I've not gone deep into the code but it's because this
Arrays.asList(notAbundantSummable).indexOf(sum)
returns -1, which it does because the value of sum is not in your array
looks like you're running into an issue because your arrays starting indexing the first element at 0 (so the last element in an array of length n is the item[n-1] ) while in a List object the index starts at 1 for the first object (the last object of a List of length n is at element n)