IndexOutOfBoundsException: index: 0 size: 0

Programming Questions

killgannon

IndexOutOfBoundsException: index: 0 size: 0

in Programming Questions • 1 year ago

I'm getting this error occasionally and it points me to two different areas of the same piece of code each time..

for (int i = bullet.size() - 1; i >= 0; i--) {
boolean remove = false;
bullet.get(i).run();
for (int j = enemy.size() - 1; j >= 0; j--) {
if (enemy.get(j).pointInside(bullet.get(i).x, bullet.get(i).y)) {
bullet.remove(i);
enemy.remove(j);
score += 100;
}
}
if (bullet.get(i).y <= 0) {
bullet.remove(i);
}
}

sometimes it points to line 8 and others line 18, I'm guessing that when the object at position 'i' is removed the other line of code still tries to use it and throws an error because it's missing but I don't know how to resolve it. I tried to use a boolean variable that would switch to true if either of the conditions were true and would then in turn call for it to be removed but that didn't work either.

Can anyone point me in the right direction please? =]

Replies(8)

Chrisir

Re: IndexOutOfBoundsException: index: 0 size: 0

1 year ago

oups,
enemy.get(j).pointInside
is very ambitious....
I recommend

currentEnemy=enemy.get(j);

same for bullet
and then

if (currentEnemy.pointInside(currentBullet.x, currentBullet.y)) {

should be better.

In line 18 same stuff please.

see
http://www.processing.org/learning/topics/arraylistclass.html

Ball ball = (Ball) balls.get(i);
ball.move();
ball.display();


I leave it at this for now.

quarks

Re: IndexOutOfBoundsException: index: 0 size: 0

1 year ago

The problem is that you are iterating over the arraylists lines 2 & 6 but then if there is a collision you are removing elements from the same arraylist (lines 9 & 10) invalidating the loop variables.

Create another 2 araylists for bullets and enemies to be removed and when you have a collision add them to these arraylists leaving them in the original lists. Then after collision detection:- for each bullet in the remove-bullet arraylist remove it from the bullet arraylist. Do the same for enemies. Then clear() the remove arraylists ready for the next collision detection.

There is a neater solution using iterators but that is more advanced.

killgannon

Re: IndexOutOfBoundsException: index: 0 size: 0

1 year ago

Thank you for the responses =]

I've never seen the clear() method before and it doesn't appear to be in the reference, how is it used?

PhiLho

Re: IndexOutOfBoundsException: index: 0 size: 0

1 year ago

someArrayList.clear() just remove all elements of the list.
See ArrayList for a more complete reference.

killgannon

Re: IndexOutOfBoundsException: index: 0 size: 0

1 year ago

I've been playing around with the code and I'm confused by how to implement quarks' suggestion. I've created the second arraylist for each but I don't understand how and where to retrieve the object from the list and how to then associate that with the object in the original lists so that the correct ones are removed.

for (int i = bullet.size() - 1; i >= 0; i--) {
bullet.get(i).run();
for (int j = enemy.size() - 1; j >= 0; j--) {
if (enemy.get(j).pointInside(bullet.get(i).x, bullet.get(i).y)) {
bulletRemoval.add(bullet.get(i));
enemyRemoval.add(enemy.get(j));
score += 100;
}
}
if (bullet.get(i).y <= 0) {
bullet.remove(i);
}
for (i = bulletRemoval.size() - 1; i >= 0; i--) {
bullet.remove(i);
}
for (i = enemyRemoval.size() - 1; i >= 0; i--) {
enemy.remove(i);
}
}

This is what I have now but it just gives me another error and the sketch freezes.

PhiLho

Re: IndexOutOfBoundsException: index: 0 size: 0

1 year ago

ArrayList has a removeAll method. I just tested it, applying the idea of quarks:

ArrayList<String> strings = new ArrayList<String>();
for (int i = 0; i < 100; i++)
{
strings.add("B" + i);
}
ArrayList<String> toRemove = new ArrayList<String>();
for (String s : strings)
{
if (random(10) < 3)
{
toRemove.add(s);
}
}
strings.removeAll(toRemove);
println(strings);

Applied to your code, it looks like:

// At global level
ArrayList<Bullet> bullets = new ArrayList<Bullet>();
ArrayList<Enemy> enemies = new ArrayList<Enemy>();
// In the check function
ArrayList<Bullet> removedBullets = new ArrayList<Bullet>();
ArrayList<Enemy> removedEnemies = new ArrayList<Enemy>();
for (Bullet bullet : bullets) {
bullet.run();
for (Enemy enemy : enemies) {
if (enemy.pointInside(bullet.x, bullet.y)) {
removedBullets.add(bullet);
bullet = null;
removedEnemies.add(enemy);
score += 100;
break; // No more enemies to check for this bullet
}
}
if (bullet != null && bullet.y <= 0) {
removedBullets.add(bullet);
}
}
bullets.remove(removedBullets);
enemies.remove(removedEnemies);

Note: I renamed your array lists to use plural, it is logical and simplifies the usage of the "foreach" loop while keeping significant names...
The above isn't the most efficient, perhaps indeed using iterators would be better, but it (should) work and it is OK for small lists.

PhiLho

Re: IndexOutOfBoundsException: index: 0 size: 0

1 year ago

To be complete, the way with iterator:

ArrayList<String> strings = new ArrayList<String>();
for (int i = 0; i < 100; i++)
{
strings.add("B" + i);
}
Iterator<String> iterator = strings.iterator();
while (iterator.hasNext())
{
String s = iterator.next();
if (random(10) < 3)
{
iterator.remove();
}
}
println(strings);

and, on your code:

Iterator<Bullet> itBull = bullets.iterator();
while (itBull.hasNext()) {
Bullet bullet = itBull.next();
bullet.run();
Iterator<Enemy> itEnem = enemies.iterator();
while (itEnem.hasNext()) {
Enemy enemy = itEnem.next();
if (enemy.pointInside(bullet.x, bullet.y)) {
itBull.remove();
bullet = null;
itEnem.remove();
score += 100;
break; // No more enemies to check for this bullet
}
}
if (bullet != null && bullet.y <= 0) {
itBull.remove();
}
}

Again, the latter is untested, obviously.

killgannon

Re: IndexOutOfBoundsException: index: 0 size: 0

1 year ago

Thank you so much, the first method didn't work and to be honest the iterator method went a little over my head but I worked it out and now it's working perfectly.

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