Java NavigableMap why is headMap return different for same methods

Question

Regarding Java TreeMap method (or NavigableMap interface):

public SortedMap<K,V> headMap(K toKey)
public NavigableMap<K,V> headMap(K toKey, boolean inclusive)

Why do they return different types? I know SortedMap is super to NavigableMap, I just want to know why an inclusive flag requires a different return.

Looking at the code makes no sense either as the version without the inclusive flag simply calls the other.

Thanks.

Answer 1

I think the reason is historic. TreeMap itself and headMap(K) date back to Java 1.2, whereas NavigableMap and headMap(K, boolean) have been introduced in Java 1.6.