You can change the separator either by setting a locale or using the DecimalFormatSymbols.
If you want the grouping separator to be a point, you can use an european locale:
NumberFormat nf = NumberFormat.getNumberInstance(Locale.GERMAN);
DecimalFormat df = (DecimalFormat)nf;
Alternatively you can use the DecimalFormatSymbols class to change the symbols that appear in the formatted numbers produced by the format method. These symbols include the decimal separator, the grouping separator, the minus sign, and the percent sign, among others:
DecimalFormatSymbols otherSymbols = new DecimalFormatSymbols(currentLocale);
otherSymbols.setDecimalSeparator(',');
otherSymbols.setGroupingSeparator('.');
DecimalFormat df = new DecimalFormat(formatString, otherSymbols);
Also, you might find this useful: http://tutorials.jenkov.com/java-internationalization/decimalformat.html explains almost everything related to your problem in a very simple and nice manner.