java 数字补齐0

String str_f = str.substring(0, 1);
int i = (Integer.parseInt(str.substring(1)) + 1);
// 数字补齐0
DecimalFormat df = new DecimalFormat("0000");
String str_b = df.format(i);
String factory_id = str_f + str_b;
System.out.println(factory_id);