V를 df.format에다가 바로 쓰세요.
@Test
public void testDecimalFormat() {
double V = 1234.2231233412354435234234;
System.out.println("original : " + V);
DecimalFormat df = new DecimalFormat("#,###.###############");
String str = df.format(V);
System.out.println("converted: " + str);
}
-- 출력--
original : 1234.2231233412354
converted: 1,234.2231233412354