For particle systems - iterator needs to be explicitly called.

[Kevin Cannon]

Since Processing 2, you need to import the iterator class explicitly to handle it.

import java.util.Iterator;

[Daniel Shiffman]

thanks for simplicity i've been removing iterator and just using a for loop from all the examples (though it's still in the book text). if you see it in one still, please file an issue and note the specific example.