1、ArrarList 转 string[] ：
ArrayList list = new ArrayList();
list.add("aaa");
list.add("bbb");
String[] arrString = (String[])list.toArray(new String[list.size()]) ;
2. String[] 转换为 ArrayList<String>：
1 ArrayList<String> list = new ArrayList<> (new String[] { "aaa", "bbb" });
3. ArrayList<String> 转换为 String：
ArrayList<String> list = new ArrayList<>();
list.add("aaa");
list.add("bbb");
String str = String.join(",", list.toArray(new String[list.size()]));
4. String 转换为 ArrayList<String> ：
1 String str ="1,2,3,4,5"; 3 ArrayList<String> b = new ArrayList( str.split(',') ) ;
