Special Maps & Collections

WeakHashMap stores keys as weak references so that entries are automatically removed when the garbage collector reclaims the key.

IdentityHashMap uses reference equality (==) instead of equals() for key comparison, so two distinct objects with the same content are treated as two different keys.

Using a mutable StringBuilder as a HashMap key fails because StringBuilder uses identity-based equals/hashCode and because mutating the key after insertion strands the entry in the wrong bucket.

EnumSet is a Set backed by a bit-vector where each bit represents one enum constant, and EnumMap is a Map backed by an array indexed by enum ordinal, both far faster and smaller than their hash-based counterparts.

NavigableMap and NavigableSet extend the sorted interfaces with closest-match lookups (floor, ceiling, lower, higher) and live-backed range views (headMap, tailMap, subMap).