A synchronized list only synchronizes methods of this list.
It means a thread won’t be able to modify the list while another thread is currently running a method from this list. The object is locked while processing method.
As an example, Let’s say two threads run addAllon your list, with 2 different lists (A=A1,A2,A3 and B=B1,B2,B3) as parameter.
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As the method is synchronized, you can be sure those lists won’t be merged randomly like
A1,B1,A2,A3,B2,B3 -
You don’t decide when a thread handover the process to the other thread. Each method call has to fully run and return before the other one could run. So you can either get
A1,A2,A3,B1,B2,B3orB1,B2,B3,A1,A2,A3(As we don’t know which thread call will run first).
In your first piece of code, both threads runs on the same time. And both try to add an element to the list. You don’t have any way to block one thread except the synchronization on the add method so nothing prevent thread 1 from running multiple add operation before handing over the process to thread 2. So your output is perfectly normal.
In your second piece of code (the uncommented one), you clearly state that a thread completely lock the list from the other thread before starting the loop. Hence, you make sure one of your thread will run the full loop before the other one could access the list.