Each iteration of your for loop overwrites the value of separatedByComma. The first iteration assigns the String "a" to it, and the second replaces it with "b".

You should Stream over the elements of the strings List in order to join the Strings that match each of them into a single output String:

String separatedByComma = 
    strings.stream()
           .flatMap(word -> occList.stream()
                                   .filter(occ -> word.equalsIgnoreCase (occ.getOccSCd()))
                                   .map (occ -> occ.getOccSNm()))
           .collect(Collectors.joining (","));

Output:

a,b

The official documentation for what you want to do: https://docs.oracle.com/javase/8/docs/api/java/util/stream/Collectors.html

 // Accumulate names into a List
 List<String> list = people.stream().map(Person::getName).collect(Collectors.toList());

 // Convert elements to strings and concatenate them, separated by commas
 String joined = things.stream()
                       .map(Object::toString)
                       .collect(Collectors.joining(", "));

For your example, you would need to do this:

 // Convert elements to strings and concatenate them, separated by commas
 String joined = persons.stream()
                       .map(Person::getName) // This will call person.getName()
                       .collect(Collectors.joining(", "));

The argument passed to Collectors.joining is optional.

In Java 8 you can use String.join() like following:

Collection<String> elements = ....;
String result = String.join(", ", elements);