public abstract SortedMap<K, V> headMap (K toKey)

Returns a view of the portion of this map whose keys are strictly less than toKey. The returned map is backed by this map, so changes in the returned map are reflected in this map, and vice-versa. The returned map supports all optional map operations that this map supports.

The returned map will throw an IllegalArgumentException on an attempt to insert a key outside its range.

Parameters:
toKey    high endpoint (exclusive) of the keys in the returned map

Returns:  a view of the portion of this map whose keys are strictly less than toKey

Exceptions:
ClassCastException    if toKey is not compatible with this map's comparator (or, if the map has no comparator, if toKey does not implement Comparable). Implementations may, but are not required to, throw this exception if toKey cannot be compared to keys currently in the map.
NullPointerException    if toKey is null and this map does not permit null keys
IllegalArgumentException    if this map itself has a restricted range, and toKey lies outside the bounds of the range