A Hash is a collection of key-value pairs. It is similar to an Array, except that indexing is done via arbitrary keys of any object type, not an integer index. The order in which you traverse a hash by either key or value may seem arbitrary, and will generally not be in the insertion order.

Hashes have a default value that is returned when accessing keys that do not exist in the hash. By default, that value is nil.

Hash uses key.eql? to test keys for equality. If you need to use instances of your own classes as keys in a Hash, it is recommended that you define both the eql? and hash methods. The hash method must have the property that a.eql?(b) implies a.hash == b.hash.

class MyClass
  attr_reader :str
  def initialize(str)
    @str = str
  end
  def eql?(o)
    o.is_a?(MyClass) && str == o.str
  end
  def hash
    @str.hash
  end
end

a = MyClass.new("some string")
b = MyClass.new("some string")
a.eql? b  #=> true

h = {}

h[a] = 1
h[a]      #=> 1
h[b]      #=> 1

h[b] = 2
h[a]      #=> 2
h[b]      #=> 2