Why is -(insertion_point - 1) returned by Collections.binarySearch when an element not present and not -insertion_point? [duplicate]

Answer 1

Because you can't have negative 0.

Consider the situation if there was no -1. If an element was found at index 0, it would return 0. If an element was not found, but it's insertion point was 0, it too would return zero. How could you distinguish between these two situations? With the addition of the -1, now they return 0 and -1 respectively, letting you distinguish.

And it is -(insertion point) - 1 which is slightly different than what your question states.

Answer 2

The documentation says:

Returns

the index of the search key, if it is contained in the list; otherwise, (-(insertion point) - 1). The insertion point is defined as the point at which the key would be inserted into the list: the index of the first element greater than the key, or list.size() if all elements in the list are less than the specified key. Note that this guarantees that the return value will be >= 0 if and only if the key is found.

The important part is that last sentence:

Note that this guarantees that the return value will be >= 0 if and only if the key is found.

If effect, you're getting two values back from binarySearch, combined in a clever way. You get information about whether the item is present (by the sign of the result), and where it belongs (the magnitude of the result).