In Java, both synchronized and concurrent collections are used to handle thread-safety in a multi-threaded environment. However, they differ in their implementation and performance characteristics.
A synchronized collection is achieved by wrapping a non-synchronized collection using the Collections.synchronizedXXX method. This method returns a synchronized view of the original collection, which means that all methods are synchronized on a common lock. This lock can result in performance overhead in cases where multiple threads try to access the collection concurrently.
On the other hand, a concurrent collection is designed to allow concurrent access by multiple threads without the need for explicit synchronization. It is implemented using various algorithms, such as lock-striping or non-blocking algorithms, to ensure thread-safety and high performance. Concurrent collections also provide atomic operations that can be performed in a single step, which makes them useful for building scalable concurrent applications.
Here is an example of how to create a synchronized and a concurrent collection in Java:
// create a synchronized list
List<String> synchronizedList = Collections.synchronizedList(new ArrayList<>());
// create a concurrent map
ConcurrentMap<String, Integer> concurrentMap = new ConcurrentHashMap<>();In general, if the application requires high concurrency and performance, a concurrent collection should be used. However, if the application does not require high concurrency and performance, or if the data access pattern is mostly sequential, a synchronized collection may be sufficient and easier to use.
It is worth noting that while concurrent collections are designed for thread-safety, they do not eliminate the need for proper synchronization in all cases. In some cases, external synchronization may still be required, such as when performing compound operations that involve multiple method calls on the collection.