== 1. new ArrayList<>(list) and List.copyOf share the elements ==
  boxes     = [Box(100), Box(2)]
  viaCtor   = [Box(100), Box(2)]
  viaCopyOf = [Box(100), Box(2)]
  after boxes.add(Box(3)): viaCtor.size() = 2

== 2. The deep version needs the element copy, so a stream or a loop ==
  boxes = [Box(-1), Box(2), Box(3)]
  deep  = [Box(100), Box(2), Box(3)]

== 3. HashMap copy constructor: new map, same values ==
  mCopy.get("k") = Box(55)

== 4. 1-D primitive arrays: clone / copyOf are true copies ==
  a = [99, 2, 3], clone = [1, 2, 3], copyOf = [1, 2, 3]

== 5. 2-D arrays: the rows are shared ==
  grid     = [[99, 2], [3, 4]]
  shallow  = [[99, 2], [3, 4]]
  deepGrid = [[1, 2], [3, 4]]

== 6. Object arrays: copyOf copies references, not objects ==
  arrCopy[0] = Box(77), same object: true