有权访问实例方法的变量

class BankAccount

  def self.create_for(first_name, last_name)
    @accounts ||= []
    @accounts << BankAccount.new(first_name, last_name)
  end

  def initialize(first_name, last_name)
    @balance = 0
    @first_name = first_name
    @last_name = last_name
  end

 def self.find_for(first_name, last_name)
    @accounts.find{|account| account.full_name == "#{first_name} #{last_name}"}
  end

  def full_name
    "#{@first_name} #{@last_name}"
  end

end
class BankAccount

  def self.create_for(first_