    001/*
    002 * Licensed to the Apache Software Foundation (ASF) under one or more
    003 * contributor license agreements. See the NOTICE file distributed with this
    004 * work for additional information regarding copyright ownership. The ASF
    005 * licenses this file to You under the Apache License, Version 2.0 (the
    006 * "License"); you may not use this file except in compliance with the License.
    007 * You may obtain a copy of the License at
    008 * http://www.apache.org/licenses/LICENSE-2.0 Unless required by applicable law
    009 * or agreed to in writing, software distributed under the License is
    010 * distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
    011 * KIND, either express or implied. See the License for the specific language
    012 * governing permissions and limitations under the License.
    013 */
    014package org.apache.commons.collections4.iterators;
    015
    016import java.util.Iterator;
    017import java.util.NoSuchElementException;
    018
    019/**
    020 * Decorates another iterator to return elements in a specific range.
    021 * <p>
    022 * The decorated iterator is bounded in the range [offset, offset+max).
    023 * The {@code offset} corresponds to the position of the first element to
    024 * be returned from the decorated iterator, and {@code max} is the maximum
    025 * number of elements to be returned at most.
    026 * <p>
    027 * In case an offset parameter other than 0 is provided, the decorated
    028 * iterator is immediately advanced to this position, skipping all elements
    029 * before that position.
    030 *
    031 * @since 4.1
    032 */
    033public class BoundedIterator<E> implements Iterator<E> {
    034
    035    /** The iterator being decorated. */
    036    private final Iterator<? extends E> iterator;
    037
    038    /** The offset to bound the first element return */
    039    private final long offset;
    040
    041    /** The max number of elements to return */
    042    private final long max;
    043
    044    /** The position of the current element */
    045    private long pos;
    046
    047    //-----------------------------------------------------------------------
    048
    049    /**
    050     * Decorates the specified iterator to return at most the given number of elements,
    051     * skipping all elements until the iterator reaches the position at {@code offset}.
    052     * <p>
    053     * The iterator is immediately advanced until it reaches the position at {@code offset},
    054     * incurring O(n) time.
    055     *
    056     * @param iterator  the iterator to be decorated
    057     * @param offset  the index of the first element of the decorated iterator to return
    058     * @param max  the maximum number of elements of the decorated iterator to return
    059     * @throws NullPointerException if iterator is null
    060     * @throws IllegalArgumentException if either offset or max is negative
    061     */
    062    public BoundedIterator(final Iterator<? extends E> iterator, final long offset, final long max) {
    063        if (iterator == null) {
    064            throw new NullPointerException("Iterator must not be null");
    065        }
    066        if (offset < 0) {
    067            throw new IllegalArgumentException("Offset parameter must not be negative.");
    068        }
    069        if (max < 0) {
    070            throw new IllegalArgumentException("Max parameter must not be negative.");
    071        }
    072
    073        this.iterator = iterator;
    074        this.offset = offset;
    075        this.max = max;
    076        pos = 0;
    077        init();
    078    }
    079
    080    /**
    081     * Advances the underlying iterator to the beginning of the bounded range.
    082     */
    083    private void init() {
    084        while (pos < offset && iterator.hasNext()) {
    085            iterator.next();
    086            pos++;
    087        }
    088    }
    089
    090    //-----------------------------------------------------------------------
    091
    092    @Override
    093    public boolean hasNext() {
    094        if (!checkBounds()) {
    095            return false;
    096        }
    097        return iterator.hasNext();
    098    }
    099
    100    /**
    101     * Checks whether the iterator is still within its bounded range.
    102     * @return {@code true} if the iterator is within its bounds, {@code false} otherwise
    103     */
    104    private boolean checkBounds() {
    105        if (pos - offset + 1 > max) {
    106            return false;
    107        }
    108        return true;
    109    }
    110
    111    @Override
    112    public E next() {
    113        if (!checkBounds()) {
    114            throw new NoSuchElementException();
    115        }
    116        final E next = iterator.next();
    117        pos++;
    118        return next;
    119    }
    120
    121    /**
    122     * {@inheritDoc}
    123     * <p>
    124     * In case an offset other than 0 was specified, the underlying iterator will be advanced
    125     * to this position upon creation. A call to {@link #remove()} will still result in an
    126     * {@link IllegalStateException} if no explicit call to {@link #next()} has been made prior
    127     * to calling {@link #remove()}.
    128     */
    129    @Override
    130    public void remove() {
    131        if (pos <= offset) {
    132            throw new IllegalStateException("remove() can not be called before calling next()");
    133        }
    134        iterator.remove();
    135    }
    136}