flatten arrays of integers without using Array.flatten

flatten.rb

class Flatten
  def initialize(array)
    @array = array
  end

  def flatten(array = @array)
    if array.is_a? Integer
      [array]
    else
      array.inject([]) { |memo, elem| memo + flatten(elem) }
    end
  end
end

flatten_test.rb

require 'minitest/autorun'
require_relative 'flatten'

class FlattenTest < Minitest::Test
  def test_flatten
    assert_equal [1,2,3,4], Flatten.new([[1,2,[3]],4]).flatten
  end
end