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 *      https://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     *
086     * @return {@code true} if the iterator is within its bounds, {@code false} otherwise
087     */
088    private boolean checkBounds() {
089        if (pos - offset + 1 > max) {
090            return false;
091        }
092        return true;
093    }
094
095    @Override
096    public boolean hasNext() {
097        if (!checkBounds()) {
098            return false;
099        }
100        return iterator.hasNext();
101    }
102
103    /**
104     * Advances the underlying iterator to the beginning of the bounded range.
105     */
106    private void init() {
107        while (pos < offset && iterator.hasNext()) {
108            iterator.next();
109            pos++;
110        }
111    }
112
113    @Override
114    public E next() {
115        if (!checkBounds()) {
116            throw new NoSuchElementException();
117        }
118        final E next = iterator.next();
119        pos++;
120        return next;
121    }
122
123    /**
124     * {@inheritDoc}
125     * <p>
126     * In case an offset other than 0 was specified, the underlying iterator will be advanced
127     * to this position upon creation. A call to {@link #remove()} will still result in an
128     * {@link IllegalStateException} if no explicit call to {@link #next()} has been made prior
129     * to calling {@link #remove()}.
130     * </p>
131     */
132    @Override
133    public void remove() {
134        if (pos <= offset) {
135            throw new IllegalStateException("remove() cannot be called before calling next()");
136        }
137        iterator.remove();
138    }
139}