List<String> arrList = new ArrayList<String>();
arrList.add("1");
arrList.add("2");
arrList.add("3");
arrList.add("4");
//list->str
String arrStr = StringUtils.join(arrList,",");
//str->list
List<String> list = Arrays.asList(arrStr.split(","));

依赖：Apache Commons的StringUtils

maven

<dependency>
    <groupId>org.apache.commons</groupId>
    <artifactId>commons-lang3</artifactId>
    <version>3.4</version>
</dependency>