The composite design pattern allows you to interact with individual objects and compositions of objects in the same way. Oftentimes, this pattern aggregates several implementations of an interface into a single, combined implementation. For example:

interface BillableItem {
  fun showPrice()
}

class Burger : BillableItem {
  override fun showPrice() = 
    println("Burger: £3.50")
}

class Fries : BillableItem {
  override fun showPrice() = 
    println("Fries: £1.50")
}

class Milkshake : BillableItem {
  override fun showPrice() = 
    println("Milkshake: £2.20")
}

class Meal(
  private vararg val items: BillableItem
) : BillableItem {
  override fun showPrice() = 
    items.forEach(BillableItem::showPrice)
}

The Meal class is an example of the composite pattern as its API hides the detail of whether the class represents a single part of a hierarchy objects, or the entire tree.