Make my circles randomly disappear and after random seconds again appear

So I'm making very simple game. Game is about some guy who jumps on rocks(circles) and rocks sometimes get covered by water and when they are, you cant stand on them otherwise you'll fall into water and drown. I'm stuck in part where i need to make those rocks disappear(be covered by water). So I need at randomized time make them disappear, for random seconds(not too long) make them "invisible" and then again they need to show up. I'm still kinda beginner and I would appreciate any kind of answer but if you could explain it to me I'd be thrilled. Here is my code: My main class

package com.pitcher654.main;

import java.awt.Canvas;
import java.awt.Color;
import java.awt.Graphics;
import java.awt.event.KeyListener;
import java.awt.image.BufferStrategy;
import java.util.Random;

import com.pitcher654.main.Game.STATE;

public class Game extends Canvas implements Runnable{

private static final long serialVersionUID = -7800496711589684767L;

public static final int WIDTH = 640, HEIGHT = WIDTH / 12 * 9;

private Thread thread;
private boolean running = false;

private Random r;
private Handler handler;
//private HUD hud;
private Menu menu;

public enum STATE {
    Menu,
    Help,
    Game
};

public STATE gameState = STATE.Menu;

public Game() {
    handler = new Handler();
    menu = new Menu(this, handler);
    this.addKeyListener(new KeyInput(handler));
    this.addMouseListener(menu);
    new Window(WIDTH, HEIGHT, "My game", this);

    //hud = new HUD();
    r = new Random();

    if(gameState == STATE.Game) {
        //handler.addObject(new Player(100, 200, ID.Player));
    }

    //handler.addObject(new Player(100, 200, ID.Player));
    //handler.addObject(new BasicEnemy(100, 200, ID.BasicEnemy));

}

public synchronized void start() {
    thread = new Thread(this);
    thread.start();
    running = true;
}

public synchronized void stop() {
    try {
        thread.join();
        running = false;
    }catch(Exception ex) { ex.printStackTrace(); }
}

public void run()
{
    this.requestFocus();
    long lastTime = System.nanoTime();
    double amountOfTicks = 60.0;
    double ns = 1000000000 / amountOfTicks;
    double delta = 0;
    long timer = System.currentTimeMillis();
    int frames = 0;
    while(running)
    {
                long now = System.nanoTime();
                delta += (now - lastTime) / ns;
                lastTime = now;
                while(delta >=1)
                        {
                            tick();
                            delta--;
                        }
                        if(running)
                            render();
                        frames++;

                        if(System.currentTimeMillis() - timer > 1000)
                        {
                            timer += 1000;
                            //System.out.println("FPS: "+ frames);
                            frames = 0;
                        }
    }
            stop();
 }  

private void tick() {
    handler.tick();
    //hud.tick();
    if(gameState == STATE.Game) {

    }else if(gameState == STATE.Menu) {
        menu.tick();
    }
}

private void render() {
    BufferStrategy bs = this.getBufferStrategy();
    if(bs == null) {
        this.createBufferStrategy(3);
        return;
    }

    Graphics g = bs.getDrawGraphics();

    g.setColor(new Color(87, 124, 212));
    g.fillRect(0, 0, WIDTH, HEIGHT);
    if(gameState == STATE.Game) {
        g.setColor(new Color(209, 155, 29));
        for(int i = 0; i < 5; i++) {
            g.fillOval(80 + (100 * i), 325, 70, 20);
        }
    }else if(gameState == STATE.Menu || gameState == STATE.Help){
        menu.render(g);
    }

    handler.render(g);

    if(gameState == STATE.Game) {

    }
    //hud.render(g);

    g.dispose();
    bs.show();
}

public static int clamp(int var, int min, int max) {
    if(var >= max)
        return var = max;
    else if(var <= max)
        return var = min;
    else 
        return var;
}

public static void main(String[] args) {
    new Game();
}
}

My Player class where I create my player:

package com.pitcher654.main;

import java.awt.Color;
import java.awt.Graphics;
import java.util.Random;

import com.pitcher654.main.Game.STATE;

public class Player extends GameObject {

    Random r = new Random();

public Player(int x, int y, ID id) {
    super(x, y, id);
    //velX = r.nextInt(5) + 1;
    //velY = r.nextInt(5);
}

public void tick() {
    x += velX;
    y += velY;
    //System.out.println(x);
    if(x == 500) x = 500;
}

public void render(Graphics g) {
    if(id == ID.Player) g.setColor(Color.white);
    if(id == ID.Player2) g.setColor(Color.blue);
    g.fillRect(x, y, 32, 32);
    g.drawLine(x + 15, y, x + 15, y + 100);
    g.drawLine(x + 15, y + 100, x, y + 135);
    g.drawLine(x + 15, y + 100, x + 33, y + 135);
    g.drawLine(x + 15, y + 70, x - 35, y + 30);
    g.drawLine(x + 15, y + 70, x + 65, y + 30);
    /*if(game.gameState == STATE.Menu) {
        g.setColor(new Color(87, 124, 212));
        g.fillRect(0, 0, Game.WIDTH, Game.HEIGHT);
    }*/
}


}

And my game Object class:

package com.pitcher654.main;

import java.awt.Graphics;

public abstract class GameObject {

protected static int x, y;
protected ID id;
protected int velX, velY;

public GameObject(int x, int y, ID id) {
    this.x = x;
    this.y = y;
    this.id = id;
}

public abstract void tick();
public abstract void render(Graphics g);

public void setX(int x) {
    this.x = x;
}
public void setY(int y) {
    this.y = y;
}
public int getX() {
    return x;
}
public int getY() {
    return y;
}
public void setID(ID id) {
    this.id = id;
}
public ID getID() {
    return id;
}
public void setVelX(int velX) {
    this.velX = velX;
}
public void setVelY(int velY) {
    this.velY = velY;
}
public int getVelX() {
    return velX;
}
public int getVelY() {
    return velY;
}
}

If you need any other class, just tell me I'll post it.