Cannot print inside forEach loop in stream java

Question

The method "combination" should make combination of input arrays. And I like to get this stream of combination and save it into a file.

public void writeDot() {
    try (PrintStream out = new PrintStream(path, "UTF-8")) {
        out.print("digraph {\n");
        String[] arr = {"hyo", "ji", "yoo", "mi", "vi", "se", "ari"};
        combination(arr, 2, 0, new String[2])
                .stream()
                .map(a -> Arrays.toString(a).join(" -> "))
                .forEach(out::print);
        out.println(";\n");
        out.println("}");
    } catch (IOException ioException) {
        ioException.printStackTrace();
    }
}

Combination methods goes like this:

public List<String[]> combination(String[] arr, int len, int startPosition, String[] result) {
    if (len == 0) {
        //System.out.println(Arrays.toString(result));
        return null;
    }
    for (int i = startPosition; i <= arr.length - len; i++) {
        result[result.length - len] = arr[i];
        combination(arr, len - 1, i + 1, result);
        list.add(result);
    }
    return list;
}

Result I expected is:

digraph {
hyo -> ji;
ji -> hyo;

and so on..
}

But I only get:

digraph {
;

}

What's wrong with my code? please help me.

Answer 1

String.join is a static method that accepts two parameters: delimiter and array of elements.

You pass no elements, so result is empty string.

Correct code is:

combination(arr, 2, 0, new String[2])
                .stream()
                .map(a->String.join(" -> ", a))
                .forEach(out::print);