Most Frequent Word (Task 70)

I'm working with Java and I'm having some problem with Task 70, possibly because I don't fully grasp the potentialities of the HashMap structure.

As far as I understand HashMaps work great at storing (.put()) and retrieving (.get()) entrances because of the whole hashing process, which makes size appear as irrelevant.

My problems, though, start very early: I can't create the list of random words because of the sheer size of it. I thought about using something like this:

public static void setWords() {
    words = new HashMap<>();              //words is a class variable
    for (int i = 1; i < 900000; i++) {
        String word = generateWord(6);    //generates a word of 6 characters using linear congruence
        if (words.get(word) != null) {
            words.put(word, words.get(word) + 1);
        } else {
            words.put(word, 1);
        }
    }
}

but the program runs for minutes and minutes to no avail. I thought the problem could be in the if block, but eveng cutting it down (for test purposes) to:

public static void setWords() {
    words = new HashMap<>();
    for (int i = 1; i < 900000; i++) {
        String word = generateWord(6);
            words.put(word, 1);
    }
}

makes no difference. Can someone point me what I'm doing wrong? Or is it normal for the list to take lots of time to generate? I feel like I'm banging my head against a wall and I'm clueless about how to approach this.

Sure, I'm now almost positive that the problem lies beneath it (I've tried adding dummy strings and it works just fine), but I can't figure out what's wrong. Or better, I do realise that there are a lot of operations being performed at every iteration, but I wouldn't know how to avoid that. Here's my code, as taken from the solution of other tasks:

//seed, count, consonants and vowels are class variables

public static String generateWord(int length) {
    String word = "";
    for (int i = 0; i < length; i++) {
        int random = linCongGen(445, 700001, 2097152, seed, count + i + 1); 
        int index;
        if (i % 2 == 0) {
            index = random % 19;
            word += consonants.charAt(index);
        } else {
            index = random % 5;
            word += vowels.charAt(index);
        }
    }
    count += length;
    return word;
}

public static int linCongGen(int a, int c, int m, int x0, int n) {
    for (int i = 0; i < n; i++) {
        x0 = (a * x0 + c) % m;
    }
    return x0;
}

Thanks for the tip, looking at this not late at night made me realise my mistake, which was possibly due to the fact that I had been using copy-pasted code from old solutions without checking it... Definitely bad practice.

Running the LCG just once per cycle, storing its last value and updating it worked liked a charm, thank you again!

Thanks, I didn't know of that class before. The rule of thumb should be using it inside loops, right? In that way the String isn't created over and over again and this positively affects performance and run-time.