See the following snippet, which demonstrates unexpected behavior

var fmt = NumberFormat.getCompactNumberInstance(Locale.US, NumberFormat.Style.SHORT)
fmt.parse("5") // returns 5
fmt.parse("5K") // returns 5000
fmt.setParseIntegerOnly(true)
fmt.parse("5K") // returns 5000
fmt.parse("5") // unexpectedly throws StringIndexOutOfBoundsException

The position in parse needs to be checked before invoking charAt.