you will find they essentially maintain a storage-object-level mutex -- every access obtains a lock upon that mutex to maintain thread-safety before going ahead with the operation. For concurrent scenarios, this is a disaster because you can only invoke one operation at a time.
Fortunately, there are few alternatives you can look at right inside the JDK. For maps, you can use:
and for lists or sets, you can look at these:
Java 6 users can also use these (Note: they are not O(1), but rather O(log(n)) operations):
However, there are few points worth knowing:
1. CopyOnWriteXxx collections are suited only for those scenarios where the read operations hugely outnumber the write operations.
2. Concurrent scenarios are better dealt with a strategy at application architecture level rather than brute force use of concurrency-optimized collections.
3. For concurrent scenarios, isolate operations with lifecycle-managed threads each dealing with immutable objects and/or small concurrent datasets rather than giant thread-safe ones. Minimize obtaining of locks rather than making everything thread-safe.
Please let me know what you think.