null keys and values: HashMap allows, ConcurrentHashMap rejects
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HashMap.put(null, ..) accepted; the null key lives in table[0]: true
HashMap.get(null) = for-null-key
HashMap.get("present-but-null") = null   containsKey = true
HashMap.get("absent")           = null   containsKey = false   <- get() alone cannot tell these two apart
ConcurrentHashMap put(null, v), put(k, null), get(null), containsKey(null), merge(k, null, f): all throw NullPointerException
compute(k, (k,v) -> null) removes the entry instead: containsKey(k) = false