    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 * <p>
    032 * In case an offset parameter other than 0 is provided, the decorated
    033 * iterator is immediately advanced to this position, skipping all elements
    034 * before that position.
    035 * </p>
    036 *
    037 * @param <E> the type of elements returned by this iterator.
    038 * @since 4.1
    039 */
    040public class BoundedIterator<E> implements Iterator<E> {
    041
    042    /** The iterator being decorated. */
    043    private final Iterator<? extends E> iterator;
    044
    045    /** The offset to bound the first element return */
    046    private final long offset;
    047
    048    /** The max number of elements to return */
    049    private final long max;
    050
    051    /** The position of the current element */
    052    private long pos;
    053
    054    /**
    055     * Decorates the specified iterator to return at most the given number of elements,
    056     * skipping all elements until the iterator reaches the position at {@code offset}.
    057     * <p>
    058     * The iterator is immediately advanced until it reaches the position at {@code offset},
    059     * incurring O(n) time.
    060     * </p>
    061     *
    062     * @param iterator  the iterator to be decorated
    063     * @param offset  the index of the first element of the decorated iterator to return
    064     * @param max  the maximum number of elements of the decorated iterator to return
    065     * @throws NullPointerException if iterator is null
    066     * @throws IllegalArgumentException if either offset or max is negative
    067     */
    068    public BoundedIterator(final Iterator<? extends E> iterator, final long offset, final long max) {
    069        if (offset < 0) {
    070            throw new IllegalArgumentException("Offset parameter must not be negative.");
    071        }
    072        if (max < 0) {
    073            throw new IllegalArgumentException("Max parameter must not be negative.");
    074        }
    075
    076        this.iterator = Objects.requireNonNull(iterator, "iterator");
    077        this.offset = offset;
    078        this.max = max;
    079        pos = 0;
    080        init();
    081    }
    082
    083    /**
    084     * Checks whether the iterator is still within its bounded range.
    085     * @return {@code true} if the iterator is within its bounds, {@code false} otherwise
    086     */
    087    private boolean checkBounds() {
    088        if (pos - offset + 1 > max) {
    089            return false;
    090        }
    091        return true;
    092    }
    093
    094    @Override
    095    public boolean hasNext() {
    096        if (!checkBounds()) {
    097            return false;
    098        }
    099        return iterator.hasNext();
    100    }
    101
    102    /**
    103     * Advances the underlying iterator to the beginning of the bounded range.
    104     */
    105    private void init() {
    106        while (pos < offset && iterator.hasNext()) {
    107            iterator.next();
    108            pos++;
    109        }
    110    }
    111
    112    @Override
    113    public E next() {
    114        if (!checkBounds()) {
    115            throw new NoSuchElementException();
    116        }
    117        final E next = iterator.next();
    118        pos++;
    119        return next;
    120    }
    121
    122    /**
    123     * {@inheritDoc}
    124     * <p>
    125     * In case an offset other than 0 was specified, the underlying iterator will be advanced
    126     * to this position upon creation. A call to {@link #remove()} will still result in an
    127     * {@link IllegalStateException} if no explicit call to {@link #next()} has been made prior
    128     * to calling {@link #remove()}.
    129     * </p>
    130     */
    131    @Override
    132    public void remove() {
    133        if (pos <= offset) {
    134            throw new IllegalStateException("remove() cannot be called before calling next()");
    135        }
    136        iterator.remove();
    137    }
    138}