Consider the goal of adding entry (k,v) to a map only if there does not yet exist
some other entry with key k. For a map M (without null values), this might be accomplished as follows.
if (M.get(k) == null)
M.put(k, v);
While this accomplishes the goal, its efficiency is less than ideal, as time will be spent on the failed search during the get call, and again during the put call (which always begins by trying to locate an existing entry with the given key).To avoid this inefficiency, some map implementations support a custom method putIfAbsent(k, v) that accomplishes this goal. Given such an implementation of putIfAbsent for the UnsortedTableMap class.