org.apache.commons.collections.iterators
Class ObjectGraphIterator

java.lang.Object
  extended by org.apache.commons.collections.iterators.ObjectGraphIterator
All Implemented Interfaces:
Iterator

public class ObjectGraphIterator
extends Object
implements Iterator

An Iterator that can traverse multiple iterators down an object graph.

This iterator can extract multiple objects from a complex tree-like object graph. The iteration starts from a single root object. It uses a Transformer to extract the iterators and elements. Its main benefit is that no intermediate List is created.

For example, consider an object graph:

                 |- Branch -- Leaf
                 |         \- Leaf
         |- Tree |         /- Leaf
         |       |- Branch -- Leaf
  Forest |                 \- Leaf
         |       |- Branch -- Leaf
         |       |         \- Leaf
         |- Tree |         /- Leaf
                 |- Branch -- Leaf
                 |- Branch -- Leaf
The following Transformer, used in this class, will extract all the Leaf objects without creating a combined intermediate list:
 public Object transform(Object input) {
   if (input instanceof Forest) {
     return ((Forest) input).treeIterator();
   }
   if (input instanceof Tree) {
     return ((Tree) input).branchIterator();
   }
   if (input instanceof Branch) {
     return ((Branch) input).leafIterator();
   }
   if (input instanceof Leaf) {
     return input;
   }
   throw new ClassCastException();
 }

Internally, iteration starts from the root object. When next is called, the transformer is called to examine the object. The transformer will return either an iterator or an object. If the object is an Iterator, the next element from that iterator is obtained and the process repeats. If the element is an object it is returned.

Under many circumstances, linking Iterators together in this manner is more efficient (and convenient) than using nested for loops to extract a list.

Note: this class is not type-safe in Java 1.5 (no generics).

Since:
Commons Collections 3.1
Version:
$Revision: 1.1.1.1 $ $Date: 2005/05/23 04:34:59 $
Author:
Matt Hall, John Watkinson, Stephen Colebourne

Field Summary
protected  Iterator currentIterator
          The current iterator
protected  Object currentValue
          The current value
protected  boolean hasNext
          Whether there is another element in the iteration
protected  Iterator lastUsedIterator
          The last used iterator, needed for remove()
protected  Object root
          The root object in the tree
protected  ArrayStack stack
          The stack of iterators
protected  Transformer transformer
          The transformer to use
 
Constructor Summary
ObjectGraphIterator(Iterator rootIterator)
          Constructs a ObjectGraphIterator that will handle an iterator of iterators.
ObjectGraphIterator(Object root, Transformer transformer)
          Constructs an ObjectGraphIterator using a root object and transformer.
 
Method Summary
protected  void findNext(Object value)
          Finds the next object in the iteration given any start object.
protected  void findNextByIterator(Iterator iterator)
          Finds the next object in the iteration given an iterator.
 boolean hasNext()
          Checks whether there are any more elements in the iteration to obtain.
 Object next()
          Gets the next element of the iteration.
 void remove()
          Removes from the underlying collection the last element returned.
protected  void updateCurrentIterator()
          Loops around the iterators to find the next value to return.
 
Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait
 

Field Detail

stack

protected final ArrayStack stack
The stack of iterators


root

protected Object root
The root object in the tree


transformer

protected Transformer transformer
The transformer to use


hasNext

protected boolean hasNext
Whether there is another element in the iteration


currentIterator

protected Iterator currentIterator
The current iterator


currentValue

protected Object currentValue
The current value


lastUsedIterator

protected Iterator lastUsedIterator
The last used iterator, needed for remove()

Constructor Detail

ObjectGraphIterator

public ObjectGraphIterator(Object root,
                           Transformer transformer)
Constructs an ObjectGraphIterator using a root object and transformer.

The root object can be an iterator, in which case it will be immediately looped around.

Parameters:
root - the root object, null will result in an empty iterator
transformer - the transformer to use, null will use a no effect transformer

ObjectGraphIterator

public ObjectGraphIterator(Iterator rootIterator)
Constructs a ObjectGraphIterator that will handle an iterator of iterators.

This constructor exists for convenience to emphasise that this class can be used to iterate over nested iterators. That is to say that the iterator passed in here contains other iterators, which may in turn contain further iterators.

Parameters:
rootIterator - the root iterator, null will result in an empty iterator
Method Detail

updateCurrentIterator

protected void updateCurrentIterator()
Loops around the iterators to find the next value to return.


findNext

protected void findNext(Object value)
Finds the next object in the iteration given any start object.

Parameters:
value - the value to start from

findNextByIterator

protected void findNextByIterator(Iterator iterator)
Finds the next object in the iteration given an iterator.

Parameters:
iterator - the iterator to start from

hasNext

public boolean hasNext()
Checks whether there are any more elements in the iteration to obtain.

Specified by:
hasNext in interface Iterator
Returns:
true if elements remain in the iteration

next

public Object next()
Gets the next element of the iteration.

Specified by:
next in interface Iterator
Returns:
the next element from the iteration
Throws:
NoSuchElementException - if all the Iterators are exhausted

remove

public void remove()
Removes from the underlying collection the last element returned.

This method calls remove() on the underlying Iterator and it may throw an UnsupportedOperationException if the underlying Iterator does not support this method.

Specified by:
remove in interface Iterator
Throws:
UnsupportedOperationException - if the remove operator is not supported by the underlying Iterator
IllegalStateException - if the next method has not yet been called, or the remove method has already been called after the last call to the next method.


Copyright © 2005-2005 Apache Software Foundation, Matt Hall, John Watkinson. All Rights Reserved.