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 * <p>
32 * In case an offset parameter other than 0 is provided, the decorated
33 * iterator is immediately advanced to this position, skipping all elements
34 * before that position.
35 * </p>
36 *
37 * @param <E> the type of elements returned by this iterator.
38 * @since 4.1
39 */
40 public class BoundedIterator<E> implements Iterator<E> {
41
42 /** The iterator being decorated. */
43 private final Iterator<? extends E> iterator;
44
45 /** The offset to bound the first element return */
46 private final long offset;
47
48 /** The max number of elements to return */
49 private final long max;
50
51 /** The position of the current element */
52 private long pos;
53
54 /**
55 * Decorates the specified iterator to return at most the given number of elements,
56 * skipping all elements until the iterator reaches the position at {@code offset}.
57 * <p>
58 * The iterator is immediately advanced until it reaches the position at {@code offset},
59 * incurring O(n) time.
60 * </p>
61 *
62 * @param iterator the iterator to be decorated
63 * @param offset the index of the first element of the decorated iterator to return
64 * @param max the maximum number of elements of the decorated iterator to return
65 * @throws NullPointerException if iterator is null
66 * @throws IllegalArgumentException if either offset or max is negative
67 */
68 public BoundedIterator(final Iterator<? extends E> iterator, final long offset, final long max) {
69 if (offset < 0) {
70 throw new IllegalArgumentException("Offset parameter must not be negative.");
71 }
72 if (max < 0) {
73 throw new IllegalArgumentException("Max parameter must not be negative.");
74 }
75
76 this.iterator = Objects.requireNonNull(iterator, "iterator");
77 this.offset = offset;
78 this.max = max;
79 pos = 0;
80 init();
81 }
82
83 /**
84 * Checks whether the iterator is still within its bounded range.
85 * @return {@code true} if the iterator is within its bounds, {@code false} otherwise
86 */
87 private boolean checkBounds() {
88 if (pos - offset + 1 > max) {
89 return false;
90 }
91 return true;
92 }
93
94 @Override
95 public boolean hasNext() {
96 if (!checkBounds()) {
97 return false;
98 }
99 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 }