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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.ArrayDeque;
import java.util.Deque;
import java.util.Iterator;
/**
* Decorates an iterator to support pushback of elements.
*
* The decorator stores the pushed back elements in a LIFO manner: the last element
* that has been pushed back, will be returned as the next element in a call to {@link #next()}.
*
* The decorator does not support the removal operation. Any call to {@link #remove()} will
* result in an {@link UnsupportedOperationException}.
*
* @since 4.0
*/
public class PushbackIterator implements Iterator {
/** The iterator being decorated. */
private final Iterator iterator;
/** The LIFO queue containing the pushed back items. */
private final Deque items = new ArrayDeque<>();
//-----------------------------------------------------------------------
/**
* Decorates the specified iterator to support one-element lookahead.
*
* If the iterator is already a {@link PushbackIterator} it is returned directly.
*
* @param the element type
* @param iterator the iterator to decorate
* @return a new peeking iterator
* @throws NullPointerException if the iterator is null
*/
public static PushbackIterator pushbackIterator(final Iterator iterator) {
if (iterator == null) {
throw new NullPointerException("Iterator must not be null");
}
if (iterator instanceof PushbackIterator) {
@SuppressWarnings("unchecked") // safe cast
final PushbackIterator it = (PushbackIterator) iterator;
return it;
}
return new PushbackIterator<>(iterator);
}
//-----------------------------------------------------------------------
/**
* Constructor.
*
* @param iterator the iterator to decorate
*/
public PushbackIterator(final Iterator iterator) {
super();
this.iterator = iterator;
}
/**
* Push back the given element to the iterator.
*
* Calling {@link #next()} immediately afterwards will return exactly this element.
*
* @param item the element to push back to the iterator
*/
public void pushback(final E item) {
items.push(item);
}
@Override
public boolean hasNext() {
return !items.isEmpty() || iterator.hasNext();
}
@Override
public E next() {
return !items.isEmpty() ? items.pop() : iterator.next();
}
/**
* This iterator will always throw an {@link UnsupportedOperationException}.
*
* @throws UnsupportedOperationException always
*/
@Override
public void remove() {
throw new UnsupportedOperationException();
}
}