    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 another iterator to return elements in a specific range.
    025 * <p>
    026 * The decorated iterator is bounded in the range [offset, offset+max).
    027 * The {@code offset} corresponds to the position of the first element to
    028 * be returned from the decorated iterator, and {@code max} is the maximum
    029 * number of elements to be returned at most.
    030 * <p>
    031 * In case an offset parameter other than 0 is provided, the decorated
    032 * iterator is immediately advanced to this position, skipping all elements
    033 * before that position.
    034 *
    035 * @param <E> the type of elements returned by this iterator.
    036 * @since 4.1
    037 */
    038public class BoundedIterator<E> implements Iterator<E> {
    039
    040    /** The iterator being decorated. */
    041    private final Iterator<? extends E> iterator;
    042
    043    /** The offset to bound the first element return */
    044    private final long offset;
    045
    046    /** The max number of elements to return */
    047    private final long max;
    048
    049    /** The position of the current element */
    050    private long pos;
    051
    052    /**
    053     * Decorates the specified iterator to return at most the given number of elements,
    054     * skipping all elements until the iterator reaches the position at {@code offset}.
    055     * <p>
    056     * The iterator is immediately advanced until it reaches the position at {@code offset},
    057     * incurring O(n) time.
    058     *
    059     * @param iterator  the iterator to be decorated
    060     * @param offset  the index of the first element of the decorated iterator to return
    061     * @param max  the maximum number of elements of the decorated iterator to return
    062     * @throws NullPointerException if iterator is null
    063     * @throws IllegalArgumentException if either offset or max is negative
    064     */
    065    public BoundedIterator(final Iterator<? extends E> iterator, final long offset, final long max) {
    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 = Objects.requireNonNull(iterator, "iterator");
    074        this.offset = offset;
    075        this.max = max;
    076        pos = 0;
    077        init();
    078    }
    079
    080    /**
    081     * Checks whether the iterator is still within its bounded range.
    082     * @return {@code true} if the iterator is within its bounds, {@code false} otherwise
    083     */
    084    private boolean checkBounds() {
    085        if (pos - offset + 1 > max) {
    086            return false;
    087        }
    088        return true;
    089    }
    090
    091    @Override
    092    public boolean hasNext() {
    093        if (!checkBounds()) {
    094            return false;
    095        }
    096        return iterator.hasNext();
    097    }
    098
    099    /**
    100     * Advances the underlying iterator to the beginning of the bounded range.
    101     */
    102    private void init() {
    103        while (pos < offset && iterator.hasNext()) {
    104            iterator.next();
    105            pos++;
    106        }
    107    }
    108
    109    @Override
    110    public E next() {
    111        if (!checkBounds()) {
    112            throw new NoSuchElementException();
    113        }
    114        final E next = iterator.next();
    115        pos++;
    116        return next;
    117    }
    118
    119    /**
    120     * {@inheritDoc}
    121     * <p>
    122     * In case an offset other than 0 was specified, the underlying iterator will be advanced
    123     * to this position upon creation. A call to {@link #remove()} will still result in an
    124     * {@link IllegalStateException} if no explicit call to {@link #next()} has been made prior
    125     * to calling {@link #remove()}.
    126     */
    127    @Override
    128    public void remove() {
    129        if (pos <= offset) {
    130            throw new IllegalStateException("remove() can not be called before calling next()");
    131        }
    132        iterator.remove();
    133    }
    134}