$ java NullHandlingDemo

=== HashMap: null key and null value are both fine ===
hm.get(null) = nullKeyValue
hm.get("k1") = null

=== ConcurrentHashMap: null key throws on put() ===
chm.put(null, "x") threw NullPointerException

=== ConcurrentHashMap: null value throws on put() ===
chm.put("k1", null) threw NullPointerException

=== ConcurrentHashMap: get(null) ALSO throws, not just put() ===
chm.get(null) threw: Cannot invoke "Object.hashCode()" because "key" is null