org.apache.commons.collections.iterators
Class ObjectGraphIterator

java.lang.Object
  org.apache.commons.collections.iterators.ObjectGraphIterator

All Implemented Interfaces:

java.util.Iterator

public class ObjectGraphIterator
extends java.lang.Object
implements java.util.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.

Since:

Commons Collections 3.1

Version:

$Revision: 647116 $ $Date: 2008-04-11 12:23:08 +0100 (Fri, 11 Apr 2008) $

Author:

Stephen Colebourne

Field Summary

protected java.util.Iterator	currentIterator The current iterator
protected java.lang.Object	currentValue The current value
protected boolean	hasNext Whether there is another element in the iteration
protected java.util.Iterator	lastUsedIterator The last used iterator, needed for remove()
protected java.lang.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(java.util.Iterator rootIterator)
Constructs a ObjectGraphIterator that will handle an iterator of iterators.

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

Method Summary

protected void	findNext(java.lang.Object value) Finds the next object in the iteration given any start object.
protected void	findNextByIterator(java.util.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.
java.lang.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 java.lang.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 java.util.Iterator currentIterator

The current iterator

currentValue

protected java.lang.Object currentValue

The current value

lastUsedIterator

protected java.util.Iterator lastUsedIterator

The last used iterator, needed for remove()

Constructor Detail

ObjectGraphIterator

public ObjectGraphIterator(java.lang.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(java.util.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(java.lang.Object value)

Finds the next object in the iteration given any start object.

Parameters:

value - the value to start from

findNextByIterator

protected void findNextByIterator(java.util.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 java.util.Iterator

Returns:

true if elements remain in the iteration

next

public java.lang.Object next()

Gets the next element of the iteration.

Specified by:

next in interface java.util.Iterator

Returns:

the next element from the iteration

Throws:

java.util.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 java.util.Iterator

Throws:

java.lang.UnsupportedOperationException - if the remove operator is not supported by the underlying Iterator

java.lang.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.