To set thousand separator, say 123,456.78 you have to use DecimalFormat:
DecimalFormat df = new DecimalFormat("#,###.00");
System.out.println(df.format(new BigDecimal(123456.75)));
System.out.println(df.format(new BigDecimal(123456.00)));
System.out.println(df.format(new BigDecimal(123456123456.78)));
Here is the result:
123,456.75
123,456.00
123,456,123,456.78
Although I set #,###.00 mask, it successfully formats the longer values too.
Note that the comma(,) separator in result depends on your locale. It may be just space( ) for Russian locale.