Check collision of 2 objects in libgdx - libgdx

Question

I am making a game in libgdx, where you shoot aliens with bullets. I have 2 ArrayLists of objects and would like to check if any of objects in bulletArrayList is colliding with any object from alienArrayList. What is the best way to do that? I was thinking of contactListener.
In the screen class I am generating objects like this:
public class PlayScreen implements Screen, InputProcessor,ContactListener {
public ArrayList<Alien> alienArrayList = new ArrayList<Alien>();
public ArrayList<Bullet> bulletArrayList = new ArrayList<Bullet>();
public void generateAlien() {
alien = new Alien();
alienArrayList.add(alien);
}
public void shootBullet(float x, float y) {
//send x,y moving coordiantes
bullet = new Bullet(x,y);
bulletArrayList.add(bullet);
}
}
In object class I have Rectangle box which i am moving like this:
public class Alien {
public Alien() {
bound = new Rectangle( x, y, alienRegion.getRegionWidth(), alienRegion.getRegionHeight());
}
public void update(float delta) {
bound.y -= speed * delta;
}
public void render(SpriteBatch batch, float delta) {
update(delta);
elapsedTime += Gdx.graphics.getDeltaTime();
alienRegion = (TextureRegion) alien.getKeyFrame(elapsedTime, true);
batch.draw(alienRegion, getBound().x, getBound().y);
}
}

Answer 1

Because you are using Rectangles in your Alien class, we can use a class called Intersector which has static methods to check for collision detection.
for(Alien alien1: alienArrayList) {
for(Alien alien2 : bulletArrayList) {
if(Intersector.overlaps(alien1.rect, alien2.rect)) {
// Collision code
}
}
}
First, we iterate through the two lists using a nested special for loop. Then we pass two Rectangles to the Intersector.overlaps(rect1, rect2). This is a static method defined in the Intersector class which will return true if the rectangles are overlapping.
Also, this code can go straight into your render method.
This code is not the most optimized because it will check 2 rects twice however, I will leave the optimization to you.
I hope that this answer was helpful and if you have any further questions please feel free to post a comment below.