如何在java中测试多个交叉点？

Question

I am currently making a tower defence game.

Simply put it works like this: green circles (enemies) move across the screen. By clicking, you can place a tower. The enemy has a rectangle hidden underneath it, and the tower has a large opaque rectangle around it indicating its hit range. If they are colliding, the enemy starts to lose health, until it dies.

Before, I used this test to see if the enemy was within range of the tower:

for(int i=0;i<enemy.length; i++) //Runs for the total amount of enemies
{
  for(int j=0; j<boxes.length;j++) //Runs for the total amount of towers placed
  {
    if(enemy[i].getEBox().intersects(boxes[j]))  //boxes[j] is the towers' range box
    {
      enemy.setHealth(enemy.getHealth()-1);
    }
  }
}

However, I would like the towers to only be able to shoot one enemy at a time, most preferably the enemy at the front. To do this I need java to detect that there are multiple enemies colliding with the rectangle and only damage the front enemy.

Here is the test I ran to do this (The enemies array value go backwards so the first one to appear is enemy[0] and the last enemy[10] for example):

for(int i=1;i<enemy.length; i++) 
{
 for(int j=0; j<boxes.length;j++) 
  {
   if(enemy[i].getEBox().intersects(boxes[j])&&!(enemy[i-1].getEBox().intersects(boxes[j]))) 
    {
  enemy.setHealth(enemy.getHealth()-1);
    }
  }
}

however, the second condition always returns a positive. How can I change my if statement to conduct a successful test?

Answer 1

To make a tower only shoot at one enemy, flip the loops and break the inner loop when shooting starts.

for(int j=0; j<boxes.length;j++) //Runs for the total amount of towers placed
{
  for(int i=0;i<enemy.length; i++) //Runs for the total amount of enemies
  {
    if(enemy[i].getEBox().intersects(boxes[j]))  //boxes[j] is the towers' range box
    {
      enemy[i].setHealth(enemy[i].getHealth()-1);
      break; // skip remaining enemies, since towers has used
             // up it's shooting capability for this round
    }
  }
}

If a tower can prioritize which enemy to shoot at, loop through all the enemies to find the best candidate, then shoot at it.

for (int j=0; j<boxes.length; j++) //Runs for the total amount of towers placed
{
   int enemyToShootIdx = -1;
   for (int i=0; i<enemy.length; i++) //Runs for the total amount of enemies
      if (enemy[i].getEBox().intersects(boxes[j]))  //boxes[j] is the towers' range box
         if (enemyToShootIdx == -1 || betterCandidate(boxes[j], enemy[i], enemy[enemyToShootIdx]))
            enemyToShootIdx = i;
   if (enemyToShootIdx != -1)
      enemy[enemyToShootIdx].setHealth(enemy[enemyToShootIdx].getHealth()-1);
}

Now you just have to implement the betterCandidate() method.

Answer 2

You could store the enemies in the order you detect them so you will know which is the closest. Like an ArrayList for detected enemies or an ID kind of variable for each enemy object, which initialized when detected. Im pretty sure there are a lot ways like this, so storing an order somehow could work. Once you have an order you can attack only that one and there is no need to test the detected objects anymore while still looking for coming enemies and keep storing them so on. In your case

if(enemy[i].getEBox().intersects(boxes[j])&&!(enemy[i- 1].getEBox().intersects(boxes[j])))

I don't really see (or maybe just too tired) why you need these conditions to attack. If your array of enemies is already in order why dont you just attack the first? If inRange && isAlive if not you go on the next one. I hope i did not misunderstand anything and probably could help a bit. :)