Cannot find a class or type named “Iterator”

Processing Beginners
3

As @glv’s already pointed out, you need to import the Iterator interface before using it:
import java.util.Iterator;

Alternatively you can use a regular for ( ; ; ) loop, but making sure to decrease its iterator variable each time you invoke method remove():

for (int i = 0; i < particles.size(); ++i) {
  final Particle p = particles.get(i);
  p.run();
  if (p.isDead())  particles.remove(i--);
}

It’s even simpler if you iterate it backwards, so you don’t have to worry about decreasing the iterator for each remove():

for (int i = particles.size(); i-- > 0; ) {
  final Particle p = particles.get(i);
  p.run();
  if (p.isDead())  particles.remove(i);
}

For a more complex but more efficient (b/c we completely avoid container’s element left-shifting) backwards technique, see this sketch’s mousePressed() code block implementation:

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