A simple Insertion Sort implementation in Java
this code is not commented
strikingly, it does not even have doc comments
given at least two classes from the same coder that do essentially the same thing, there should be an 'interface' - look for one you can use (if as a template), especially from the Java API
SECOND_NUMBER_INDEX is close to meaningless
as a "genuine" constant, SECOND_NUMBER_INDEX should be static
int[] sort(int[] numbers) would then not use any instance (data) members and should be a class/static method instead of an instance one
if iterations was called ordered or sorted, the condition of the outer loop read close to a loop invariant
having found the first element no bigger than currentNumber, continuing to index 0 isn't called for - just place currentNumber and break the inner loop, no need for newIndex


Use a foreach-loop in main().