    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;
    020
    021/**
    022 * Decorates another iterator to skip the first N elements.
    023 * <p>
    024 * In case an offset parameter other than 0 is provided, the decorated
    025 * iterator is immediately advanced to this position, skipping all elements
    026 * before that position.
    027 * </p>
    028 *
    029 * @param <E> the type of elements returned by this iterator.
    030 * @since 4.1
    031 */
    032public class SkippingIterator<E> extends AbstractIteratorDecorator<E> {
    033
    034    /** The offset to bound the first element return */
    035    private final long offset;
    036
    037    /** The position of the current element */
    038    private long pos;
    039
    040    /**
    041     * Decorates the specified iterator to skip all elements until the iterator
    042     * reaches the position at {@code offset}.
    043     * <p>
    044     * The iterator is immediately advanced until it reaches the position at {@code offset},
    045     * incurring O(n) time.
    046     *
    047     * @param iterator  the iterator to be decorated
    048     * @param offset  the index of the first element of the decorated iterator to return
    049     * @throws NullPointerException if iterator is null
    050     * @throws IllegalArgumentException if offset is negative
    051     */
    052    public SkippingIterator(final Iterator<E> iterator, final long offset) {
    053        super(iterator);
    054
    055        if (offset < 0) {
    056            throw new IllegalArgumentException("Offset parameter must not be negative.");
    057        }
    058
    059        this.offset = offset;
    060        this.pos = 0;
    061        init();
    062    }
    063
    064    /**
    065     * Skips the given number of elements.
    066     */
    067    private void init() {
    068        while (pos < offset && hasNext()) {
    069            next();
    070        }
    071    }
    072
    073    @Override
    074    public E next() {
    075        final E next = super.next();
    076        pos++;
    077        return next;
    078    }
    079
    080    /**
    081     * {@inheritDoc}
    082     * <p>
    083     * In case an offset other than 0 was specified, the underlying iterator will be advanced
    084     * to this position upon creation. A call to {@link #remove()} will still result in an
    085     * {@link IllegalStateException} if no explicit call to {@link #next()} has been made prior
    086     * to calling {@link #remove()}.
    087     */
    088    @Override
    089    public void remove() {
    090        if (pos <= offset) {
    091            throw new IllegalStateException("remove() can not be called before calling next()");
    092        }
    093        super.remove();
    094    }
    095
    096}