产生随机数时出现问题

package ClosestPoints;

import java.applet.Applet;
import java.awt.Color;
import java.awt.Graphics;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.Random;

public class ClosestPoints extends Applet implements ActionListener {
    int i, j, a, b, n, A[][];

    @Override
    public void paint(Graphics g) {
        // TODO Auto-generated method stub
        // super.paint(g);
        Random r = new Random();
        n = Math.abs(r.nextInt(18)) + 2; // 随机产生点的个数（2~20）
        A = new int[n][2];
        // 随机产生坐标
        for (i = 0; i < n; i++) {
            A[i][0] = Math.abs(r.nextInt(1000));
            A[i][1] = Math.abs(r.nextInt(1000));
            System.out.println("A" + (i + 1) + "(" + A[i][0] + "," + A[i][1]
                    + ")");
        }

        // 描点
        for (i = 0; i < n; i++) {
            g.setColor(Color.red);
            g.drawOval(A[i][0], A[i][1], 5, 5);
            g.fillOval(A[i][0], A[i][1], 6, 6);
            g.setColor(Color.blue);
            g.drawString("A" + (i + 1) + "(" + A[i][0] + "," + A[i][1] + ")",
                    A[i][0] - 25, A[i][1] + 20);
        }

    }

    @Override
    public void actionPerformed(ActionEvent e) {
        // TODO Auto-generated method stub

    }

}