1/*
2 * Copyright (C) 2009 The Guava Authors
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
7 *
8 * http://www.apache.org/licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
15 */
16
17package com.google.common.collect.testing;
18
19import java.util.Arrays;
20import java.util.Collection;
21import java.util.Iterator;
22
23/**
24 * An implementation of {@code Iterable} which throws an exception on all
25 * invocations of the {@link #iterator()} method after the first, and whose
26 * iterator is always unmodifiable.
27 *
28 * <p>The {@code Iterable} specification does not make it absolutely clear what
29 * should happen on a second invocation, so implementors have made various
30 * choices, including:
31 *
32 * <ul>
33 * <li>returning the same iterator again
34 * <li>throwing an exception of some kind
35 * <li>or the usual, <i>robust</i> behavior, which all known {@link Collection}
36 *     implementations have, of returning a new, independent iterator
37 * </ul>
38 *
39 * Because of this situation, any public method accepting an iterable should
40 * invoke the {@code iterator} method only once, and should be tested using this
41 * class. Exceptions to this rule should be clearly documented.
42 *
43 * <p>Note that although your APIs should be liberal in what they accept, your
44 * methods which <i>return</i> iterables should make every attempt to return
45 * ones of the robust variety.
46 *
47 * <p>This testing utility is not thread-safe.
48 *
49 * @author Kevin Bourrillion
50 */
51public final class MinimalIterable<E> implements Iterable<E> {
52  /**
53   * Returns an iterable whose iterator returns the given elements in order.
54   */
55  public static <E> MinimalIterable<E> of(E... elements) {
56    // Make sure to get an unmodifiable iterator
57    return new MinimalIterable<E>(Arrays.asList(elements).iterator());
58  }
59
60  /**
61   * Returns an iterable whose iterator returns the given elements in order.
62   * The elements are copied out of the source collection at the time this
63   * method is called.
64   */
65  @SuppressWarnings("unchecked") // Es come in, Es go out
66  public static <E> MinimalIterable<E> from(final Collection<E> elements) {
67    return (MinimalIterable) of(elements.toArray());
68  }
69
70  private Iterator<E> iterator;
71
72  private MinimalIterable(Iterator<E> iterator) {
73    this.iterator = iterator;
74  }
75
76  @Override
77  public Iterator<E> iterator() {
78    if (iterator == null) {
79      // TODO: throw something else? Do we worry that people's code and tests
80      // might be relying on this particular type of exception?
81      throw new IllegalStateException();
82    }
83    try {
84      return iterator;
85    } finally {
86      iterator = null;
87    }
88  }
89}
90