public class NumberUtil {
 static String format(int num, int length) {
        StringBuilder sb = new StringBuilder();
        for (int i = 1; i <= length; i++) {
            sb.append("0");
        }
        DecimalFormat df = new DecimalFormat(sb.toString());
        return df.format(num);
    }

    public static void main(String[] args) {
        System.out.println(format(153,2));
        System.out.println(format(153,3));
        System.out.println(format(153,5));
    }
}