Two threads, two increments, forced interleaving: read, read, write, write
==========================================================================

expected final value:                  2
HashMap                get + put:      1
synchronizedMap        get + put:      1
ConcurrentHashMap      get + put:      1
ConcurrentHashMap      merge:          2
(each map call is individually thread-safe; the get-then-put PAIR is not atomic)
