The main difference between an immutable object (all properties final) and an effectively immutable object (properties aren’t final but can’t be changed) is safe publication. You can safely publish an immutable object in a multi threaded context without having to worry about adding synchronization, thanks to the guarantees provided by the Java Memory Model for … Read more
For HotSpot: The object header consists of a mark word and a klass pointer. The mark word has word size (4 byte on 32 bit architectures, 8 byte on 64 bit architectures) and the klass pointer has word size on 32 bit architectures. On 64 bit architectures the klass pointer either has word size, but … Read more
The FAQ is not the authority on the matter; the JLS is. Section 17.4.4 specifies synchronizes-with relationships, which feed into happens-before relationships (17.4.5). The relevant bullet point is: An unlock action on monitor m synchronizes-with all subsequent lock actions on m (where “subsequent” is defined according to the synchronization order). Since m here is the … Read more
The key point of the program order rule is: in a thread. Imagine this simple program (all variables initially 0): T1: x = 5; y = 6; T2: if (y == 6) System.out.println(x); From T1’s perspective, an execution must be consistent with y being assigned after x (program order). However from T2’s perspective this does … Read more
If you look at the underlying array reference you’ll see it’s marked as volatile. When a write operation occurs (such as in the above extract) this volatile reference is only updated in the final statement via setArray. Up until this point any read operations will return elements from the old copy of the array. The … Read more
Obviously the write to currentPos doesn’t happen-before the read of it, but I don’t see how that can be the issue. currentPos = new Point(currentPos.x+1, currentPos.y+1); does a few things, including writing default values to x and y (0) and then writing their initial values in the constructor. Since your object is not safely published … Read more