    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.NoSuchElementException;
    020
    021import org.apache.commons.collections4.ResettableIterator;
    022
    023/**
    024 * <code>SingletonIterator</code> is an {@link java.util.Iterator} over a single
    025 * object instance.
    026 *
    027 * @since 2.0
    028 */
    029public class SingletonIterator<E>
    030        implements ResettableIterator<E> {
    031
    032    /** Whether remove is allowed */
    033    private final boolean removeAllowed;
    034    /** Is the cursor before the first element */
    035    private boolean beforeFirst = true;
    036    /** Has the element been removed */
    037    private boolean removed = false;
    038    /** The object */
    039    private E object;
    040
    041    /**
    042     * Constructs a new <code>SingletonIterator</code> where <code>remove</code>
    043     * is a permitted operation.
    044     *
    045     * @param object  the single object to return from the iterator
    046     */
    047    public SingletonIterator(final E object) {
    048        this(object, true);
    049    }
    050
    051    /**
    052     * Constructs a new <code>SingletonIterator</code> optionally choosing if
    053     * <code>remove</code> is a permitted operation.
    054     *
    055     * @param object  the single object to return from the iterator
    056     * @param removeAllowed  true if remove is allowed
    057     * @since 3.1
    058     */
    059    public SingletonIterator(final E object, final boolean removeAllowed) {
    060        super();
    061        this.object = object;
    062        this.removeAllowed = removeAllowed;
    063    }
    064
    065    //-----------------------------------------------------------------------
    066    /**
    067     * Is another object available from the iterator?
    068     * <p>
    069     * This returns true if the single object hasn't been returned yet.
    070     *
    071     * @return true if the single object hasn't been returned yet
    072     */
    073    @Override
    074    public boolean hasNext() {
    075        return beforeFirst && !removed;
    076    }
    077
    078    /**
    079     * Get the next object from the iterator.
    080     * <p>
    081     * This returns the single object if it hasn't been returned yet.
    082     *
    083     * @return the single object
    084     * @throws NoSuchElementException if the single object has already
    085     *    been returned
    086     */
    087    @Override
    088    public E next() {
    089        if (!beforeFirst || removed) {
    090            throw new NoSuchElementException();
    091        }
    092        beforeFirst = false;
    093        return object;
    094    }
    095
    096    /**
    097     * Remove the object from this iterator.
    098     *
    099     * @throws IllegalStateException if the {@code next} method has not
    100     *        yet been called, or the {@code remove} method has already
    101     *        been called after the last call to the {@code next}
    102     *        method.
    103     * @throws UnsupportedOperationException if remove is not supported
    104     */
    105    @Override
    106    public void remove() {
    107        if (removeAllowed) {
    108            if (removed || beforeFirst) {
    109                throw new IllegalStateException();
    110            }
    111            object = null;
    112            removed = true;
    113        } else {
    114            throw new UnsupportedOperationException();
    115        }
    116    }
    117
    118    /**
    119     * Reset the iterator to the start.
    120     */
    121    @Override
    122    public void reset() {
    123        beforeFirst = true;
    124    }
    125
    126}