There are several ways to achieve this;
1. Using getCurrencyInstance()
NumberFormat format = NumberFormat.getCurrencyInstance();
format.setMaximumFractionDigits(0);
format.setCurrency(Currency.getInstance("EUR"));
format.format(1000000);
2. Using DecimalFormat
NumberFormat formatter = new DecimalFormat("#,###");
double myNumber = 1000000;
String formattedNumber = formatter.format(myNumber);
//formattedNumber is equal to 1,000,000
3. Using getNumberInstance()
int number = 1000000000;
String str = NumberFormat.getNumberInstance(Locale.US).format(number);
//str = 1,000,000,000
Done.