In Java 8 or later:

String listString = String.join(", ", list);

In case the list is not of type String, a joining collector can be used:

String listString = list.stream().map(Object::toString)
                        .collect(Collectors.joining(", "));

Using Java 8 you can do this in a very clean way:

String.join(delimiter, elements);

This works in three ways:

1) directly specifying the elements

String joined1 = String.join(",", "a", "b", "c");

2) using arrays

String[] array = new String[] { "a", "b", "c" };
String joined2 = String.join(",", array);

3) using iterables

List<String> list = Arrays.asList(array);
String joined3 = String.join(",", list);

You may use a StringJoiner for that solution.

List<String> mainList = new ArrayList<>();  //The one to contain all Strings from yourList
List<String> yourList = Arrays.asList("Hey", "how", "are", "you");    //The list of Strings

StringJoiner joiner = new StringJoiner(" "); //Space as delimiter, strings will be separated with spaces
for (String s : yourlist) {
    joiner.add(s);
}
mainList.add(joiner.toString());    //Add the new concatenated String to your list, in a single string