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 *  A <code>Hash</code> is a dictionary-like collection of unique keys and 

 *  their values. Also called associative arrays, they are similar to

 *  Arrays, but where an <code>Array</code> uses integers 

 *  as its index, a <code>Hash</code> allows you to use any object type. 

 *

 *  Hashes enumerate their values in the order that

 *  the corresponding keys were inserted. 

 *

 *  A Hash can be easily created by using its implicit form:

 *

 *    grades = {"Jane Doe" => 10, "Jim Doe" => 6}

 *  

 *  Hashes allow an alternate syntax form when your keys are always symbols.

 *  Instead of 

 *

 *    options = {:font_size => 10, :font_family => "Arial"}

 * 

 *  You could write it as: 

 *

 *    options = {font_size: 10, font_family: "Arial"}

 * 

 *  Each named key is a symbol you can access in hash:

 *

 *    options[:font_size]  #=> 10

 *

 *  A Hash can also be created through its <code>new</code> method:

 *

 *    grades = Hash.new

 *    grades["Dorothy Doe"] = 9

 *  <code>nil</code>. You can setup its default value by sending it as

 *  an argument on the Hash initialization: 

 *

 *    grades = Hash.new(0)

 *

 *  Or by using its <code>default</code> method:

 *

 *    grades = {"Timmy Doe" => 8}

 *    grades.default = 0

 *     

 *  Accessing a Hash requires using its key:

 *

 *    puts grades["Jane Doe"] #=> 10

 *   

 *  === Common Uses

 *

 *  Hashes are an easy way to represent data structures, such as

 *

 *    books         = {}

 *    books[:matz]  = "The Ruby Language"

 *    books[:black] = "The Well-Grounded Rubyist"

 * 

 *  Hashes are also commonly used as a way to have named parameters

 *  in functions. Note that no brackes are used below. If a hash is 

 *  the last argument on a method call, no braces are needed,  

 *  thus creating a really clean interface: 

 *  

 *    Person.create(name: "John Doe", age: 27)

 *

 *    def self.create(params)

 *      @name = params[:name]

 *      @age  = params[:age]

 *    end

 *