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is it enough to use only local variables and no instance variables. Thus only using memory on the stack (per thread).
But what happens when you create a new MyObject that is local to the method. Doesn’t the new object get created on the heap ? Is it thread safe becuase the reference to it is local (thread safe) ?
It is thread safe because if it is only referenced by variables in that particular method (it is, as you said, a local variable), then no other threads can possibly have a reference to the object, and therefore cannot change it.
Imagine you and I are pirates (threads). You go and bury your booty (the object) on an island (the heap), keeping a map to it (the reference). I happen to use the same island for burying my booty, but unless you give me your map, or I go digging all over the island (which isn’t allowed on the island of Java), I can’t mess with your stash.