Tell me about Java Collections API: hierarchy, structures, implementations.

Java Collections API is a set of interfaces and classes for working with collections of objects, providing convenient data structures and algorithms.

Hierarchy of main interfaces:

• Collection — basic interface for groups of objects

  • List — ordered collection, allows duplicates (e.g., ArrayList, LinkedList)
  • Set — collection of unique elements (e.g., HashSet, TreeSet)
  • Queue — collection for processing elements in order (e.g., LinkedList, PriorityQueue)

• Map — key-value mapping, does not inherit from Collection (e.g., HashMap, TreeMap)

Main implementations:

• ArrayList — dynamic array, fast access by index
• LinkedList — doubly linked list, efficient addition/removal in the middle
• HashSet — based on hash table, provides fast search
• TreeSet — based on red-black tree, elements are sorted
• HashMap — hash table for key-value pairs, fast access
• TreeMap — sorted map based on a tree

Java Collections API also includes utilities for working with collections (Collections class), supports iterators, stream operations, and provides flexibility and performance when working with data.