4. The basic concepts (abstractions) for storage and structuring of object instances include:
Bag (multi-sets) - A bag is an unordered collection of elements which may include duplicate elements. (Duplicate has a very specific meaning- the equals() method defined in the Comparator interface.)
Set - A set is an unordered collection of elements. Duplicate elements are not allowed.
List - A list is an ordered collection of elements.
Map - A map is an unordered collection of key-value pairs. The keys in a map must be unique.