参考：
https://www.cnblogs.com/zp-uestc/p/10188752.html

将逗号分隔的字符串转换为List

String str = "a,b,c";  

List<String> result = Arrays.asList(str.split(","));

将List转换为逗号分隔的字符串

利用Guava的Joiner

List<String> list = new ArrayList<String>();  
list.add("a");  
list.add("b");  
list.add("c");  

String str = Joiner.on(",").join(list);  

利用Apache Commons的StringUtils

List<String> list = new ArrayList<String>();  
list.add("a");  
list.add("b");  
list.add("c");  

String str = StringUtils.join(list.toArray(), ",");