What data structures in Java are synchronized, and which are not?

In Java, some data structures are synchronized (thread-safe), while others are not.

Synchronized:

• Vector — a synchronized version of ArrayList.
• Hashtable — a synchronized version of HashMap.
• Stack — inherits from Vector, so it is also synchronized.
• Collections wrapped with Collections.synchronizedList(), synchronizedMap(), etc.
• Classes from the java.util.concurrent package, such as ConcurrentHashMap, CopyOnWriteArrayList.

Non-synchronized:

• ArrayList
• LinkedList
• HashMap
• HashSet

If you need to use a non-synchronized collection in a multithreaded environment, you can wrap it in a synchronized collection using methods from Collections or use specialized classes from java.util.concurrent.

Example:

List<String> list = Collections.synchronizedList(new ArrayList<>());