尝试从ArrayList中删除元素

Question

I'm attempting to create a class which will display text for a couple of seconds and then disappear.

I'm using LWJGL and in my main class MetalCLicker i have a for loop which cycles through the popups.

            for(PopUp pop: popups){
            pop.tick(pop);
        }

popup class: (problem at bottom in tick method) public class PopUp {

MetalClicker game;

int x;
float y, lifetime;
String line1, line2, line3;
Color color;

private UnicodeFont font;


public PopUp(MetalClicker game, int x, int y, float lifetime, String line1, String line2, String line3, Color color){
    this.x = x;
    this.y = y;
    this.lifetime = lifetime*game.fps;
    this.line1 = line1;
    this.line2 = line2;
    this.line3 = line3;
    this.color = color;
    this.game = game;


    font = new UnicodeFont(new java.awt.Font ("Vani", Font.BOLD, 12));
    font.getEffects().add(new ColorEffect(java.awt.Color.white));
    font.addNeheGlyphs();
    try {
        font.loadGlyphs();
    } catch (SlickException e) {
        e.printStackTrace();
    }
}

public void render(){
    font.drawString(x - (line1.length()/2), y, line1, color);
    font.drawString(x - (line2.length()/2), y+14, line2, color);
    font.drawString(x - (line3.length()/2), y+28, line3, color);
}

public void tick(PopUp pop){
    y -= 3/lifetime;

    lifetime -= 1;
    if (lifetime == 0) game.popups.remove(pop); //problem resides here
    else render();
}
}

The program crashes when lifetime hits 0, thus attempting to remove the element. Putting prints before and after the remove line successfully prints out the line, So I'm confused now :(

I have tried using this in the tick method so i switched to sending the actual element in the parameter.

There is no error in the console, but the debug tells me ArrayList$Itr.next() line: 831 ArrayList$Itr.checkForComodification() line: 859 [local variables unavailable]

inside Thread [main]

I'll update post with more info if needed, but i cant think of what to say to help you, help me.

And info on how i can go about without using MetalCLicker game in my method parameters would be cool.

Answer 1

you're trying to remove elements while iterating through the list, thus invalidating the iterator. You cannot use this:

for(PopUp pop: popups) {
  popups.remove(pop); // effectively, due to tick
}

or even, safely, this:

for(var i=0, last=popups.size(); i<last; i++) {
  PopUp pop = popups.get(i);
  popups.remove(pop); // next "i++" will skip over an item
}

However, you CAN use this:

for(var i=popups.size()-1; i>=0; i--) {
  PopUp pop = popups.get(i);
  popups.remove(pop);
}

because any removals now happen in the part of the array list that will not be touched by the next iteration

Answer 2

You can't loop around a collection and remove an item directly from it at the same time. You have two options here, you can copy this collection to a new collection for the loop alone or you can, instead of using a foreach , use the collection iterator directly and send the iterator as a parameter to the tick method, then you can call the remove method at the collection iterator and it won't throw this exception.

So it's either change the foreach to:

for(PopUp pop : popups.clone()){
    pop.tick(pop);
}

Or use an iterator instead of a foreach:

Iterator<PopUp> iterator = popups.iterator();
while ( iterator.hasNext() ) {
  PopUp pop = iterator.next();
  pop.tick(iterator);
}

And at the tick implementation:

public void tick(Iterator it){
    y -= 3/lifetime;

    lifetime -= 1;
    if (lifetime == 0) {
      it.remove();
    }
    else render();
}

Both have mostly the same effect, but I'd say the second option is better.