Straightforward. FIFO, Queue. Size control: when reaching capacity, q.poll().

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
class MovingAverage {

    Queue<Integer> q;
    int size = 0, sum = 0;
    
    /** Initialize your data structure here. */
    public MovingAverage(int size) {
        q = new LinkedList<>();
        this.size = size;
    }
    
    public double next(int val) {
        if (q.size() == size) {
            sum -= q.poll();
        }
        q.offer(val);
        sum += val;
        return (double) sum / q.size();
    }
}

/**
 * Your MovingAverage object will be instantiated and called as such:
 * MovingAverage obj = new MovingAverage(size);
 * double param_1 = obj.next(val);
 */