String str = "a,b,c";  

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

2、将List转换为逗号分隔的字符串

（1） 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);  

（2）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(), ",");