How to generate random numbers that re generate...?

I am making a game in java in which a disc is thrown through a hoop. The hoop's Y position is a random number and i would like to make this number change every time another hoop is generated.

Here is my code:

import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.RenderingHints;
import java.awt.event.KeyListener;
import java.awt.event.KeyEvent;
import javax.swing.JPanel;
import javax.swing.JFrame;
import javax.swing.JOptionPane;
import java.util.Random;

@
SuppressWarnings("serial")
public class DiscHoopToss extends JPanel {

  int x = 40;
  int y = 150;
  int xm = 0;
  int ym = 0;
  Random rng = new Random();
  int r = rng.nextInt((220 - 20) + 1) + 20;

  public DiscHoopToss() {
    addKeyListener(new KeyListener() {

      @
      Override
      public void keyTyped(KeyEvent e) {}

      @
      Override
      public void keyPressed(KeyEvent e) {
        if (e.getKeyCode() == KeyEvent.VK_UP) {
          ym = -4;
          xm = 3;
        }
      }

      @
      Override
      public void keyReleased(KeyEvent e) {
        if (e.getKeyCode() == KeyEvent.VK_UP) {
          ym = 2;
          xm = 3;
        }
      }
    });
    setFocusable(true);
  }




  private void moveDisc() {
    x = x + xm;
    y = y + ym;

    if (y == 0) {
      JOptionPane.showMessageDialog(this, "Don't let the disc go off the screen!", "Game Over", JOptionPane.ERROR_MESSAGE);
      y = 150;
      x = 40;
      xm = 0;
      ym = 0;
    }

    if (y == getHeight() - 20) {
      JOptionPane.showMessageDialog(this, "Don't let the disc go off the screen!", "Game Over", JOptionPane.ERROR_MESSAGE);
      y = 150;
      x = 40;
      ym = 0;
      xm = 0;
    }

  }


  @
  Override
  public void paint(Graphics g) {
    super.paint(g);
    Graphics2D g2d = (Graphics2D) g;
    g2d.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);

    g2d.drawOval(x, r, 25, 55);

    g2d.fillOval(650, y, 50, 20);
  }
  public static void main(String[] args) throws InterruptedException {
    JFrame frame = new JFrame("Toss the disc into the hoop!");
    DiscHoopToss game = new DiscHoopToss();
    frame.add(game);
    frame.setSize(750, 350);
    frame.setResizable(false);
    frame.setVisible(true);
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

    while (true) {
      game.moveDisc();
      game.repaint();
      Thread.sleep(10);
    }
  }
}