    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.Iterator;
    020import java.util.NoSuchElementException;
    021
    022import org.apache.commons.collections4.ResettableIterator;
    023
    024/**
    025 * {@code SingletonIterator} is an {@link Iterator} over a single
    026 * object instance.
    027 *
    028 * @param <E> The type of elements returned by this iterator.
    029 * @since 2.0
    030 */
    031public class SingletonIterator<E>
    032        implements ResettableIterator<E> {
    033
    034    /** Whether remove is allowed */
    035    private final boolean removeAllowed;
    036
    037    /** Is the cursor before the first element */
    038    private boolean beforeFirst = true;
    039
    040    /** Has the element been removed */
    041    private boolean removed;
    042
    043    /** The object */
    044    private E object;
    045
    046    /**
    047     * Constructs a new {@code SingletonIterator} where {@code remove}
    048     * is a permitted operation.
    049     *
    050     * @param object  The single object to return from the iterator
    051     */
    052    public SingletonIterator(final E object) {
    053        this(object, true);
    054    }
    055
    056    /**
    057     * Constructs a new {@code SingletonIterator} optionally choosing if
    058     * {@code remove} is a permitted operation.
    059     *
    060     * @param object  The single object to return from the iterator
    061     * @param removeAllowed  true if remove is allowed
    062     * @since 3.1
    063     */
    064    public SingletonIterator(final E object, final boolean removeAllowed) {
    065        this.object = object;
    066        this.removeAllowed = removeAllowed;
    067    }
    068
    069    /**
    070     * Is another object available from the iterator?
    071     * <p>
    072     * This returns true if the single object hasn't been returned yet.
    073     *
    074     * @return true if the single object hasn't been returned yet
    075     */
    076    @Override
    077    public boolean hasNext() {
    078        return beforeFirst && !removed;
    079    }
    080
    081    /**
    082     * Gets the next object from the iterator.
    083     * <p>
    084     * This returns the single object if it hasn't been returned yet.
    085     *
    086     * @return The single object
    087     * @throws NoSuchElementException if the single object has already
    088     *    been returned
    089     */
    090    @Override
    091    public E next() {
    092        if (!beforeFirst || removed) {
    093            throw new NoSuchElementException();
    094        }
    095        beforeFirst = false;
    096        return object;
    097    }
    098
    099    /**
    100     * Remove the object from this iterator.
    101     *
    102     * @throws IllegalStateException if the {@code next} method has not
    103     *        yet been called, or the {@code remove} method has already
    104     *        been called after the last call to the {@code next}
    105     *        method.
    106     * @throws UnsupportedOperationException if remove is not supported
    107     */
    108    @Override
    109    public void remove() {
    110        if (!removeAllowed) {
    111            throw new UnsupportedOperationException();
    112        }
    113        if (removed || beforeFirst) {
    114            throw new IllegalStateException();
    115        }
    116        object = null;
    117        removed = true;
    118    }
    119
    120    /**
    121     * Reset the iterator to the start.
    122     */
    123    @Override
    124    public void reset() {
    125        beforeFirst = true;
    126    }
    127
    128}