Question

I’m making a simple tower defense game in Swing and I’ve run into a performance problem when I try to put many sprites (more than 20) on screen.

The whole game takes place on a JPanel which has setIgnoreRepaint(true).
Here is the paintComponent method (con is the Controller):

public void paintComponent(Graphics g){
    super.paintComponent(g);
    //Draw grid
    g.drawImage(background, 0, 0, null);
    if (con != null){
        //Draw towers
        for (Tower t : con.getTowerList()){
            t.paintTower(g);
        }
        //Draw targets
        if (con.getTargets().size() != 0){
            for (Target t : con.getTargets()){
                t.paintTarget(g);
            }
            //Draw shots
            for (Shot s : con.getShots()){
                s.paintShot(g);
            }
        }
    }
}

The Target class simple paints a BufferedImage at its current location. The getImage method doesn’t create a new BufferedImage, it simply returns the Controller class’s instance of it:

public void paintTarget(Graphics g){
    g.drawImage(con.getImage("target"), getPosition().x - 20, getPosition().y - 20, null);
}

Each target runs a Swing Timer to calculate its position. This is the ActionListener it calls:

public void actionPerformed(ActionEvent e) {
    if (!waypointReached()){
        x += dx;
        y += dy;
        con.repaintArea((int)x - 25, (int)y - 25, 50, 50);
    }
    else{
        moving = false;
        mover.stop();
    }
}

private boolean waypointReached(){
    return Math.abs(x - currentWaypoint.x) <= speed && Math.abs(y - currentWaypoint.y) <= speed;
}

Other than that, repaint() is only called when placing a new tower.

How can I improve the performance?

Answer 1

Each target runs a Swing Timer to calculate its position. This is the ActionListener it calls:

This may be your problem – having each target/bullet (I assume?) responsible for keeping track of when to update itself and draw itself sounds like quite a bit of work. The more common approach is to have a loop along the lines of

while (gameIsRunning) {
  int timeElapsed = timeSinceLastUpdate();
  for (GameEntity e : entities) {
    e.update(timeElapsed);
  }
  render(); // or simply repaint in your case, I guess
  Thread.sleep(???); // You don't want to do this on the main Swing (EDT) thread though
}

Essentially, an object further up the chain has the responsibility to keep track of all entities in your game, tell them to update themselves, and render them.