_reader :name
  attr_writer :name
  def initialize(name)
    @name = name
  end
end
```

Ruby does something like this for us automatically:

```rb
def name
  @name
end

def name=(value)
  @name = value
end
```

Like magic, we can read and write variables as we please! We just pass our instance variables (as symbols) to `attr_reader` or `attr_writer`.

(That `name=` might look funny, but you're allowed to put an `=` sign in a method name. That's just a Ruby convention saying, "hey, this method sets a value!")

attr_reader, attr_writer

Let's build a class! 

```rb
class ClassName
  def method_name(parameter)
    @class_variable = parameter
  end
end
```

1. First we tell Ruby that we want to construct our class `ClassName`. Don't forget the `end` to end your class!
2. Inside our `ClassName` is where we define our methods. Here we have `method_name` followed by `(parameter)`, When we use this method it will look like this `method_name("hello!")`
3. If our parameter above were "Hello!" it would be assigned to our variable `@class_variable` on the following line.
4. Again, don't forget to `end` your `method_name`!

Quick Review: Building a Class

Module Magic

What we did in the last exercise might not have seemed strange to you, but think about it: we mixed together the behaviors of a class and a module!

When a module is used to mix additional behavior and information into a class, it's called a **mixin**. Mixins allow us to customize a class without having to rewrite code!

The Marriage of Modules and Classes

We can do more than just `require` a module, however. We can also `include` it!

Any class that `include`s a certain module can use those module's methods!

A nice effect of this is that you no longer have to prepend your constants and methods with the module name. Since everything has been pulled in, you can simply write `PI` instead of `Math::PI`.

Feeling Included

Perfect! But not all our Ruby methods should be public.

Just as we use `public` to announce our public methods, we use `private` to specify our private ones:
```rb
class ClassName
  # Some class stuff

  public
  # Public methods go here
  def public_method; end

  private
  # Private methods go here
  def private_method; end
end
```

`private` methods are just that: they're private to the object where they are
defined. This means you can only call these methods from other code inside
the object. Another way to say this is that the method cannot be called with an *explicit receiver*. You've been using receivers all along—these are the objects on which methods are called! Whenever you call `object.method`, `object` is the receiver of the `method`.

In order to access private information, we have to create public methods that know how to get it. This separates the private *implementation* from the public *interface*, and we'll go over this in more detail in the next two exercises.

Private! Keep Out!

You can think of a **module** as a toolbox that contains a set methods and constants. There are lots and lots of Ruby tools you might want to use, but it would clutter the interpreter to keep them around all the time. For that reason, we keep a bunch of them in modules and only pull in those module toolboxes when we need the constants and methods inside!

You can think of modules as being very much like classes, only modules can't create instances and can't have subclasses. They're just used to store things!

What's a Module?

If we want to both read and write a particular variable, there's an even shorter shortcut than using `attr_reader` and `attr_writer`. We can use `attr_accessor` to make a variable readable *and* writeable in one fell swoop.

attr_accessor

Perfect!

Some modules, like `Math`, are already present in the interpreter. Others need to be explicitly brought in, however, and we can do this using `require`. We can do this simply by typing
```rb
require 'module'
```
We want to use the Ruby `Date` module to show today's date, but we haven't `require`d it yet!

A Few Requirements

Great work! Now we'll get into the details of public and private methods.

Methods are public by default in Ruby, so if you don't specify `public` or `private`, your methods will be public. In this case, however, we want to make it clear to people reading our code which methods are public. We do this by putting `public` before our method definitions, like so:
```rb
class ClassName
  # Some class stuff
  public
  def public_method
    # public_method stuff
  end
end
```
Note that everything after the `public` keyword through the `end` of the class definition will now be public unless we say otherwise. (We'll say otherwise in the next exercise.)

Going Public

Private Affairs

Whereas `include` mixes a module's methods in at the instance level (allowing instances of a particular class to use the methods), the `extend` keyword mixes a module's methods at the *class* level. This means that *class itself* can use the methods, as opposed to *instances* of the class.

Extend Your Knowledge

One of the main purposes of modules is to separate methods and constants into named spaces. This is called (conveniently enough) **namespacing**, and it's how Ruby doesn't confuse `Math::PI` and `Circle::PI`.

See that double colon we just used? That's called the **scope resolution operator**, which is a fancy way of saying it tells Ruby *where* you're looking for a specific bit of code. If we say `Math::PI`, Ruby knows to look inside the `Math` module to get that `PI`, not any other `PI` (such as the one we created in `Circle`).

Let's get some practice in with an existing Ruby module: `Math`.


Resolve to Keep Learning

Almost there! Our review wouldn't be complete without a little mixin magic.

Mixin for the Win

Now you understand why we said mixins could give us the ability to mimic inheriting from more than one class: by mixing in traits from various modules as needed, we can add any combination of behaviors to our classes we like!

Imitating Multiple Inheritance

All right! Let's do a little practice with `public` methods.

A Matter of Public Knowledge

You can pull in pre-existing modules (we'll show you how soon!), but you can also make your own. Modules are super easy to make! You just use the `module` keyword, like so:
```rb
module ModuleName
  # Bits 'n pieces
end
```
Like class names, module names are written in CapitalizedCamelCase, rather than lowercase_with_underscores.

It doesn't make sense to include variables in modules, since variables (by definition) change (or vary). Constants, however, are supposed to always stay the same, so including helpful constants in modules is a great idea.

Ruby doesn't *make* you keep the same value for a constant once it's initialized, but it will warn you if you try to change it. Ruby constants are written in ALL_CAPS and are separated with underscores if there's more than one word.

An example of a Ruby constant is `PI`, which lives in the `Math` module and is approximately equal to 3.141592653589793. We created our own `PI` in the previous exercise, but don't worry: because they're in separate modules, Ruby knows to keep them separate.