Synchronization of Collections

Question

Why should we use only the Collection objects returned using the related method calls in "java.util.Collections" class in the synchronization block of our code???
Instead, why cant we use the ordinary Collection objects obtained without executing these methods calls for the same???
fun_one

Answer 1

To sum up, why the object passed into the synchronization block be the synchronized version, though we're specifically passing the collection object in the synchronized block.I'm still not entirely sure what you mean. I'm confused primarily by the talk of "passing" objects in/into synchronized blocks. A code example would be good here.
The way I read it your question is this: why use an inherently synchronized Collection (e.g. Collections.synchronizedCollection(new ArrayList())) rather than access an unsynchronized collection from within synchronized blocks (e.g. synchronized(list) { list.get(0); }).
The answer to that would be - "because it's putting all burden on users of the collection to synchronize correctly". Such explicitly synchronized code will be prone to subtle bugs. Even if your code is correct, you might pass the collection to someone else's code (e.g. library) that will know nothing of your bespoke synchronization scheme and therefore will access the collection incorrectly.