Collections. Binarysearch (list, K key) clarification Java

In view of the following declaration, it is taken from this Oracle Java Tutorial and related to the binarysearch() method of class Collections:

Why does the return value of binarysearch () not only return a negative index, but not a negative index minus 1? (BOLD part quoted above)

In short: why (– (insertion point) – 1) and not just (– (insertion point))?

Thank you in advance

Solution

That's because – (insertion point) is not clear You will not be able to distinguish between:

>Items found in bit 0; > Item not found, insertion point 0

When using – (insertion point) – 1, the above two cases will result in different return values (0 and - 1)