001/*
002 * Licensed to the Apache Software Foundation (ASF) under one or more
003 * contributor license agreements.  See the NOTICE file distributed with
004 * this work for additional information regarding copyright ownership.
005 * The ASF licenses this file to You under the Apache License, Version 2.0
006 * (the "License"); you may not use this file except in compliance with
007 * the License.  You may obtain a copy of the License at
008 *
009 *      http://www.apache.org/licenses/LICENSE-2.0
010 *
011 * Unless required by applicable law or agreed to in writing, software
012 * distributed under the License is distributed on an "AS IS" BASIS,
013 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
014 * See the License for the specific language governing permissions and
015 * limitations under the License.
016 */
017package org.apache.commons.collections4.iterators;
018
019import java.util.Iterator;
020import java.util.NoSuchElementException;
021import java.util.Objects;
022
023/**
024 * Decorates an iterator to support one-element lookahead while iterating.
025 * <p>
026 * The decorator supports the removal operation, but an {@link IllegalStateException}
027 * will be thrown if {@link #remove()} is called directly after a call to
028 * {@link #peek()} or {@link #element()}.
029 *
030 * @param <E> the type of elements returned by this iterator.
031 * @since 4.0
032 */
033public class PeekingIterator<E> implements Iterator<E> {
034
035    /**
036     * Decorates the specified iterator to support one-element lookahead.
037     * <p>
038     * If the iterator is already a {@link PeekingIterator} it is returned directly.
039     *
040     * @param <E>  the element type
041     * @param iterator  the iterator to decorate
042     * @return a new peeking iterator
043     * @throws NullPointerException if the iterator is null
044     */
045    public static <E> PeekingIterator<E> peekingIterator(final Iterator<? extends E> iterator) {
046        Objects.requireNonNull(iterator, "iterator");
047        if (iterator instanceof PeekingIterator<?>) {
048            @SuppressWarnings("unchecked") // safe cast
049            final PeekingIterator<E> it = (PeekingIterator<E>) iterator;
050            return it;
051        }
052        return new PeekingIterator<>(iterator);
053    }
054
055    /** The iterator being decorated. */
056    private final Iterator<? extends E> iterator;
057
058    /** Indicates that the decorated iterator is exhausted. */
059    private boolean exhausted;
060
061    /** Indicates if the lookahead slot is filled. */
062    private boolean slotFilled;
063
064    /** The current slot for lookahead. */
065    private E slot;
066
067    /**
068     * Constructs a new instance.
069     *
070     * @param iterator  the iterator to decorate
071     */
072    public PeekingIterator(final Iterator<? extends E> iterator) {
073        this.iterator = iterator;
074    }
075
076    /**
077     * Returns the next element in iteration without advancing the underlying iterator.
078     * If the iterator is already exhausted, null will be returned.
079     *
080     * @return the next element from the iterator
081     * @throws NoSuchElementException if the iterator is already exhausted according to {@link #hasNext()}
082     */
083    public E element() {
084        fill();
085        if (exhausted) {
086            throw new NoSuchElementException();
087        }
088        return slot;
089    }
090
091    private void fill() {
092        if (exhausted || slotFilled) {
093            return;
094        }
095        if (iterator.hasNext()) {
096            slot = iterator.next();
097            slotFilled = true;
098        } else {
099            exhausted = true;
100            slot = null;
101            slotFilled = false;
102        }
103    }
104
105    @Override
106    public boolean hasNext() {
107        if (exhausted) {
108            return false;
109        }
110        return slotFilled || iterator.hasNext();
111    }
112
113    @Override
114    public E next() {
115        if (!hasNext()) {
116            throw new NoSuchElementException();
117        }
118        final E x = slotFilled ? slot : iterator.next();
119        // reset the lookahead slot
120        slot = null;
121        slotFilled = false;
122        return x;
123    }
124
125    /**
126     * Returns the next element in iteration without advancing the underlying iterator.
127     * If the iterator is already exhausted, null will be returned.
128     * <p>
129     * Note: this method does not throw a {@link NoSuchElementException} if the iterator
130     * is already exhausted. If you want such a behavior, use {@link #element()} instead.
131     * <p>
132     * The rationale behind this is to follow the {@link java.util.Queue} interface
133     * which uses the same terminology.
134     *
135     * @return the next element from the iterator
136     */
137    public E peek() {
138        fill();
139        return exhausted ? null : slot;
140    }
141
142    /**
143     * {@inheritDoc}
144     *
145     * @throws IllegalStateException if {@link #peek()} or {@link #element()} has been called
146     *   prior to the call to {@link #remove()}
147     */
148    @Override
149    public void remove() {
150        if (slotFilled) {
151            throw new IllegalStateException("peek() or element() called before remove()");
152        }
153        iterator.remove();
154    }
155
156}