Returns the greatest key less than or equal to the given key,
or {@code null} if there is no such key.
@param key the key
@return the greatest key less than or equal to {@code key},
or {@code null} if there is no such key
@throws ClassCastException if the specified key cannot be compared
with the keys currently in the map
@throws NullPointerException if the specified key is null
and this map does not permit null keys
Returns the greatest key less than or equal to the given key, or {@code null} if there is no such key.
@param key the key @return the greatest key less than or equal to {@code key}, or {@code null} if there is no such key @throws ClassCastException if the specified key cannot be compared with the keys currently in the map @throws NullPointerException if the specified key is null and this map does not permit null keys