Java: StringBuffer实现数字每三位用逗号隔开

package io;

public class T {
  public static void main(String[] args) {
    String b = "124356789123456";
    String bb = "124356789123456.124356789123456";
    System.out.println(split(b));
    System.out.println(split(bb));
  }

  private static String split(String s) {
    // StringBuilder stringBuilder = new StringBuilder(s);
    StringBuffer stringBuffer = new StringBuffer().append(s);

    // 对"."前分隔
    int index = stringBuffer.lastIndexOf(".");
    if (index == -1) {
      index = stringBuffer.length() - 1 - 2;  // 没有"."时, 要从最后一个数字索引位置往前移两位插入
    } else {
      index -= 3;  // 有"."时, 因为"."非数字, 从index往前移三位插入
    }

    for (; index > 0; index -= 3)
      stringBuffer.insert(index, ',');

    //  对"."后分隔
    index = stringBuffer.indexOf(".");  // 必须重新计算
    // System.out.println("index = " + index);
    // System.out.println("stringBuffer = " + stringBuffer);
    if (index != -1) {
      for (index += 4; index < stringBuffer.length(); index += 4)
        stringBuffer.insert(index, ',');
    }

    return new String(stringBuffer);
  }
}