Populating an existing Set<String> via Java 8 streams

I am new to Java streams and functional programming and I wonder if my question will ever even have a valid use case or it is just not meaningful.
I have the following code that gives me a Set<String> from an array

    public static void main(String[] args) {
        String []arr = {"a","abc","b","cd"};
        Set<String> dict = new HashSet<>();
        Arrays.stream(arr).map(item->{
            System.out.println(item);
            dict.add(item);
            return dict;
        });

//        dict = Arrays.stream(arr).collect(Collectors.toSet());
        System.out.println(dict);
    }

This gives me an empty dict. If I do it the way I have in second last line, it works. Is there a way to mutate already instantiated dict that was instantiated outside stream that it gets populated in the map function?

Stream.map() is an intermediate operation, meaning it only stages a step of the pipeline, but it doesn't do anything when you call it. You need a terminal operation to put the stream to work. In your case, you can use forEachOrdered():

Arrays.stream(arr)
        .peek(System.out::println)
        .forEachOrdered(dict::add);

Note: I'm assuming there's a reason you need to modify an existing set rather than produce a new one. Otherwise, just use collect() as suggested in the other answer.

1. Before the question got edited, the commented part didn't compile because dict must be either final or effectively final. See this answer for more information.
2. One of the few rules of using Stream API is to avoid side effects. The JavaDoc for java.util.stream package describes it well with examples:
   > Side-effects in behavioral parameters to stream operations are, in general, discouraged, as they can often lead to unwitting violations of the statelessness requirement, as well as other thread-safety hazards.
3. The gist of each Stream is to return a new collection/value through operations such as filtering, mapping, reducing, or advanced collecting. It was never meant to modify an existing one. The correct way how to transform the array arr into a set is:

   Set<String> dict = Arrays.stream(arr)
         .peek(item -> System.out.println(item)) // optional for debugging
         .collect(Collectors.toSet());