SongList容器

class Song
  def initialize(name, artist, duration)
    @name=name
    @artist=artist
    @duration=duration
  end
  
  attr_accessor :name, :duration, :artist
end

class KaraokeSong < Song
  def initialize(name, artist, duration, lyrics)
    super(name, artist, duration)
    @lyrics=lyrics
  end
    
  attr_writer :lyrics
  attr_reader :lyrics
    
end

class SongList
  
  def initialize
    @songs= Array. new
  end
  
  def append(song)
    @songs.push(song)
    self
  end
  
  def delete_first
    @songs.shift
  end
  
  def delete_last
    @songs.pop
  end
  
  def [](index)
    @songs[index]
  end
end

require 'test/unit'
class TestSongList < Test::Unit::TestCase
  def test_delete
    list = SongList. new
    s1 =Song. new('title1','artist1',1)
    s2 =Song. new('title2','artist2',2)
    s3 =Song. new('title3','artist3',3)
    s4 =Song. new('title4','artist4',4)
    
    list.append(s1).append(s2).append(s3).append(s4)
    
    assert_equal(s1, list[0])
    assert_equal(s3,list[2])
    assert_nil(list[9])

    assert_equal(s1,list.delete_first)
    assert_equal(s2,list.delete_first)
    assert_equal(s4,list.delete_last)
    assert_equal(s3,list.delete_last)
  end
end