Code challenge

/** Length of the word */
private static final int WORD_LENGTH = 6;

/** The words most differ in at least 3 positions */
private static final int MINIMUM_CHAR_DIFFERENCE = 3;

/** Newly created word */
private final char[] newWord = new char[WORD_LENGTH];

/** List with the filtered items */
private final List<String> words = new ArrayList<>();



/**
 * Generates all possible combinations and adds it to the permutations list if the generated word is valid
 * @param validChars dictionary of possible chars
 */
private void combine(char[] validChars) {
    combine(validChars,0);
}


/**
 * Generates all possible combinations and adds it to the permutations list if the generated word is valid
 * @param validChars dictionary of possible chars
 * @param position next position
 */
private void combine(char[] validChars, int position) {
    if (position == newWord.length) {
        addToList(newWord);
    } else {
        for (char validChar : validChars) {
            newWord[position] = validChar;
            combine(validChars, position + 1);
        }
    }
}


/**
 * Add the word to the complete list
 */
private void addToList(char[] newWord) {
    String word = charArrayToString(newWord);
    if (isWordValid(word)) {
        words.add(word);
    }
}


/**
 * Convert char array to String
 * @param word char array to be converted
 * @return a String formed by the char array
 */
private String charArrayToString(char[] word) {
    return String.valueOf(word);
}


/**
 * Check char by char if the new word does not match the added ones in 3 or more positions
 * @param newWord new word
 * @return true if the new word has minimum of 3 differences compared to other words
 */
private boolean isWordValid(String newWord) {
    //Iterate the list of words
    for(String word : words) {
        int charCounter = getNumberOfDifferentChars(word, newWord);
        //The counter is bigger or equal than 3, which means the word is valid.
        if(charCounter <= MINIMUM_CHAR_DIFFERENCE) {
            return false;
        }
    }
    return true;
}


/**
 * Compare two strings and return the number of different positions between them
 * @param word to be compared
 * @param newWord to be compared
 * @return number of different positions between them
 */
private int getNumberOfDifferentChars(String word, String newWord) {
    int charCounter = 0;
    //Iterate the chars from each word
    for(int i = 0; i < WORD_LENGTH; i++) {
        char charFromWord = word.charAt(i);
        char charFromNewWord = newWord.charAt(i);
        //Increment the counter if the chars are different in the same position
        if(charFromWord != charFromNewWord) {
            charCounter++;
        }
    }
    return charCounter;
}


/**
 * Print the result
 */
private void print() {
    int i = 1;
    for(String word : words) {
        System.out.println("(" + i + ") : " + word);
        i++;
    }
}


/**
 * Main method
 * @param args
 */
public static void main(String[] args) {
    char[] validChars = {'A','C','G','T'};
    SecondTrial permutation = new SecondTrial();
    permutation.combine(validChars);
    permutation.print();
}