How to set thousands separator in Java?

This should work (untested, based on JavaDoc):

DecimalFormat formatter = (DecimalFormat) NumberFormat.getInstance(Locale.US);
DecimalFormatSymbols symbols = formatter.getDecimalFormatSymbols();

symbols.setGroupingSeparator(' ');
formatter.setDecimalFormatSymbols(symbols);
System.out.println(formatter.format(bd.longValue()));

According to the JavaDoc, the cast in the first line should be save for most locales.

try this code to format as used in Brazil:

    DecimalFormat df = new DecimalFormat(
      "#,##0.00", 
      new DecimalFormatSymbols(new Locale("pt", "BR")));

    BigDecimal value = new BigDecimal(123456.00);

    System.out.println(df.format(value.floatValue()));

    // results: "123.456,00"

BigDecimal bd = new BigDecimal(300000);

NumberFormat formatter = NumberFormat.getInstance(new Locale("en_US"));

System.out.println(formatter.format(bd.longValue()));

EDIT

To get custom grouping separator such as space, do this:

DecimalFormatSymbols symbols = DecimalFormatSymbols.getInstance();
symbols.setGroupingSeparator(' ');

DecimalFormat formatter = new DecimalFormat("###,###.##", symbols);
System.out.println(formatter.format(bd.longValue()));

You can use format function with ",";

int no = 124750;
String str = String.format("%,d", no);

//str = 124,750

"," includes locale-specific grouping characters.

docs