How do you make sure the 2 balls are removed after the collision?

Question

I am a beginner and I am making a space shooter game in Netbeans. I want if the spacecraft's bullet hits the enemy's bullet that the 2 bullets are removed. I have created an Arraylist of bullets and bulletsEnemy. Then I created a method hitByBullet that should ensure that I can search for the bullets that collided. Then in the method bulletsHit (), the bullets should be removed.

My enemies shoot in a loop and don't move. My problem is that the bullets go through each other. The radius of the bullets is 5.

These is the 2 methods that I have written

'''

public int hitByBullet(){
   for(int i=0; i<bullets.size(); i++){
       for(int j= 0;j< bulletsEnemy.size(); j++){
          if( bullets.get(i).getY() - bulletsEnemy.get(j).getY()<= 5)
           if(bullets.get(i).getX() -bulletsEnemy.get(j).getX()<= 5){ 
           return i;
           }
       }
        
   }
   return -1;
}

'''

public boolean bulletsHit(){
     if (hitByBullet()!= -1){               
         bullets.remove(this);  //with 'this' I refer too 'i'
         bulletsEnemy.remove(this);
         return true;
     }
     return false;
}

'''

[This is how it looks like] (https://i.sstatic.net/4XsCR.png)

Answer 1

I will try to answer in a way to let you find your way to implement what you want. There are several frameworks out there which will help you with this and as you are new to programming there are a lot of things you could change/do better. But I will ignore these things.

I think your are trying to check if one 5x5 bullet intersects with another 5x5 bullet.

To make it easy let us asume your bullets are squares:

In your hitByBullet you should check your square centers (i asume that are x,y of your bullets) distance in both dimensions:

Sth. like this should check the collision:

if(Math.abs(bullets.get(i).getY() - bulletsEnemy.get(j).getY()) < 5 && 
    Math.abs(bullets.get(i).getX() - bulletsEnemy.get(j).getX()) < 5) { 
       return i;
}

Depending of the size of your bullets you need to change the distance check ( <=5). If your bullets have a size of 5 px it should be < 10. If your bullets are 10x10 you should check against < 20 ...

Regarding a more realistic scenario, that your bullets are circles (e.g. resulting of a 5x5 square px or a radius r=5):

You need to check if the distance between your two bullet centers is smaller than the bullet radius * 2.

That will bring you to use the theorem of pythagoras:

https://en.wikipedia.org/wiki/Pythagorean_theorem

The distance between your bullet centers is c. a is the distance in dimensional x and b the distance in dimensional y. You don't need to take care about negative dimensional values cause you need pow them by 2 which will always give you a positive value.

To check for collision you should calculate the distance c as follows:

c = SquareRootOf(a^2+b^2)

Math.sqrt(Math.pow(bullets.get(i).getX() - bulletsEnemy.get(j).getX()),2) + Math.pow(bullets.get(i).getY() - bulletsEnemy.get(j).getY(),2)) < 10

I think you should read basics about collision detection and geometry. Just two possible points of information:

An easy to start example with code:

https://zetcode.com/javagames/collision/

Circle intersection:

https://www.geeksforgeeks.org/check-two-given-circles-touch-intersect/

Regarding the removal of the bullets I asume you have some kind of game loop in which you frequently draw the bullets and calculate the collision (call the bulletsHit method):

I will make an assumption to make things easier: The movement speed of your bullets is not faster than one pixel per loop. Otherwise I think you need to take care of your loop circle time and calculate the positions of all elements and also take care about bullets that won't intersect in one loop circle because they could have passed by from one loop circle to the next loop circle. I'm not familiar with game dev, so maybe there are more things to cover.

Regarding this you should change things:

1. The method hitByBullet() will only return one collision per call. You could store your collidedBullets in two sets and after removing them from your bullets and bulletsEnemy list clear these temporary sets.

2. I don't understand what you are doing with "this" and your result of hitByBullet.

I would do it like this

public void hitByBullet(){
  for(int i=0; i<bullets.size(); i++){
   for(int j= 0;j< bulletsEnemy.size(); j++){
      if( Math.sqrt(Math.pow(bullets.get(i).getX() - bulletsEnemy.get(j).getX()),2) + Math.pow(bullets.get(i).getY() - bulletsEnemy.get(j).getY(),2)) < 10){ 
         bulletsToDelete.add(bullets.get(i));
         bulletsEnemyToDelete.add(bulletsEnemy.get(j));
       }
     }
   }
}

public boolean bulletsHit(){
   hitByBullet();
   bulletsToDelete.forEach(bullet -> bullets.remove(bullet);
   bulletsEnemyToDelete.forEach(bullet -> bulletsEnemy.remove(bullet);
   boolean bulletsHit = !bulletsToDelete.isEmpty();
   bulletsToDelete.clear();
   bulletsEnemyToDelete.clear();
   return bulletsHit;
}

As I mentioned before there are a lots of things you should improve:

• general structure of your code
• naming of your method (e.g. bulletsHit sounds like only checking if bullets are hit but you are also removing them)
• double checking of bullets

Greetings