When should I use SynchronousQueue over LinkedBlockingQueue

new SynchronousQueue()
new LinkedBlockingQueue(1)

What is the difference? When I should use SynchronousQueue against LinkedBlockingQueue with capacity 1?

the SynchronousQueue is more of a handoff, whereas the LinkedBlockingQueue just allows a single element. The difference being that the put() call to a SynchronousQueue will not return until there is a corresponding take() call, but with a LinkedBlockingQueue of size 1, the put() call (to an empty queue) will return immediately.

I can't say that i have ever used the SynchronousQueue directly myself, but it is the default BlockingQueue used for the Executors.newCachedThreadPool() methods. It's essentially the BlockingQueue implementation for when you don't really want a queue (you don't want to maintain any pending data).

As far as I understand code above do the same things.

No, the code is not the same at all.

Sync.Q. requires to have waiter(s) for offer to succeed. LBQ will keep the item and offer will finish immediately even if there is no waiter.

SyncQ is useful for tasks handoff. Imagine you have a list w/ pending task and 3 threads available waiting on the queue, try offer() with 1/4 of the list if not accepted the thread can run the task on its own. [the last 1/4 should be handled by the current thread, if you wonder why 1/4 and not 1/3]

Think of trying to hand the task to a worker, if none is available you have an option to execute the task on your own (or throw an exception). On the contrary w/ LBQ, leaving the task in the queue doesn't guarantee any execution.

Note: the case w/ consumers and publishers is the same, i.e. the publisher may block and wait for consumers but after offer or poll returns, it ensures the task/element is to be handled.

One reason to use SynchronousQueue is to improve application performance. If you must have a hand-off between threads, you will need some synchronization object. If you can satisfy the conditions required for its use, SynchronousQueue is the fastest synchronization object I have found. Others agree. See: Implementation of BlockingQueue: What are the differences between SynchronousQueue and LinkedBlockingQueue

SynchronousQueue works in a similar fashion with following major differences: 1) The size of SynchronousQueue is 0 2) put() method will only insert an element if take() method will be able to fetch that element from the queue at the same moment i.e. an element cannot be inserted if the consumer take() call is going to take some time to consume it.

SynchronousQueue - Insert only when someone will receive it at that moment itself.