关于iterator的一点疑惑

书中代码：

//: holding/SimpleIteration.java
import typeinfo.pets.*;
import java.util.*;

public class SimpleIteration {
  public static void main(String[] args) {
    List<Pet> pets = Pets.arrayList(12);
    Iterator<Pet> it = pets.iterator();
    while(it.hasNext()) {
      Pet p = it.next();
      System.out.print(p.id() + ":" + p + " ");
    }
    System.out.println();
    // A simpler approach, when possible:
    for(Pet p : pets)
      System.out.print(p.id() + ":" + p + " ");
    System.out.println();	
    // An Iterator can also remove elements:
    it = pets.iterator();
    for(int i = 0; i < 6; i++) {
      it.next();
      it.remove();
    }
    System.out.println(pets);
  }
} /* Output:
0:Rat 1:Manx 2:Cymric 3:Mutt 4:Pug 5:Cymric 6:Pug 7:Manx 8:Cymric 9:Rat 10:EgyptianMau 11:Hamster
0:Rat 1:Manx 2:Cymric 3:Mutt 4:Pug 5:Cymric 6:Pug 7:Manx 8:Cymric 9:Rat 10:EgyptianMau 11:Hamster
[Pug, Manx, Cymric, Rat, EgyptianMau, Hamster]
*///:~

我主要困惑于hasNext()、next()函数的返回以及内部操作：

while(it.hasNext()) {
    Pet p = it.next();
    System.out.print(p.id() + ":" + p + " ");
}

看书看到这里的时候有些困惑，然后在后面看到了一个实现了Iterator接口的内部类，读完这段代码之后，就理解了其运行原理：

//: holding/NonCollectionSequence.java
import typeinfo.pets.*;
import java.util.*;

class PetSequence {
  protected Pet[] pets = Pets.createArray(8);
}

public class NonCollectionSequence extends PetSequence {
  public Iterator<Pet> iterator() {
    return new Iterator<Pet>() {
      private int index = 0;
      public boolean hasNext() {
        return index < pets.length;
      }
      public Pet next() { return pets[index++]; }
      public void remove() { // Not implemented
        throw new UnsupportedOperationException();
      }
    };
  }
  public static void main(String[] args) {
    NonCollectionSequence nc = new NonCollectionSequence();
    InterfaceVsIterator.display(nc.iterator());
  }
} /* Output:
0:Rat 1:Manx 2:Cymric 3:Mutt 4:Pug 5:Cymric 6:Pug 7:Manx
*///:~

观察这段代码：

return new Iterator<Pet>() {
    private int index = 0;
    public boolean hasNext() {
    return index < pets.length;
    }
    public Pet next() { return pets[index++]; }
    public void remove() { // Not implemented
    throw new UnsupportedOperationException();
    }
};

由hasNext()函数的实现可以猜测List中的元素的索引是从0开始的（事实也确实如此），然后是next()函数，看到index++这块代码，我们就理解了，next()函数返回的当前索引位置的元素，然后还有一个将索引递增的操作。