synchronized block for an Integer object

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syncObject is changing each time you ++ it (the ++ is converting it to a primitive int, incrementing it, and then autoboxing it back to the Integer object. Integer objects are immutable … once they are created, they cannot change.

Bottom ine is that you are not using the same syncPObj in all the threads, different threads use different syncObjects at different times to sync on.

use one object as the synchronization (call it syncObj), and declare it as a final Object:

private static final Object syncObject = new Object();

Then your counter should be a primitive (int) for perofrmance, call it ‘counter’ or something.

Synchronize on syncObject, and increment counter.

Edit: as per @jsn, the done flag is also broken in that your code has a ‘tight loop’ on the isAllDone() method, and that is bad practice. You should use thread[i].join() to wait (blocking) on each thread’s completion, and then check the status from that. Using an ExecutorService is the ‘right way’.

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