“UnsafeSequence can be fixed by making getNext a synchronized method, as shown in Sequence in Listing 1.2,[3] thus preventing the unfortunate interaction in Figure 1.1. (Exactly why this works is the subject of Chapters 2 and 3.) [3] @GuardedBy is described in Section 2.4; it documents the synchronization policy for Sequence. Listing 1.2. Thread-safe Sequence Generator. In the absence of synchronization, the compiler, hardware, and runtime are allowed to take substantial liberties with the timing and ordering of actions, such as caching variables in registers or processor-local caches where they are temporarily (or even permanently) invisible to other threads. These tricks are in aid of better performance and are generally desirable, but they place a burden on the developer to clearly identify where data is being shared across threads so that these optimizations do not undermine safety. (Chapter 16 gives the gory details on exactly what ordering guarantees the JVM makes and how synchronization affects those guarantees,”