Java 2D Game Programming : Missile shooting is too fast?

Question

Hello I'm fairly new to programming and this is my first time posting here so any help would be appreciated so: my problem is that I"m trying to create some kind of 2D shooter game in java but I don't know if my simple game loop is good because when i shoot a missile it shoots a one every 20 ms and it's too fast and shoots a ton of missiles at once so is there any way to adjust it ? Like to keep some delay between every missile and the other?? and please tell me if i have problems or bad programming in my code !!

this is my game panel where most of the game happens and where my loop and adding missiles method in

public class GamePanel extends JPanel implements KeyListener {
Measurments mesure = new Measurments();
int panel_width = mesure.getUniversalWidth();
int panel_height = mesure.getUniversalHeight();
Timer timer;
Random rand = new Random();

ArrayList<Enemy> enemies = new ArrayList<>();
ArrayList<Missile> missiles = new ArrayList<>();

Player player = new Player(0, 0);

boolean up = false;
boolean down = false;
boolean right = false;
boolean left = false;

boolean isShooting = false;
boolean isRunning = true;

public boolean gameRunning() {
    return isRunning;
}

int count = 5;
int missilesCount = 6;

public GamePanel() {

    timer = new Timer(20, new ActionListener() {

        public void actionPerformed(ActionEvent e) {
            StartGame();
            repaint();

        }

    });

    setSize(panel_width, panel_height);

    addKeyListener(this);

    timer.start();

    for (int i = 0; i < count; i++) {
        addEnemy(new Enemy(rand.nextInt(750), rand.nextInt(500)));
    }
}

public void paintComponent(Graphics g) {
    super.paintComponent(g);
    Graphics2D g2d = (Graphics2D) g;
    player.paint(g2d);
    for (int i = 0; i < enemies.size(); i++) {

        Enemy temp = enemies.get(i);
        temp.paint(g2d);
    }
    for (int i = 0; i < missiles.size(); i++) {
        Missile mis = missiles.get(i);
        mis.paint(g2d);
        mis.behave();
    }
}

public void StartGame() {
    if (isRunning) {
        runGame();
        setBackground(Color.YELLOW);

    } else {
        setBackground(Color.BLACK);

    }
}
public void runGame() {
    update();

};

public void update() {
    player.checkBorders();

    checkColls();

    if (up) {
        player.updateUp();
    }
    if (down) {
        player.updateDown();
    }
    if (right) {
        player.updateRight();
    }
    if (left) {
        player.updateLeft();
    }
    if (isShooting) {
        for (int i = 0; i < 5; i++) {
            missiles.add(new Missile(player.getX() + 16, player.getY() + 16));
        }

    }
    for (int i = 0; i < missiles.size(); i++) {
        Missile temp = missiles.get(i);
        if (temp.getX() == panel_width) {
            RemoveMissile(temp);
        }
    }

}

public void addEnemy(Enemy e) {
    enemies.add(e);

}

public void removeEnemy(Enemy e) {
    enemies.remove(e);
}

public void addMissile(Missile e) {
    missiles.add(e);
}

public void RemoveMissile(Missile e) {
    missiles.add(e);
}

public void checkColls() {
    for (int i = 0; i < enemies.size(); i++) {
        Enemy tempEnm = enemies.get(i);
        for (int e = 0; e < missiles.size(); e++) {
            Missile tempMis = missiles.get(e);
            if (tempMis.missileRect().intersects(tempEnm.enemyRect())) {
                enemies.remove(tempEnm);
                missiles.remove(tempMis);
            }
        }
    }

}



public void keyPressed(KeyEvent e) {
    int key = e.getKeyCode();

    if (key == e.VK_UP) {
        up = true;
    }
    if (key == e.VK_DOWN) {
        down = true;
    }
    if (key == e.VK_RIGHT) {
        right = true;
    }
    if (key == e.VK_LEFT) {
        left = true;
    }
    if (key == e.VK_ENTER) {
        isRunning = true;
    }
    if (key == e.VK_SPACE) {
        isShooting = true;

    }

}

public void keyReleased(KeyEvent e) {
    int key = e.getKeyCode();
    if (key == e.VK_UP) {
        up = false;
    }
    if (key == e.VK_DOWN) {
        down = false;
    }
    if (key == e.VK_RIGHT) {
        right = false;
    }
    if (key == e.VK_LEFT) {
        left = false;
    }
    if (key == e.VK_SPACE) {
        isShooting = false;
    }

}

public void keyTyped(KeyEvent e) {

}

}

Thanks in advance !!

Answer 1

private long fired = 0L;

public void update() {
   ...
   // firing missiles: only if the missile count is less than the max., and the elapsed
   // time is more than a limit (100 ms)
   if ( isShooting && missiles.size() < missilesCount &&
        ( System.currentTimeMilis() - this.fired ) > 100 ) {
      missiles.add( new Missile( player.getX() + 16, player.getY() + 16 ) );
      // time of last firing
      this.fired = System.currentTimeMilis();
   }
   ...
}

public void RemoveMissile(Missile e) {
   // as Guest is asked in another answer, this method should remove, not add...
   missiles.remove(e);
}