Java で記述されたコードを Ruby で再現

gift_point.rb

class GiftPoint
  attr_reader :value

  MIN_POINT = 0
  STANDARD_MEMBERSHIP_POINT = 3_000
  PREMIUM_MEMBERSHIP_POINT = 10_000

  def initialize(point)
    if point < MIN_POINT
      raise RuntimeError, "ポイントが0以上ではありません"
    end

    @value = point
  end

  private_class_method :new
  private_constant :MIN_POINT, :STANDARD_MEMBERSHIP_POINT, :PREMIUM_MEMBERSHIP_POINT

  class << self
    def for_standard_membership
      new(STANDARD_MEMBERSHIP_POINT)
    end

    def for_premium_membership
      new(PREMIUM_MEMBERSHIP_POINT)
    end
  end
end

standard_membership_point = GiftPoint.for_standard_membership
premium_membership_point = GiftPoint.for_premium_membership

p standard_membership_point.value # => 3000
p premium_membership_point.value # => 10000

# It gets error:
gift_point = GiftPoint.new(100)
# => private method `new' called for GiftPoint:Class (NoMethodError)`
p gift_point.value