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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.collections4.iterators;
import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
import java.util.NoSuchElementException;
import org.apache.commons.collections4.FluentIterable;
/**
* Provides an interleaved iteration over the elements contained in a
* collection of Iterators.
*
* Given two {@link Iterator} instances {@code A} and {@code B}, the
* {@link #next} method on this iterator will switch between {@code A.next()}
* and {@code B.next()} until both iterators are exhausted.
*
* @since 4.1
*/
public class ZippingIterator implements Iterator {
/** The {@link Iterator}s to evaluate. */
private final Iterator> iterators;
/** The next iterator to use for next(). */
private Iterator nextIterator = null;
/** The last iterator which was used for next(). */
private Iterator lastReturned = null;
// Constructors
// ----------------------------------------------------------------------
/**
* Constructs a new ZippingIterator that will provide
* interleaved iteration over the two given iterators.
*
* @param a the first child iterator
* @param b the second child iterator
* @throws NullPointerException if either iterator is null
*/
@SuppressWarnings("unchecked")
public ZippingIterator(final Iterator a, final Iterator b) {
this(new Iterator[] {a, b});
}
/**
* Constructs a new ZippingIterator that will provide
* interleaved iteration over the three given iterators.
*
* @param a the first child iterator
* @param b the second child iterator
* @param c the third child iterator
* @throws NullPointerException if either iterator is null
*/
@SuppressWarnings("unchecked")
public ZippingIterator(final Iterator a,
final Iterator b,
final Iterator c) {
this(new Iterator[] {a, b, c});
}
/**
* Constructs a new ZippingIterator that will provide
* interleaved iteration of the specified iterators.
*
* @param iterators the array of iterators
* @throws NullPointerException if any iterator is null
*/
public ZippingIterator(final Iterator... iterators) {
// create a mutable list to be able to remove exhausted iterators
final List> list = new ArrayList<>();
for (final Iterator iterator : iterators) {
if (iterator == null) {
throw new NullPointerException("Iterator must not be null.");
}
list.add(iterator);
}
this.iterators = FluentIterable.of(list).loop().iterator();
}
// Iterator Methods
// -------------------------------------------------------------------
/**
* Returns {@code true} if any child iterator has remaining elements.
*
* @return true if this iterator has remaining elements
*/
@Override
public boolean hasNext() {
// the next iterator has already been determined
// this might happen if hasNext() is called multiple
if (nextIterator != null) {
return true;
}
while(iterators.hasNext()) {
final Iterator childIterator = iterators.next();
if (childIterator.hasNext()) {
nextIterator = childIterator;
return true;
}
// iterator is exhausted, remove it
iterators.remove();
}
return false;
}
/**
* Returns the next element from a child iterator.
*
* @return the next interleaved element
* @throws NoSuchElementException if no child iterator has any more elements
*/
@Override
public E next() throws NoSuchElementException {
if (!hasNext()) {
throw new NoSuchElementException();
}
final E val = nextIterator.next();
lastReturned = nextIterator;
nextIterator = null;
return val;
}
/**
* Removes the last returned element from the child iterator that produced it.
*
* @throws IllegalStateException if there is no last returned element, or if
* the last returned element has already been removed
*/
@Override
public void remove() {
if (lastReturned == null) {
throw new IllegalStateException("No value can be removed at present");
}
lastReturned.remove();
lastReturned = null;
}
}