1 /*
2 * Licensed to the Apache Software Foundation (ASF) under one or more
3 * contributor license agreements. See the NOTICE file distributed with
4 * this work for additional information regarding copyright ownership.
5 * The ASF licenses this file to You under the Apache License, Version 2.0
6 * (the "License"); you may not use this file except in compliance with
7 * the License. You may obtain a copy of the License at
8 *
9 * http://www.apache.org/licenses/LICENSE-2.0
10 *
11 * Unless required by applicable law or agreed to in writing, software
12 * distributed under the License is distributed on an "AS IS" BASIS,
13 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
14 * See the License for the specific language governing permissions and
15 * limitations under the License.
16 */
17 package org.apache.commons.collections4.iterators;
18
19 import java.util.Iterator;
20 import java.util.NoSuchElementException;
21 import java.util.Objects;
22
23 /**
24 * Decorates another iterator to return elements in a specific range.
25 * <p>
26 * The decorated iterator is bounded in the range [offset, offset+max).
27 * The {@code offset} corresponds to the position of the first element to
28 * be returned from the decorated iterator, and {@code max} is the maximum
29 * number of elements to be returned at most.
30 * <p>
31 * In case an offset parameter other than 0 is provided, the decorated
32 * iterator is immediately advanced to this position, skipping all elements
33 * before that position.
34 *
35 * @param <E> the type of elements returned by this iterator.
36 * @since 4.1
37 */
38 public class BoundedIterator<E> implements Iterator<E> {
39
40 /** The iterator being decorated. */
41 private final Iterator<? extends E> iterator;
42
43 /** The offset to bound the first element return */
44 private final long offset;
45
46 /** The max number of elements to return */
47 private final long max;
48
49 /** The position of the current element */
50 private long pos;
51
52 /**
53 * Decorates the specified iterator to return at most the given number of elements,
54 * skipping all elements until the iterator reaches the position at {@code offset}.
55 * <p>
56 * The iterator is immediately advanced until it reaches the position at {@code offset},
57 * incurring O(n) time.
58 *
59 * @param iterator the iterator to be decorated
60 * @param offset the index of the first element of the decorated iterator to return
61 * @param max the maximum number of elements of the decorated iterator to return
62 * @throws NullPointerException if iterator is null
63 * @throws IllegalArgumentException if either offset or max is negative
64 */
65 public BoundedIterator(final Iterator<? extends E> iterator, final long offset, final long max) {
66 if (offset < 0) {
67 throw new IllegalArgumentException("Offset parameter must not be negative.");
68 }
69 if (max < 0) {
70 throw new IllegalArgumentException("Max parameter must not be negative.");
71 }
72
73 this.iterator = Objects.requireNonNull(iterator, "iterator");
74 this.offset = offset;
75 this.max = max;
76 pos = 0;
77 init();
78 }
79
80 /**
81 * Checks whether the iterator is still within its bounded range.
82 * @return {@code true} if the iterator is within its bounds, {@code false} otherwise
83 */
84 private boolean checkBounds() {
85 if (pos - offset + 1 > max) {
86 return false;
87 }
88 return true;
89 }
90
91 @Override
92 public boolean hasNext() {
93 if (!checkBounds()) {
94 return false;
95 }
96 return iterator.hasNext();
97 }
98
99 /**
100 * Advances the underlying iterator to the beginning of the bounded range.
101 */
102 private void init() {
103 while (pos < offset && iterator.hasNext()) {
104 iterator.next();
105 pos++;
106 }
107 }
108
109 @Override
110 public E next() {
111 if (!checkBounds()) {
112 throw new NoSuchElementException();
113 }
114 final E next = iterator.next();
115 pos++;
116 return next;
117 }
118
119 /**
120 * {@inheritDoc}
121 * <p>
122 * In case an offset other than 0 was specified, the underlying iterator will be advanced
123 * to this position upon creation. A call to {@link #remove()} will still result in an
124 * {@link IllegalStateException} if no explicit call to {@link #next()} has been made prior
125 * to calling {@link #remove()}.
126 */
127 @Override
128 public void remove() {
129 if (pos <= offset) {
130 throw new IllegalStateException("remove() can not be called before calling next()");
131 }
132 iterator.remove();
133 }
134 }