public class Solution {
public static void main(String[] args) throws InterruptedException {
    Counter counter1 = new Counter();
    Counter counter2 = new Counter();
    Counter counter3 = new Counter();
    Counter counter4 = new Counter();

    counter1.start();
    counter2.start();
    counter3.start();
    counter4.start();

    counter1.join();
    counter2.join();
    counter3.join();
    counter4.join();


    for (int i = 1; i <= 100; i++) {
        if (values[i] != 1) {
            System.out.println("Массив values содержит элементы неравные 1");
            break;
        }
    }
}

public static Integer count = 0;
public static int[] values = new int[105];

static {
    for (int i = 0; i < 105; i++) {
        values[i] = 0;
    }
}

public synchronized static void incrementCount() {
    count++;
}

public synchronized static int getCount() {
    return count;
}

public static class Counter extends Thread {
    @Override
    public void run() {
        do {

---->>> synchronized (Counter.class) {  <<<---
                incrementCount();
                values[getCount()]++;
            }

            try {
                Thread.sleep(1);
            } catch (InterruptedException e) {
            }
        } while (getCount() < 100);
    }
}
}