Question

My question is about what are the fundamental/concrete data structure (like array) used in implementing abstract data structure implementations like variations maps/trees?

I’m looking for what’s used really in java collection, not theoretical answers.

Answer 1

Based on quick code review of Sun/Oracle JDK. You can easily find the details yourself.

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Lists/queues

ArrayList

Growing Object[] elementData field. Can hold 10 elements by default, grows by around 50% when cannot hold more objects, copying the old array to a bigger new one. Does not shrink when removing items.

LinkedList

Reference to Entry which in turns hold reference to actual element, previous and next element (if any).

ArrayDeque

Similar to ArrayList but also holding two pointers to internal E[] elements array – head and tail. Both adding and removing elements on either side is just a matter of moving these pointers. The array grows by 200% when is too small.

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Maps

HashMap

Growing Entry[] table field holding so called buckets. Each bucket contains linked list of entries having the same hash of the key module table size.

TreeMap

Entry<K,V> root reference holding the root of the red-black balanced tree.

ConcurrentHashMap

Similar to HashMap but access to each bucket (called segment) is synchronized by an independent lock.

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Sets

TreeSet

Uses TreeMap underneath (!)

HashSet

Uses HashMap underneath (!)

BitSet

Uses long[] words field to be as memory efficient as possible. Up to 64 bits can be stored in one element.