通常会这么写：
public static void main(String[] args) {
String strs = "";
String[] arr = new String[]{"aa", "cc", "bb"}; // 转换前的字符串数组
StringBuilder sb = new StringBuilder();
for (String ele : arr) {
if (sb.length() > 0) {
sb.append(",");
}
sb.append(ele);
}
strs = sb.toString(); // 转换后的逗号分隔字符串
System.out.println(strs);
}更简单的写法：
public static void main(String[] args) {
String[] arr = new String[]{"aa", "cc", "bb"}; // 转换前的字符串数组
String strs = String.join(",", arr); // 转换后的逗号分隔字符串
System.out.println(strs);
} 
