    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 * </p>
    025 *
    026 * @since 4.1
    027 */
    028public class SkippingIterator<E> extends AbstractIteratorDecorator<E> {
    029
    030    /** The offset to bound the first element return */
    031    private final long offset;
    032
    033    /** The position of the current element */
    034    private long pos;
    035
    036    //-----------------------------------------------------------------------
    037
    038    /**
    039     * Decorates the specified iterator to skip all elements until the iterator
    040     * reaches the position at {@code offset}.
    041     * <p>
    042     * The iterator is immediately advanced until it reaches the position at {@code offset},
    043     * incurring O(n) time.
    044     *
    045     * @param iterator  the iterator to be decorated
    046     * @param offset  the index of the first element of the decorated iterator to return
    047     * @throws NullPointerException if iterator is null
    048     * @throws IllegalArgumentException if offset is negative
    049     */
    050    public SkippingIterator(final Iterator<E> iterator, final long offset) {
    051        super(iterator);
    052
    053        if (offset < 0) {
    054            throw new IllegalArgumentException("Offset parameter must not be negative.");
    055        }
    056
    057        this.offset = offset;
    058        this.pos = 0;
    059        init();
    060    }
    061
    062    /**
    063     * Skips the given number of elements.
    064     */
    065    private void init() {
    066        while (pos < offset && hasNext()) {
    067            next();
    068        }
    069    }
    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}