Return type from a Comparator

The return value (not type, the type is int) tells the caller (the thing sorting the data):

-1 : o1 < o2
0 : o1 == o2
+1 : o1 > o2

If you always return the same value (o, 1, -1) for the comparator, regardless of it's inputs, then you're not using it correctly. You need to base the value returned on the values passed in. The idea is that the data structure (or sorter) calls the comparison function any time it needs to order two elements, to find out what order to put them in.

Its worth noting that the positive/negative integer values (-1, +1) don't need to be 1, they can be any positive/negative numbers. It's just common practice to return -1/+1.

You are confusing return-type and return-value. The return-type is int. The return value is described in the documentation:

Returns a negative integer, zero, or a positive integer as the first argument is less than, equal to, or greater than the second.