How can I format a String number to have commas and round?

You might want to look at the DecimalFormat class; it supports different locales (eg: in some countries that would get formatted as 1.000.500.000,57 instead).

You also need to convert that string into a number, this can be done with:

double amount = Double.parseDouble(number);

Code sample:

String number = "1000500000.574";
double amount = Double.parseDouble(number);
DecimalFormat formatter = new DecimalFormat("#,###.00");

System.out.println(formatter.format(amount));