Better Design?.rb

# - I've assumed you've separated the items into two arrays
#   because you want to select those items at some point later.
#   I've refactored to 'filter' the items on the way out, rather
#   than filtering them on the way in.
# - Also added a :type method rather than class checking.
# - You could even use Set instead of [] here

class Foo
  def initialize(initial_items = [])
    @items = initial_items
  end

  def add(item)
    @items << item
  end

  def a
    select(:a)
  end

  def b
    select(:b)
  end

  private

  def select(type)
    @items.select { |item| item.type == type }
  end
end

# If you wanted to create more types in the future, say :c, you
# could dynamically generate the accessors?

class A
  def initialize(name="Unknown")
    @name = "A: #{name}"
    @time = Time.now
  end
  
  def type
    :a
  end
end

class B
  def initialize(name="Unknown")
    @name = "B: #{name}"
    @time = Time.now
  end
  
  def type
    :b
  end
end

foo = Foo.new
foo.add(A.new)
foo.add(B.new)