100.times.each_with_object([]) do |i, a|
  a << Thread.new do
    p i
    Thread.sleep 1
  end
end.each { |t| t.join }

class A
  attr_reader :threads

  def initialize
    @threads = 100.times.each_with_object([]) do |i, a|
      a << Thread.new do
        p i
        Thread.sleep 1
      end
    end
  end
end

A.new.threads.each { |t| t.join }