public static void main(String[] args) throws InterruptedException {
    Last100 s = new Last100();
    for (int i = 0; i < 2000; i++) {
        int bound = 250; // average 150
        if (i > 1000) {
            bound = 550; // average 300
        }
        int time = ThreadLocalRandom.current().nextInt(50, bound);
        System.err.println(s.add(time));
        Thread.sleep(10);
    }
}

private static class Last100 {
    private static final int ELEMENTS = 100;

    private int sum;
    private int idx;
    private int[] vals = new int[ELEMENTS];
    private int numOfElements = 0;

    public synchronized int add(int val) {
        int prev = vals[idx];
        vals[idx] = val;
        idx++;
        if (idx == ELEMENTS) {
            idx = 0;
        }
        if (numOfElements < ELEMENTS) {
            numOfElements++;
        }
        sum += val - prev;
        return sum / numOfElements;
    }
}