In Java, any object can be used to synchronize any data, e.g.

  synchronized(foobar_object){ foo(); }
  synchronized(foobar_object){ bar(); }
  synchronized(foobar_object){ baz(); }

Will have foo, bar, baz methods well behaved in any data that they share regardless of whether they are foobar methods or methods of any other class(es). It is exactly analogous to the S(a) -> S(a) synchronizing instruction from the article that establishes a happens-before partitioning each thread into before/after the S(a).

The only time synchronized(explicit_object) relates to anything else is when also using the keyword where `synchronized void foo()` is equivalent (with a minor performance difference) to `synchronized(this) { ... }` wrapping the entire body of the foo method.