📚 COLLECTIONS

Term Glossary

• ArrayList vs LinkedList

  Pick ArrayList. Almost always.
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• HashMap

  O(1) average get/put, unordered.
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• ConcurrentHashMap

  Thread-safe hash map, lock-striped.
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• TreeMap

  Sorted by key (red-black tree).
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• LinkedHashMap

  Insertion-ordered HashMap.
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• EnumMap

  Map keyed by enum, backed by array.
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• ConcurrentSkipListMap

  Thread-safe sorted map.
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• Iterator vs ListIterator

  Forward vs bidirectional cursor.
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• fail-fast

  Iterators detect mod during iteration.
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• Collectors.groupingBy

  Stream → Map<K, List<V>>.
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• Map.Entry

  Key-value pair view inside a map.
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• Deque / ArrayDeque

  Double-ended queue (use as a stack).
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• CopyOnWriteArrayList

  Snapshot on every write.
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• Comparator.comparing

  Build a comparator from a key extractor.
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• BlockingQueue

  Thread-safe queue with blocking ops.
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