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

What is the best way to format the following number that is given to me as a String?

String number = "1000500000.574" //assume my value will always be a String

I want this to be a String with the value: 1,000,500,000.57

How can I format it as such?

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));

This can also be accomplished using String.format(), which may be easier and/or more flexible if you are formatting multiple numbers in one string.

    String number = "1000500000.574";
    Double numParsed = Double.parseDouble(number);

    System.out.println(String.format("The input number is: %,.2f", numParsed));
    // Or
    String numString = String.format("%,.2f", numParsed);

For the format string "%,.2f" - "," means separate digit groups with commas, and ".2" means round to two places after the decimal.

For reference on other formatting options, see https://docs.oracle.com/javase/tutorial/java/data/numberformat.html

Once you've converted your String to a number, you can use

// format the number for the default locale
NumberFormat.getInstance().format(num)

or

// format the number for a particular locale
NumberFormat.getInstance(locale).format(num)