为什么类型不匹配啊？

import java.util.*;

public class Tm58 {
    public static void main(String[] args) {
        Random rand = new Random(47);
        Integer[] a = new Integer[rand.nextInt(20)];
        System.out.println("length of a= " + a.length);
        for (int i = 0; i < a.length; i++)
             a[i] = rand.nextInt(500);
            System.out.println(Arrays.toString(a));

    }

}