threads=8, increments per thread=200,000, rounds=5, cores=2
Counts below VARY from run to run. Only the 'expected' column is fixed.

HashMap  get+put                             expected 1,600,000  actual per round: 228,496 400,000 287,091 1,200,000 451,357
                                             lost updates per round: min 400,000, max 1,371,504
Collections.synchronizedMap  get+put         expected 1,600,000  actual per round: 282,588 398,788 918,560 433,831 691,994
                                             lost updates per round: min 681,440, max 1,317,412
ConcurrentHashMap  get+put                   expected 1,600,000  actual per round: 214,096 666,429 605,316 446,831 389,717
                                             lost updates per round: min 933,571, max 1,385,904
HashMap + synchronized(map) around get+put   expected 1,600,000  actual per round: 1,600,000 1,600,000 1,600,000 1,600,000 1,600,000
                                             lost updates per round: min 0, max 0
ConcurrentHashMap  merge(KEY, 1, Integer::sum) expected 1,600,000  actual per round: 1,600,000 1,600,000 1,600,000 1,600,000 1,600,000
                                             lost updates per round: min 0, max 0
ConcurrentHashMap  compute(KEY, v+1)         expected 1,600,000  actual per round: 1,600,000 1,600,000 1,600,000 1,600,000 1,600,000
                                             lost updates per round: min 0, max 0

-- second kind of damage: threads insert DIFFERENT keys, nobody overwrites anybody --
HashMap  put of distinct keys (size())       expected 400,000  actual per round: 399,522 400,000 400,000 400,000 400,000
                                             lost updates per round: min 0, max 478
HashMap exceptions thrown inside worker threads, per round: [null, null, null, null, null]
ConcurrentHashMap  put of distinct keys (size()) expected 400,000  actual per round: 400,000 400,000 400,000 400,000 400,000
                                             lost updates per round: min 0, max 0