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/*
* Licensed to the Apache Software Foundation (ASF) under one or more
* contributor license agreements. See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* The ASF licenses this file to You under the Apache License, Version 2.0
* (the "License"); you may not use this file except in compliance with
* the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.apache.commons.collections.iterators;
import java.util.Iterator;
import java.util.NoSuchElementException;
import org.apache.commons.collections.ArrayStack;
import org.apache.commons.collections.Transformer;
/**
* 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 $Revision$ $Date$
*
* @author Stephen Colebourne
*
* @deprecated Apache Commons Collections version 3.x is being deprecated from AEMaaCS. The upgraded version 4.4 of Commons Collections is already included as replacement. Customers are advised to upgrade to this version of the library. Please note: the package name was changed to org.apache.commons.collections4. Further note that there are AEM APIs currently exposing the old collections classes; these will be updated in upcoming releases.
*/
@Deprecated(since = "2021-04-30")
public class ObjectGraphIterator implements Iterator {
/**
* The stack of iterators
*/
protected final ArrayStack stack = new ArrayStack(8);
/**
* The root object in the tree
*/
protected Object root;
/**
* The transformer to use
*/
protected Transformer transformer;
/**
* Whether there is another element in the iteration
*/
protected boolean hasNext = false;
/**
* The current iterator
*/
protected Iterator currentIterator;
/**
* The current value
*/
protected Object currentValue;
/**
* The last used iterator, needed for remove()
*/
protected Iterator lastUsedIterator;
// -----------------------------------------------------------------------
/**
* 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.
*
* @param root the root object, null will result in an empty iterator
* @param transformer the transformer to use, null will use a no effect transformer
*/
public ObjectGraphIterator(Object root, Transformer transformer) {
super();
if (root instanceof Iterator) {
this.currentIterator = (Iterator) root;
} else {
this.root = root;
}
this.transformer = transformer;
}
/**
* 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.
*
* @param rootIterator the root iterator, null will result in an empty iterator
*/
public ObjectGraphIterator(Iterator rootIterator) {
super();
this.currentIterator = rootIterator;
this.transformer = null;
}
// -----------------------------------------------------------------------
/**
* Loops around the iterators to find the next value to return.
*/
protected void updateCurrentIterator() {
if (hasNext) {
return;
}
if (currentIterator == null) {
if (root == null) {
// do nothing, hasNext will be false
} else {
if (transformer == null) {
findNext(root);
} else {
findNext(transformer.transform(root));
}
root = null;
}
} else {
findNextByIterator(currentIterator);
}
}
/**
* Finds the next object in the iteration given any start object.
*
* @param value the value to start from
*/
protected void findNext(Object value) {
if (value instanceof Iterator) {
// need to examine this iterator
findNextByIterator((Iterator) value);
} else {
// next value found
currentValue = value;
hasNext = true;
}
}
/**
* Finds the next object in the iteration given an iterator.
*
* @param iterator the iterator to start from
*/
protected void findNextByIterator(Iterator iterator) {
if (iterator != currentIterator) {
// recurse a level
if (currentIterator != null) {
stack.push(currentIterator);
}
currentIterator = iterator;
}
while (currentIterator.hasNext() && hasNext == false) {
Object next = currentIterator.next();
if (transformer != null) {
next = transformer.transform(next);
}
findNext(next);
}
if (hasNext) {
// next value found
} else if (stack.isEmpty()) {
// all iterators exhausted
} else {
// current iterator exhausted, go up a level
currentIterator = (Iterator) stack.pop();
findNextByIterator(currentIterator);
}
}
// -----------------------------------------------------------------------
/**
* Checks whether there are any more elements in the iteration to obtain.
*
* @return true if elements remain in the iteration
*/
public boolean hasNext() {
updateCurrentIterator();
return hasNext;
}
/**
* Gets the next element of the iteration.
*
* @return the next element from the iteration
* @throws NoSuchElementException if all the Iterators are exhausted
*/
public Object next() {
updateCurrentIterator();
if (hasNext == false) {
throw new NoSuchElementException("No more elements in the iteration");
}
lastUsedIterator = currentIterator;
Object result = currentValue;
currentValue = null;
hasNext = false;
return result;
}
/**
* 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.
*
* @throws UnsupportedOperationException
* if the remove operator is not supported by the underlying Iterator
* @throws 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.
*/
public void remove() {
if (lastUsedIterator == null) {
throw new IllegalStateException("Iterator remove() cannot be called at this time");
}
lastUsedIterator.remove();
lastUsedIterator = null;
}
}