• List: items in insertion order ArrayList.new [-3, 1, 4, 1, 5, 9] # [-3, 1, 4, 1, 5, 9] • Set: unique items in no particular order HashSet.new [-3, 1, 4, 1, 5, 9] # [1, -3, 4, 5, 9] o SortedSet: unique items with given ordering TreeSet.new [-3, 1, 4, 1, 5, 9] # [-3, 1, 4, 5, 9] • Map: unique keys (in no particular order) with values HashMap.new 'foo'=>0, 'bar'=>1, 'baz'=>2 # {baz=2, foo=0, bar=1} o SortedMap: unique keys (with ordering) with values TreeMap.new 'foo'=>0, 'bar'=>1, 'baz'=>2 # {bar=1, baz=2, foo=0}