方法“ combination”应组合输入数组。而且我想获得这种组合流并将其保存到文件中。

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,new String[2])
                .stream()
                .map(a -> Arrays.toString(a).join(" -> "))
                .forEach(out::print);
        out.println(";\n");
        out.println("}");
    } catch (IOException ioException) {
        ioException.printStackTrace();
    }
}

组合方法如下：

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;
}

我期望的结果是：

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

and so on..
}

但我只能得到：

digraph {
;

}

我的代码有什么问题？请帮助我。