card.rb

class Reward::Card
  def initialize(params)
    @type     = params[:type]
    @username = params[:username]
    @password = params[:password]
    @klass    = Object.const_get(@type.capitalize)
  end

  def reward_balance
    begin
      @klass.card_balance
    rescue => e
      puts e.message
      puts "unknown class: #{@klass}"
    end
  end
end

class Starbucks < Reward
  def self.card_balance
    puts "call to starbucks card_balance."
    puts @username
    puts @password
  end
end

class United < Reward
  def self.card_balance
    puts "call to united card_balance."
    puts @type
  end
end