public class ArrayQueue<E> implements Queue<E> {
    public static final int CAPACITY = 1000;

    private E[] data;
    private int f = 0;
    private int sz = 0;

    public ArrayQueue() {
        this(CAPACITY);
    }

    public ArrayQueue(int capacity) {
        if (capacity <= 0) {
            throw new IllegalArgumentException("Capacity must be positive.");
        }
        data = (E[]) new Object[capacity];
    }

    @Override
    public int size() {
        return sz;
    }

    @Override
    public boolean isEmpty() {
        return sz == 0;
    }

    @Override
    public void enqueue(E e) {
        if (sz == data.length) {
            throw new IllegalStateException("Queue is full.");
        }
        int avail = (f + sz) % data.length;
        data[avail] = e;
        sz++;
    }

    @Override
    public E first() {
        if (isEmpty()) return null;
        return data[f];
    }

    @Override
    public E dequeue() {
        if (isEmpty()) return null;
        E answer = data[f];
        data[f] = null;
        f = (f + 1) % data.length;
        sz--;
        return answer;
    }

    @Override
    public String toString() {
        StringBuilder result = new StringBuilder("[");
        for (int i = 0; i < sz; i++) {
            if (i > 0) result.append(", ");
            result.append(data[(f + i) % data.length]);
        }
        return result.append("]").toString();
    }
}