Collections Comparison

There are three families of collections. List is an ordered sequence that allows duplicates and lets you access by index — ArrayList is the default, and LinkedList also acts as a deque. Set stores unique elements, no duplicates. Map maps unique keys to values. The trick that ties everything together is a single pattern that repeats inside both Set and Map. The Hash variant — HashSet and HashMap — is unordered and operates in constant time, so it's the fastest and the default. The Linked variant — LinkedHashSet and LinkedHashMap — preserves insertion order, and LinkedHashMap can also use access order, which is what you use to build an LRU cache. The Tree variant — TreeSet and TreeMap — keeps everything sorted, with log-n operations, and it supports range queries like headMap and tailMap. So once you understand HashSet versus LinkedHashSet versus TreeSet, you automatically understand the three maps too. To choose, I ask three questions. First, do I allow duplicates? If yes, List; if no, Set. Do I look things up by a key? Then Map. Second, does order matter? If insertion order matters, use a Linked variant; if I need sorting, use a Tree variant; otherwise use Hash for speed. Third, what's my access pattern — random access points to ArrayList, end-heavy points to a deque. That three-question framework, plus the Hash-Linked-Tree pattern, lets me place any collection quickly and quote the right complexity for each.