public static void main(String[] args) {
        var chars = "abcdefghijklmnop".toCharArray();
        int variations = 7;

        var length = chars.length;
        var list = new LinkedList<String>();
        StringBuilder out = new StringBuilder();

        for (int i = 0; i < variations; i++) out.append(chars[0]);

        top:
        while (true) {
            list.add(out.toString());
            if (list.size() % 10000 == 0) {
                append(list);
                list.clear();
            }
            for (int pos = variations - 1; pos >= 0; pos--) {
                char c = out.charAt(pos);
                int cPos = Arrays.binarySearch(chars, c);
                if (cPos < length - 1) {
                    // We can just increment this one and we're done
                    out.setCharAt(pos, chars[cPos + 1]);
                    continue top;
                } else {
                    // 9-> 10: Every 'lower' digit is reset to first (0).
                    out.setCharAt(pos, chars[0]);
                }
            }
            break;
        }
    }

    public static void append(List<String> combinations) {
        try {
            Files.write(Paths.get("combinations.txt"), String.join("\n", combinations).getBytes(),
                    StandardOpenOption.APPEND, StandardOpenOption.CREATE);
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
    }

Ukladani do souboru combinations.txt. A nepotrebujes ani zadne gigabajty heapu. Jako mustr jsem pouzil jeden z tvych kodu, co jsi jsem dal. Ten ktery se mi zdal nejlepsi...