SE450: A Linked List: External Iterator [5/25] Previous pageContentsNext page

External iterators must be careful if the collection is mutable.

The important part here is how MutListObj and ListIterator coordinate on changeCount.

The example looks unusual because MutList is coded using the immutable List class.

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

/* public */
interface MutList {
  public Iterator newIterator();
  public void insert(int i); // insert i at head of list
  public void delete(int i); // delete first occurence of i in the list
}

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

/* public */
class MutListF {
  private MutListF() {}
  public static final MutList newI() {
    return new MutListObj();
  }
}

class MutListObj implements MutList, Changeable {
  private List l = ListF.nil;
  private int changeCount = 0;
  public int changeCount() { return changeCount; }
  public Iterator newIterator() { return l.newIterator(this); }
  public String toString() { return l.toString(); }
  public void insert(int i) {
    changeCount++;
    l = l.insert(i);
  }
  public void delete(int i) {
    changeCount++;
    l = l.delete(i);
  }
}

/*
 *************************************************************************
 * Immutable List classes.
 *************************************************************************
 */
interface Changeable {
  public int changeCount();
}

interface List {
  public Iterator newIterator(Changeable parent);
  public List insert(int i);
  public List delete(int i);
}

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);
  }
}

class Nil implements List {
  Nil() {}
  public String toString() { return "nil"; }
  public Iterator newIterator(Changeable parent) {
    return new NullIterator();
  }
  public List insert(int i) {
    return ListF.cons(i, this);
  }
  public List delete(int i) {
    throw new NoSuchElementException();
  }
}

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(Changeable parent) {
    return new ListIterator(this, parent);
  }
  public List insert(int i) {
    return ListF.cons(i, this);
  }
  public List delete(int i) {
    if (hd == i) {
      return tl;
    } else {
      List new_tl = tl.delete(i);
      return ListF.cons(hd, new_tl);
    }
  }
}

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

class ListIterator implements Iterator {
  private List node;
  private Changeable parent;
  private int changeCount;
  ListIterator(List node, Changeable parent) {
    this.node = node;
    this.parent = parent;
    this.changeCount = parent.changeCount();
  }
  public boolean hasNext() {
    if (changeCount != parent.changeCount())
      throw new ConcurrentModificationException();
    return node != ListF.nil;
  }
  public int next() {
    if (changeCount != parent.changeCount())
      throw new ConcurrentModificationException();
    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) {
    MutList test = MutListF.newI();
    test.insert(3);
    test.insert(2);
    test.insert(1);
    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);

    for (Iterator i = test.newIterator(); i.hasNext(); )
      test.insert(4);
  }
}

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