Java拼接字符串时如何去掉最后一个多余的逗号

String str[] = { "hello", "beijing", "world", "shenzhen" };
StringBuffer buf = new StringBuffer();
for (int i = 0; i < str.length; i++) {
    buf.append(str[i]).append(",");
}
if (buf.length() > 0) {
    //方法一  : substring
    System.out.println(buf.substring(0, buf.length()-1));
    //方法二 ：replace
    System.out.println(buf.replace(buf.length() - 1, buf.length(), ""));
    //方法三： deleteCharAt
    System.out.println(buf.deleteCharAt(buf.length()-1));
}

public class test {
    public static void main(String[] args) {
        //声明一个数组
        String[] str = new String[]{"1", "2", "3", "4"};
        //创建一个StringBuffer的对象
        StringBuffer str2 = new StringBuffer();
        //循环遍历数组
        for (int i = 0; i < str.length; i++) {
            //用append()方法拼接
            str2.append(str[i] + ",");
        }
        //用substring()方法截取掉最后一个 ","
        String str3 = str2.substring(0, str2.length() - 1);
        //输出拼接好的字符串
        System.out.println(str3 + "=================");
    }
}