Why is HashMap not thread-safe, and what should you use instead in concurrent code?

HashMap performs no internal synchronization, so concurrent put() calls from multiple threads can update the same bucket at the same time and silently lose one of the updates. In older versions of Java, concurrent resizing of a HashMap could even corrupt its internal bucket chains into a circular structure, causing get() to spin forever in an infinite loop. The recommended replacement is ConcurrentHashMap, which supports lock-free reads, uses fine-grained per-bucket locking for writes, and provides atomic compound operations like compute(), merge(), and computeIfAbsent(). It's also worth avoiding Collections.synchronizedMap() as a fix, because it wraps every single operation, including reads, in one coarse-grained lock, and compound operations performed on it (like check-then-act) still require external synchronization to be correct.