I have been wondering for some time whether it is allowable within best practice to refrain from using the
Some Map implementations are allowed to have null values, eg HashMap, in this case if
null it does not guarantee that there is no entry in the map associated with this key.
So if you want to know if a map contains a key use
Map.containsKey. If you simply need a value mapped to a key use
Map.containsKey will be useless and will affect performance. Moreover, in case of concurrent access to a map (eg
ConcurrentHashMap), after you tested
Map.containsKey(key) there is chance that the entry will removed by another thread before you call