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);

Answer 2

It appears you have some issues with your combination method which are not helping your case because you're recursively calling it with only ever making one new result array. Instead, try a better combination method - ideally use a well tested library for this, like apache commons or guava.

public static List<String[]> makePairsFromArray(String[] arr) {
    List<String[]> list = new ArrayList<>();

    for(int i = 0; i < arr.length - 1; i++) {
        for(int j = i + 1; j < arr.length; j++) {
             String[] pair = new String[2];
             pair[0] = arr[i]; 
             pair[1] = arr[j];
             list.add(pair);
             String[] opp = new String[2];
             opp[0] = arr[j];
             opp[1] = arr[i];
             list.add(opp);
        }
    }
    return list;
}

Additionally, you will need to update the map(join) method call as described by talex.