Map Operations: compute, merge, putIfAbsent

Java 8 Map methods (computeIfAbsent, merge, putIfAbsent, getOrDefault, compute) replace the manual containsKey-then-put pattern with single atomic operations that are concurrency-safe in ConcurrentHashMap.

putIfAbsent always constructs the value before checking the key (eager), while computeIfAbsent runs the mapping function only when the key is absent (lazy), making computeIfAbsent the right choice for expensive value creation.