public int indexOf(final Object e) {
        for (int i = 0; i < size; ++i) {
            if (e.equals(entries[i])) {
                return i;
            }
        }
        return -1;
    }
Expose the index in the internal array of a given object. -1 is returned
if the object is not present in the internal array.

@param e
           an object to check whether exists in this set
@return the index of the argument e in the internal array, or -1 if the
        object is not present
vespajlib/src/main/java/com/yahoo/collections/ArraySet.java