SE450: A Linked List: External Iterator [1/25] Previous pageContentsNext page

This is an external iterator over a linked list.

Here we use a null object or sentinel to mark the end of the list. (This is a degenerate composite.)

This means that we can sensibly ask an empty list for an iterator. We cannot do this if the empty list is represented by null.

file:iterator/listone/Main.java [source] [doc-public] [doc-private]
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package iterator.listone;
import java.util.NoSuchElementException;

/* public */
interface List {
  public Iterator newIterator();
}

/* public */
class ListF {
  private ListF() {}
  public static final List nil = new Nil(); /* Singleton */
  public static final List cons(int hd, List tl) /* Factory */ {
    return new Cons(hd, tl);
  }
}

/* public */
interface Iterator {
  public boolean hasNext();
  public int next();
}

/*
 *************************************************************************
 * List classes.
 *************************************************************************
 */
class Nil implements List {
  Nil() {}
  public String toString() { return "nil"; }
  public Iterator newIterator() { return new NullIterator(); }
}

class Cons implements List {
  final int hd;
  final List tl;
  Cons(int hd, List tl) { this.hd = hd; this.tl = tl; }
  public String toString() { return hd + "::" + tl.toString(); }
  public Iterator newIterator() { return new ListIterator(this); }
}

class NullIterator implements Iterator {
  NullIterator() { }
  public boolean hasNext() { return false; }
  public int next() { throw new NoSuchElementException(); }
}

class ListIterator implements Iterator {
  private List node;
  ListIterator(List node) { this.node = node; }
  public boolean hasNext() {
    return node != ListF.nil;
  }
  public int next() {
    if (! hasNext())
      throw new NoSuchElementException();
    int result = ((Cons)node).hd;
    node = ((Cons)node).tl;
    return result;
  }
}

/*
 *************************************************************************
 * A test case.
 *************************************************************************
 */
public class Main {
  public static void main(String[] args) {
    List test = ListF.cons(1, ListF.cons(2, ListF.cons(3, ListF.nil)));
    System.out.println(test);

    int sum=0;
    for (Iterator i = test.newIterator(); i.hasNext(); )
      sum += i.next();
    System.out.println(sum);

    List rev=ListF.nil;
    for (Iterator i = test.newIterator(); i.hasNext(); )
      rev = ListF.cons(i.next(),rev);
    System.out.println(rev);
  }
}

Note that the attributes must be package private so that the external iterator can access them.

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