Concurrent Collections

A plain HashMap used as a shared cache in multi-threaded code causes infinite loops (Java 7), silent data corruption (Java 8+), and stale reads because HashMap has no synchronization or visibility guarantees.

Concurrent writes to a plain HashMap during resize create a circular linked list in Java 7 (infinite loop, 100% CPU, no exception) or corrupt the node array in Java 8+ (lost entries, wrong results, silent data corruption).

ConcurrentSkipListMap is the thread-safe sorted map: O(log n) with ordered keys and range queries via a lock-free skip list; use it over ConcurrentHashMap only when you actually need sorting or navigation.