Tell about the hierarchy of collections in Java.

The hierarchy of collections in Java is built around the java.util.Collection interface, which is extended by several main interfaces:

• Collection — the basic interface for all collections.
  • List — an ordered list of elements, allows duplicates.
    • ArrayList — a dynamic array.
    • LinkedList — a doubly linked list.
    • Vector, Stack — outdated implementations.
  • Set — a collection of unique elements, unordered.
    • HashSet — based on a hash table.
    • LinkedHashSet — maintains insertion order.
    • TreeSet — a sorted set.
  • Queue — a queue where elements are processed in FIFO order.
    • LinkedList — implements Queue.
    • PriorityQueue — a priority queue.

Besides Collection, there is the Map interface (not inherited from Collection), which stores key-value pairs:

• Map
  • HashMap — a hash table.
  • LinkedHashMap — maintains insertion order.
  • TreeMap — a sorted map.

Thus, the collection hierarchy allows choosing the appropriate data structure depending on requirements for order, uniqueness, and performance.