Simply use indexOf on string. It accepts a string as well:
System.out.println("Starting position of your substring in string: " + longSt.indexOf(substring));
Note in this example you have here, the starting position is 3, not four. But that differs from the starting index versus the position within the string. Arrays are 0 based in java, so that would indicate that the character array for this string starts its substring in position 3, which is the fourth character of the string.
Once you get the index you can use that as a part of the substring method to determine start and end positions (with length's involved).