Design Patterns Java™ Workbook

Chapter 28. Iterator

public abstract int depth();
public abstract boolean hasNext();
public abstract Object next();
}

The ComponentIterator class includes a visited set that can keep track of whether
an iterator has already traversed a node in a composite. For example, the aerial shell process
flow starts with a composite make step that contains a rework step that contains, again, the
make step. When you write an iterator for a composite that may contain cycles, you must
ensure that you do not step into an infinite loop.

As with composites, the terminal class for a composite iterator is comparatively simple:

package com.oozinoz.util;
import java.util.*;
public class LeafIterator extends ComponentIterator
{

public LeafIterator(Object node, Set visited)
{
super(node, visited);
}
public int depth()
{
return 0;
}

public boolean hasNext()
{
return !visited.contains(node);
}
public Object next()
{
if (visited.contains(node))
{
return null;
}
visited.add(node);
return node;
}
}

Note that the depth of an iterator is different from the depth of a node in a tree.

The depth of an iterator depends on the depth of its subiterator. Leaf iterators do not have
subiterators, so their depth is always 0. The value of depth() for a CompositeIterator
object is 1 plus the depth of its subiterator:

public int depth()
{
if (subiterator != null)
{
return subiterator.depth() + 1;
}
return 0;
}