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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 com.feilong.lib.collection4.iterators;
import java.util.Collection;
import java.util.Iterator;
import java.util.LinkedList;
import java.util.Queue;
/**
* An IteratorChain is an Iterator that wraps a number of Iterators.
*
* This class makes multiple iterators look like one to the caller. When any
* method from the Iterator interface is called, the IteratorChain will delegate
* to a single underlying Iterator. The IteratorChain will invoke the Iterators
* in sequence until all Iterators are exhausted.
*
* Under many circumstances, linking Iterators together in this manner is more
* efficient (and convenient) than reading out the contents of each Iterator
* into a List and creating a new Iterator.
*
* Calling a method that adds new Iterator after a method in the Iterator
* interface has been called will result in an UnsupportedOperationException.
*
* NOTE: As from version 3.0, the IteratorChain may contain no iterators. In
* this case the class will function as an empty iterator.
*
* NOTE: As from version 4.0, the IteratorChain stores the iterators in a queue
* and removes any reference to them as soon as they are not used anymore. Thus
* the methods {@code setIterator(Iterator)} and {@code getIterators()} have been
* removed and {@link #size()} will return the number of remaining iterators in
* the queue.
*
* @param
*
* @since 2.1
*/
public class IteratorChain implements Iterator{
/** The chain of iterators */
private final Queue> iteratorChain = new LinkedList<>();
/** The current iterator */
private Iterator currentIterator = null;
/**
* The "last used" Iterator is the Iterator upon which next() or hasNext()
* was most recently called used for the remove() operation only
*/
private Iterator lastUsedIterator = null;
/**
* ComparatorChain is "locked" after the first time compare(Object,Object)
* is called
*/
private boolean isLocked = false;
//-----------------------------------------------------------------------
/**
* Construct an IteratorChain with no Iterators.
*
* You will normally use {@link #addIterator(Iterator)} to add some
* iterators after using this constructor.
*/
public IteratorChain(){
super();
}
/**
* Construct an IteratorChain with a single Iterator.
*
* This method takes one iterator. The newly constructed iterator will
* iterate through that iterator. Thus calling this constructor on its own
* will have no effect other than decorating the input iterator.
*
* You will normally use {@link #addIterator(Iterator)} to add some more
* iterators after using this constructor.
*
* @param iterator
* the first child iterator in the IteratorChain, not null
* @throws NullPointerException
* if the iterator is null
*/
public IteratorChain(final Iterator iterator){
super();
addIterator(iterator);
}
/**
* Constructs a new IteratorChain over the two given iterators.
*
* This method takes two iterators. The newly constructed iterator will
* iterate through each one of the input iterators in turn.
*
* @param first
* the first child iterator in the IteratorChain, not null
* @param second
* the second child iterator in the IteratorChain, not null
* @throws NullPointerException
* if either iterator is null
*/
public IteratorChain(final Iterator first, final Iterator second){
super();
addIterator(first);
addIterator(second);
}
/**
* Constructs a new IteratorChain over the array of iterators.
*
* This method takes an array of iterators. The newly constructed iterator
* will iterate through each one of the input iterators in turn.
*
* @param iteratorChain
* the array of iterators, not null
* @throws NullPointerException
* if iterators array is or contains null
*/
public IteratorChain(final Iterator...iteratorChain){
super();
for (final Iterator element : iteratorChain){
addIterator(element);
}
}
/**
* Constructs a new IteratorChain over the collection of
* iterators.
*
* This method takes a collection of iterators. The newly constructed
* iterator will iterate through each one of the input iterators in turn.
*
* @param iteratorChain
* the collection of iterators, not null
* @throws NullPointerException
* if iterators collection is or contains null
* @throws ClassCastException
* if iterators collection doesn't contain an
* iterator
*/
public IteratorChain(final Collection> iteratorChain){
super();
for (final Iterator iterator : iteratorChain){
addIterator(iterator);
}
}
//-----------------------------------------------------------------------
/**
* Add an Iterator to the end of the chain
*
* @param iterator
* Iterator to add
* @throws IllegalStateException
* if I've already started iterating
* @throws NullPointerException
* if the iterator is null
*/
public void addIterator(final Iterator iterator){
checkLocked();
if (iterator == null){
throw new NullPointerException("Iterator must not be null");
}
iteratorChain.add(iterator);
}
/**
* Returns the remaining number of Iterators in the current IteratorChain.
*
* @return Iterator count
*/
public int size(){
return iteratorChain.size();
}
/**
* Determine if modifications can still be made to the IteratorChain.
* IteratorChains cannot be modified once they have executed a method from
* the Iterator interface.
*
* @return true if IteratorChain cannot be modified, false if it can
*/
public boolean isLocked(){
return isLocked;
}
/**
* Checks whether the iterator chain is now locked and in use.
*/
private void checkLocked(){
if (isLocked == true){
throw new UnsupportedOperationException(
"IteratorChain cannot be changed after the first use of a method from the Iterator interface");
}
}
/**
* Lock the chain so no more iterators can be added. This must be called
* from all Iterator interface methods.
*/
private void lockChain(){
if (isLocked == false){
isLocked = true;
}
}
/**
* Updates the current iterator field to ensure that the current Iterator is
* not exhausted
*/
protected void updateCurrentIterator(){
if (currentIterator == null){
if (iteratorChain.isEmpty()){
currentIterator = EmptyIterator. emptyIterator();
}else{
currentIterator = iteratorChain.remove();
}
// set last used iterator here, in case the user calls remove
// before calling hasNext() or next() (although they shouldn't)
lastUsedIterator = currentIterator;
}
while (currentIterator.hasNext() == false && !iteratorChain.isEmpty()){
currentIterator = iteratorChain.remove();
}
}
//-----------------------------------------------------------------------
/**
* Return true if any Iterator in the IteratorChain has a remaining element.
*
* @return true if elements remain
*/
@Override
public boolean hasNext(){
lockChain();
updateCurrentIterator();
lastUsedIterator = currentIterator;
return currentIterator.hasNext();
}
/**
* Returns the next Object of the current Iterator
*
* @return Object from the current Iterator
* @throws java.util.NoSuchElementException
* if all the Iterators are
* exhausted
*/
@Override
public E next(){
lockChain();
updateCurrentIterator();
lastUsedIterator = currentIterator;
return currentIterator.next();
}
/**
* Removes from the underlying collection the last element returned by the
* Iterator. As with next() and hasNext(), this method calls remove() on the
* underlying Iterator. Therefore, this method 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.
*/
@Override
public void remove(){
lockChain();
if (currentIterator == null){
updateCurrentIterator();
}
lastUsedIterator.remove();
}
}