Convert comma separated String to List

List<String> items = Arrays.asList(str.split("\\s*,\\s*"));

The above code splits the string on a delimiter defined as: zero or more whitespace, a literal comma, zero or more whitespace which will place the words into the list and collapse any whitespace between the words and commas.

Please note that this returns simply a wrapper on an array: you CANNOT for example .remove() from the resulting List. For an actual ArrayList you must further use new ArrayList<String>.

If you're using Java 8, you can:

int[] numbers = Arrays.asList(numbersArray.split(",")).stream()
  .map(String::trim)
  .mapToInt(Integer::parseInt).toArray();

If not, I think your approach is the best option available.

Using Java 8 you could do it in one line:

String addressID = addresses
           .stream()
           .map(a -> String.valueOf(a.addressId))
           .collect(Collectors.joining(","));

The ArrayList returned by Arrays.asList is not java.util.ArrayList. It's java.util.Arrays.ArrayList. So you can't cast it to java.util.ArrayList.

You need to pass the list to the constructor of java.util.ArrayList class:

List<String> items = new ArrayList<String>(Arrays.asList(CommaSeparated.split("\\s*,\\s*")));

or, you can simply assign the result:

List<String> items = Arrays.asList(CommaSeparated.split("\\s*,\\s*"));

but mind you, Arrays.asList returns a fixed size list. You cannot add or remove anything into it. If you want to add or remove something, you should use the 1st version.

P.S: You should use List as reference type instead of ArrayList.