public class NumberList {
 
  public static void main(String[] args) {
    // 1. List Declaration (List of Integers)
    List<Integer> numbers = new List<Integer>();
 
    // 2. Adding elements (Numbers) to the list
    numbers.add(3);
    numbers.add(7);
    numbers.add(12);
 
    // 3. Working with Strings: Create a message string
    String message = 'The list contains these numbers: ';
 
    // 4. Looping through the list and adding numbers to the string
    for (Integer num : numbers) {
      message += num + ', ';  // Concatenating number with comma and space
    }
 
    // 5. Removing the last comma and space (optional)
    message = message.substring(0, message.length() - 2); // Removing the last characters
 
    // 6. Printing the final message with list elements
    System.debug(message);
  }
}

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