Assume a list:

List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5);

Using String.join() with map()

String result = numbers.stream()
    .map(String::valueOf)
    .collect(Collectors.joining(","));

System.out.println(result);

Output:

1,2,3,4,5

Summary:

• map(String::valueOf) converts integers to strings.
• Collectors.joining(",") concatenates them with commas.
• Works for any list of numeric or object values with a proper toString() implementation.