In which Java package are thread-safe collections located?

java.util.concurrent.

This package contains a set of collection classes designed for efficient and safe operation in multithreaded environments. Key examples:

• ConcurrentHashMap
• CopyOnWriteArrayList
• ConcurrentLinkedQueue
• ConcurrentSkipListMap
• ConcurrentSkipListSet

These collections provide atomic operations or use other mechanisms (such as immutable copies) to avoid issues with concurrent access, such as data races or deadlocks, without requiring explicit synchronization from the user in most cases.

There are also thread-safe "wrappers" for standard collections in the java.util package (e.g., Collections.synchronizedList, Collections.synchronizedMap), but collections from java.util.concurrent are generally preferred for high-performance multithreaded applications due to better scalability and efficiency.