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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.Iterator;
import java.util.NoSuchElementException;
/**
* Decorates an iterator to support one-element lookahead while iterating.
*
* The decorator supports the removal operation, but an {@link IllegalStateException}
* will be thrown if {@link #remove()} is called directly after a call to
* {@link #peek()} or {@link #element()}.
*
* @since 4.0
*/
public class PeekingIterator implements Iterator {
/** The iterator being decorated. */
private final Iterator iterator;
/** Indicates that the decorated iterator is exhausted. */
private boolean exhausted = false;
/** Indicates if the lookahead slot is filled. */
private boolean slotFilled = false;
/** The current slot for lookahead. */
private E slot;
//-----------------------------------------------------------------------
/**
* Decorates the specified iterator to support one-element lookahead.
*
* If the iterator is already a {@link PeekingIterator} 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 PeekingIterator peekingIterator(final Iterator iterator) {
if (iterator == null) {
throw new NullPointerException("Iterator must not be null");
}
if (iterator instanceof PeekingIterator) {
@SuppressWarnings("unchecked") // safe cast
final PeekingIterator it = (PeekingIterator) iterator;
return it;
}
return new PeekingIterator<>(iterator);
}
//-----------------------------------------------------------------------
/**
* Constructor.
*
* @param iterator the iterator to decorate
*/
public PeekingIterator(final Iterator iterator) {
this.iterator = iterator;
}
private void fill() {
if (exhausted || slotFilled) {
return;
}
if (iterator.hasNext()) {
slot = iterator.next();
slotFilled = true;
} else {
exhausted = true;
slot = null;
slotFilled = false;
}
}
//-----------------------------------------------------------------------
@Override
public boolean hasNext() {
if (exhausted) {
return false;
}
return slotFilled || iterator.hasNext();
}
/**
* Returns the next element in iteration without advancing the underlying iterator.
* If the iterator is already exhausted, null will be returned.
*
* Note: this method does not throw a {@link NoSuchElementException} if the iterator
* is already exhausted. If you want such a behavior, use {@link #element()} instead.
*
* The rationale behind this is to follow the {@link java.util.Queue} interface
* which uses the same terminology.
*
* @return the next element from the iterator
*/
public E peek() {
fill();
return exhausted ? null : slot;
}
/**
* Returns the next element in iteration without advancing the underlying iterator.
* If the iterator is already exhausted, null will be returned.
*
* @return the next element from the iterator
* @throws NoSuchElementException if the iterator is already exhausted according to {@link #hasNext()}
*/
public E element() {
fill();
if (exhausted) {
throw new NoSuchElementException();
}
return slot;
}
@Override
public E next() {
if (!hasNext()) {
throw new NoSuchElementException();
}
final E x = slotFilled ? slot : iterator.next();
// reset the lookahead slot
slot = null;
slotFilled = false;
return x;
}
/**
* {@inheritDoc}
*
* @throws IllegalStateException if {@link #peek()} or {@link #element()} has been called
* prior to the call to {@link #remove()}
*/
@Override
public void remove() {
if (slotFilled) {
throw new IllegalStateException("peek() or element() called before remove()");
}
iterator.remove();
}
}