`Hashtable.contains(Object)` and `ConcurrentHashMap.contains(Object)` are legacy
methods for testing if the given object is a value in the hash table. They are
often mistaken for `containsKey`, which checks whether the given object is a
*key* in the hash table.

If you intended to check whether the given object is a key in the hash table,
use `containsKey` instead. If you really intended to check whether the given
object is a value in the hash table, use `containsValue` for clarity.
