Java Collection

Shorthand Mapping

• List  = Random (index-based access)

• Set   = Unique (no duplicates)

• Tree  = Ordered (sorted order)

• Hash  = Quick (fast lookup via hashing)

• Linked = Easy insert/remove

Legacy Classes → Modern Replacement

• Vector → ArrayList

  • Resizable array, synchronized; Thread-safe but slower.

• Stack → Deque (ArrayDeque preferred)

  • LIFO (last-in, first-out) stack; Extends Vector, so also synchronized.

• Hashtable → HashMap (or ConcurrentHashMap for thread safety)

  • Key–value pairs, synchronized; Doesn’t allow null keys or values

List.of() vs Arrays.asList()

// Returns java.util.ImmutableCollections.ListN (internal immutable class)

List<String> list1 = List.of("Alice", "Bob", "Charlie");

• Immutable (cannot add, remove, or modify) → Inherently thread-safe

• Does not all null → Throws NullPointerException

// java.util.Arrays$ArrayList (backed by the original array)

List<String> list2 = Arrays.asList("Alice", "Bob", "Charlie");

• Fixed-size (cannot add/remove, but can modify elements)

• Allows null