org.apache.commons.collections.iterators

Class ObjectGraphIterator

Implemented Interfaces:
Iterator

public class ObjectGraphIterator
extends java.lang.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.

Version:
$Revision: 1.3 $ $Date: 2004/05/03 11:50:30 $
Author:
Stephen Colebourne
Since:
Commons Collections 3.1

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.

Field Details

currentIterator

protected Iterator currentIterator
The current iterator

currentValue

protected Object currentValue
The current value

hasNext

protected boolean hasNext
Whether there is another element in the iteration

lastUsedIterator

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

root

protected Object root
The root object in the tree

stack

protected final ArrayStack stack
The stack of iterators

transformer

protected Transformer transformer
The transformer to use

Constructor Details

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

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

Method Details

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.
Returns:
true if elements remain in the iteration

next

public Object next()
Gets the next element of the iteration.
Returns:
the next element from the iteration

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.


updateCurrentIterator

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

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