    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;
    017
    018/**
    019 * Decorates another iterator to skip the first N elements.
    020 * <p>
    021 * In case an offset parameter other than 0 is provided, the decorated
    022 * iterator is immediately advanced to this position, skipping all elements
    023 * before that position.
    024 *
    025 * @since 4.1
    026 */
    027public class SkippingIterator<E> extends AbstractIteratorDecorator<E> {
    028
    029    /** The offset to bound the first element return */
    030    private final long offset;
    031
    032    /** The position of the current element */
    033    private long pos;
    034
    035    //-----------------------------------------------------------------------
    036
    037    /**
    038     * Decorates the specified iterator to skip all elements until the iterator
    039     * reaches the position at {@code offset}.
    040     * <p>
    041     * The iterator is immediately advanced until it reaches the position at {@code offset},
    042     * incurring O(n) time.
    043     *
    044     * @param iterator  the iterator to be decorated
    045     * @param offset  the index of the first element of the decorated iterator to return
    046     * @throws NullPointerException if iterator is null
    047     * @throws IllegalArgumentException if offset is negative
    048     */
    049    public SkippingIterator(final Iterator<E> iterator, final long offset) {
    050        super(iterator);
    051
    052        if (offset < 0) {
    053            throw new IllegalArgumentException("Offset parameter must not be negative.");
    054        }
    055
    056        this.offset = offset;
    057        this.pos = 0;
    058        init();
    059    }
    060
    061    /**
    062     * Skips the given number of elements.
    063     */
    064    private void init() {
    065        while (pos < offset && hasNext()) {
    066            next();
    067        }
    068    }
    069
    070    //-----------------------------------------------------------------------
    071
    072    @Override
    073    public E next() {
    074        final E next = super.next();
    075        pos++;
    076        return next;
    077    }
    078
    079    /**
    080     * {@inheritDoc}
    081     * <p>
    082     * In case an offset other than 0 was specified, the underlying iterator will be advanced
    083     * to this position upon creation. A call to {@link #remove()} will still result in an
    084     * {@link IllegalStateException} if no explicit call to {@link #next()} has been made prior
    085     * to calling {@link #remove()}.
    086     */
    087    @Override
    088    public void remove() {
    089        if (pos <= offset) {
    090            throw new IllegalStateException("remove() can not be called before calling next()");
    091        }
    092        super.remove();
    093    }
    094
    095}