Iterator

Iterator, an interface in java.util for iterating sequence of objects.

public interface Iterator {
    boolean hasNext();
    Object next();
    void remove();                          // The remove() method is optional.
  }

An Iterator is like a bookmark.
 Just as you can have many bookmarks in a book, you can have many Iterators iterating over the same data structure, each one independent of the others.

One Iterator can advance without disturbing other Iterators that are iterating over the same data structure.
The first time next() is called on a newly constructed Iterator, it returns the first item in the sequence. Each subsequent time next() is called, it returns the next item in the sequence.
 After the Iterator has returned every item in the sequence, every subsequent call to next() throws an exception and halts with an error message. (I find this annoying; I would prefer an interface in which next() returns null. The Java library designers disagree.)

To help you avoid triggering an exception, hasNext() returns true if the Iterator has more items to return, or false if it has already returned every item in the sequence. It is usually considered good practice to check hasNext() before calling next(). (In the next lecture we'll learn how to catch exceptions; that will give us an alternative way to prevent our program from crashing when next() throws an exception.)

 There is usually no way to reset an Iterator back to the beginning of the sequence. Instead, you construct a new Iterator.

Most data structures that support Iterators "implement" another interface in
java.util called "Iterable".

public interface Iterable {
    Iterator iterator();
  }

怎么用，一般来说，如果想遍历一个数据结构DS，就call DS.iteratre(); 这个method会constructs and returns DSIterator whose fields are initialized so it is ready to return the first item in DS.

例子:

/* list/SListIterator.java */

package list;
import java.util.*;

public class SListIterator implements Iterator {
  SListNode n;

  public SListIterator(SList l) {
    n = l.head;
  }

  public boolean hasNext() {
    return n != null;
  }

  public Object next() {
    if (n == null) {
      /* We'll learn about throwing exceptions in the next lecture. */
      throw new NoSuchElementException();                       // In java.util
    }
    Object i = n.item;
    n = n.next;
    return i;
  }

  public void remove() {
    /* Doing it the lazy way.  Remove this, motherf! */
    throw new UnsupportedOperationException("Nice try, bozo."); // In java.lang
  }
}

/* list/SList.java */

package list;
import java.util.*;

public class SList implements Iterable {
  SListNode head;
  int size;

  public Iterator iterator() {
    return new SListIterator(this);
  }

  [other methods here]
}