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 *      https://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.ListIterator;
020import java.util.NoSuchElementException;
021
022import org.apache.commons.collections4.ResettableListIterator;
023
024/**
025 * {@code SingletonIterator} is an {@link ListIterator} over a single
026 * object instance.
027 *
028 * @param <E> The type of elements returned by this iterator.
029 * @since 2.1
030 */
031public class SingletonListIterator<E> implements ResettableListIterator<E> {
032
033    private boolean beforeFirst = true;
034    private boolean nextCalled;
035    private boolean removed;
036    private E object;
037
038    /**
039     * Constructs a new {@code SingletonListIterator}.
040     *
041     * @param object  The single object to return from the iterator
042     */
043    public SingletonListIterator(final E object) {
044        this.object = object;
045    }
046
047    /**
048     * Always throws {@link UnsupportedOperationException}.
049     *
050     * @param obj Ignored.
051     * @throws UnsupportedOperationException Always thrown.
052     */
053    @Override
054    public void add(final E obj) {
055        throw new UnsupportedOperationException("add() is not supported by this iterator");
056    }
057
058    /**
059     * Is another object available from the iterator?
060     * <p>
061     * This returns true if the single object hasn't been returned yet.
062     *
063     * @return true if the single object hasn't been returned yet
064     */
065    @Override
066    public boolean hasNext() {
067        return beforeFirst && !removed;
068    }
069
070    /**
071     * Is a previous object available from the iterator?
072     * <p>
073     * This returns true if the single object has been returned.
074     *
075     * @return true if the single object has been returned
076     */
077    @Override
078    public boolean hasPrevious() {
079        return !beforeFirst && !removed;
080    }
081
082    /**
083     * Gets the next object from the iterator.
084     * <p>
085     * This returns the single object if it hasn't been returned yet.
086     *
087     * @return The single object
088     * @throws NoSuchElementException if the single object has already
089     *    been returned
090     */
091    @Override
092    public E next() {
093        if (!beforeFirst || removed) {
094            throw new NoSuchElementException();
095        }
096        beforeFirst = false;
097        nextCalled = true;
098        return object;
099    }
100
101    /**
102     * Returns the index of the element that would be returned by a subsequent
103     * call to {@code next}.
104     *
105     * @return 0 or 1 depending on current state.
106     */
107    @Override
108    public int nextIndex() {
109        return beforeFirst ? 0 : 1;
110    }
111
112    /**
113     * Gets the previous object from the iterator.
114     * <p>
115     * This returns the single object if it has been returned.
116     *
117     * @return The single object
118     * @throws NoSuchElementException if the single object has not already
119     *    been returned
120     */
121    @Override
122    public E previous() {
123        if (beforeFirst || removed) {
124            throw new NoSuchElementException();
125        }
126        beforeFirst = true;
127        return object;
128    }
129
130    /**
131     * Returns the index of the element that would be returned by a subsequent
132     * call to {@code previous}. A return value of -1 indicates that the iterator is currently at
133     * the start.
134     *
135     * @return 0 or -1 depending on current state.
136     */
137    @Override
138    public int previousIndex() {
139        return beforeFirst ? -1 : 0;
140    }
141
142    /**
143     * Remove the object from this iterator.
144     *
145     * @throws IllegalStateException if the {@code next} or {@code previous}
146     *        method has not yet been called, or the {@code remove} method
147     *        has already been called after the last call to {@code next}
148     *        or {@code previous}.
149     */
150    @Override
151    public void remove() {
152        if (!nextCalled || removed) {
153            throw new IllegalStateException();
154        }
155        object = null;
156        removed = true;
157    }
158
159    /**
160     * Reset the iterator back to the start.
161     */
162    @Override
163    public void reset() {
164        beforeFirst = true;
165        nextCalled = false;
166    }
167
168    /**
169     * Sets sets the value of the singleton.
170     *
171     * @param object  The object to set
172     * @throws IllegalStateException if {@code next} has not been called
173     *          or the object has been removed
174     */
175    @Override
176    public void set(final E object) {
177        if (!nextCalled || removed) {
178            throw new IllegalStateException();
179        }
180        this.object = object;
181    }
182
183}