Ruby(2) - Variables and Class

2.1 Variables 
   There are four kinds of variables in Ruby, and varialbes are not required to be declared before use.

Type	Name Convention	Example	Comments
local variable	starting with a lowercase letter.	name	like variables in function
instance variable	preceded by an ``at'' sign (``@'').	@name	like variables in Java Class
class variable	preceded by two ``at'' sign (``@'').	@@name	like static variables in Java Class
constant	start with an uppercase letter.	Name

2.2 Class 
    2.2.1 New a class instance
    In Ruby you should create a instance of a class by calling the class's new method. The new method will invoke the matched initialize method in the class according the parameters you provided to the new method.

#－－－－－－－－－－－－－Class Example－－－－－－－－－－－－－－－－
class Song
  def initialize(name, artist, duration)         # Constructor
    @name     = name
    @artist   = artist
    @duration = duration
  end

  @@plays = 0                                    # Class variable

  def initialize(name, artist, duration, plays)  # Constructor
    @name     = name
    @artist   = artist
    @duration = duration
    @plays    = 0
  end

  def play                                       # Instance Method
    @plays += 1
    @@plays += 1
    "This  song: #@plays plays. Total #@@plays plays."  # Last
  end
  
  def Song.setPlays(times)                       # Class Method
    @@plays = times
  end
end

    2.2.2 Attributes
    Attribute can be defined in two ways:
   1. use attr_reader and attr_writer.
       2. define method
#------------------Attribute Example--------------------------------------
class Song
  #attr_reader :name, :artist, :duration         # Attribute name readonly

  def name                                       # Attribute name readonly
    @name 
  end  

  def artist                                     # Attribute artist readonly
    @artist 
  end  

  def duration                                   # Attribute duration readonly
    @duration 
  end  

  #attr_writer :duration                         # Attribute duration writable
  def duration=(newDuration) 
    @duration = newDuration 
  end 
end

    Virtual attribute : a kind of attribute not associated with an instance variable directly
class Song 
  def durationInMinutes 
    @duration/60.0   # force floating point 
  end 
  def durationInMinutes=(value) 
    @duration = (value*60).to_i 
  end 
end 

    2.2.3 Access Control
    The only way to change an object's state in Ruby is by calling one of its methods., no class variable can be accessed directly.
         Three access level:

• Public methods can be called by anyone---there is no access control. Methods are public by default (except for initialize, which is always private).
• Protected methods can be invoked only by objects of the defining class and its subclasses. Access is kept within the family.
• Private methods cannot be called with an explicit receiver. Because you cannot specify an object when using them, private methods can be called only in the defining class and by direct descendents within that same object.

#-------------------------Specifying Access Control------------------------------
class MyClass

      def method1    # default is 'public'
        #...
      end

  protected          # subsequent methods will be 'protected'

      def method2    # will be 'protected'
        #...
      end

  private            # subsequent methods will be 'private'

      def method3    # will be 'private'
        #...
      end

  public             # subsequent methods will be 'public'

      def method4    # and this will be 'public'
        #...
      end
end

#---------------------Another way-----------------------------
class MyClass

  def method1
  end

  # ... and so on

  public    :method1, :method4
  protected :method2
  private   :method3
end