How to change properties of an object from an array list?

Question

Basically, I am creating a top down shooter in java, and for the bullets there is a bullet object with all the properties and update methods and stuff. I hava decided to use an array list to store the bullets in once the mouse is pressed and an instance of the object has been created. The problem is that I do not know how to identify elements in an array list. Here is a snippet of some of the code when i was using just a simple array:

addMouseListener(new MouseAdapter(){
    public void mousePressed(MouseEvent e){
        pressedX = e.getX();
        pressedY = e.getY();


        bullets[bulletCount] = new Bullet(player.x, player.y));
        double angle = Math.atan2((pressedY - player.y),(pressedX - player.x));
    bullets[bulletCount].dx = Math.cos(angle)*5;
    bullets[bulletCount].dy = Math.sin(angle)*5;
    bulletCount++;  


    }

Any help is greatly appreciated! Thanks in advance!

what are you trying to identify, a specific bullet?

twain249
– twain249

2012-03-17 20:39:30 +00:00
Commented Mar 17, 2012 at 20:39 — twain249
– twain249, Commented Mar 17, 2012 at 20:39

rsp · Accepted Answer · 2012-03-17 21:32:02Z

3

You could just change anything like this:

bullets[index].foo

to

bullets.get(index).foo

However, in the code you've given, we can do better.

So:

addMouseListener(new MouseAdapter() {
    public void mousePressed(MouseEvent e) {
        int pressedX = e.getX();
        int pressedY = e.getY();

        Bullet bullet = new Bullet(player.x, player.y);
        double angle = Math.atan2((pressedY - player.y), (pressedX - player.x));
        bullet.dx = Math.cos(angle)*5;
        bullet.dy = Math.sin(angle)*5;
        bullets.add(bullet);
    }
}

Now that's still accessing fields in the bullet directly, which doesn't seem like a very good idea to me. I would suggest that you either use properties for dx and dy - or possibly a single property taking a Velocity (which would basically be a vector of dx and dy) - or you make that part of the constructor:

addMouseListener(new MouseAdapter() {
    public void mousePressed(MouseEvent e) {
        // Again, ideally don't access variables directly
        Point playerPosition = new Point(player.x, player.y);
        Point touched = new Point(e.getX(), e.getY());

        // You'd need to put this somewhere else if you use an existing Point
        // type.
        double angle = touched.getAngleTowards(playerPosition);
        // This method would have all the trigonometry.
        Velocity velocity = Velocity.fromAngleAndSpeed(angle, 5);

        bullets.add(new Bullet(playerPosition, velocity));
    }
}

edited Mar 17, 2012 at 21:32

rsp

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answered Mar 17, 2012 at 20:39

Jon Skeet

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1 Comment

hazard1994 Over a year ago

Jon Skeet you're my hero!! Works a treat! Thanks for the help!!

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