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}