char[] elements = {'a', 'b', 'c'}
k = 4

public static ArrayList<String> printAllKLength(char[] elements, int nrElements, int patternLength) {
    ArrayList<String> patternVariations = new ArrayList<String>();
    patternVariations = printAllKLengthRec(elements, "", nrElements, patternLength, patternVariations);

    return patternVariations;
}

public static ArrayList<String> printAllKLengthRec(char[] elements, String prefix, int nrElements, int patternLength, ArrayList<String> patternVariations) {

    if (patternLength == 0) {
        patternVariations.add(prefix);
        //System.out.println(prefix);
        return patternVariations;
    }

    for (int i = 0; i < nrElements; ++i) {
        String newPrefix = prefix + elements[i];
        printAllKLengthRec(elements, newPrefix, nrElements, patternLength - 1, patternVariations);
    }
    return patternVariations;
}