public class Main {
    public static void main(String[] args) {
	double number = 1234567.91011;
	char separator = ',';
	char decimal = '.';
	int maxDecimalPlaces = 3;
	int blockSize = 3;

	System.out.println(format(number, separator, decimal, maxDecimalPlaces, blockSize));
    }

    public static String format(double number, char separator, char decimal, int maxDeci, int blockSize) {
	int n = (int)number;
	char[] nStr = ("" + n).toCharArray();

	int d = (int)Math.round((number-n)*Math.pow(10, maxDeci));
	String dStr = decimal + "" + d;

	String newNStr = "";
	int numbersToStartWith = nStr.length%blockSize;
	int blocksOfNumbersAfterwards = nStr.length/blockSize;

	for (int k = 0 ; k < numbersToStartWith ; k++)
	    newNStr += nStr[k];

	for (int i = 0 ; i < blocksOfNumbersAfterwards ; i++) {
	    if (numbersToStartWith != 0 || i != 0)
		newNStr += separator;
	    for (int j = 0 ; j < blockSize ; j++) {
		newNStr += nStr[numbersToStartWith + i*blockSize + j];
	    }
	}

	return newNStr + dStr;
    }
}