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
17 package com.google.common.collect.testing;
18
19 import com.google.common.annotations.GwtCompatible;
20 import java.util.Arrays;
21 import java.util.Collection;
22 import java.util.Iterator;
23
24 /**
25 * An implementation of {@code Iterable} which throws an exception on all invocations of the {@link
26 * #iterator()} method after the first, and whose iterator is always unmodifiable.
27 *
28 * <p>The {@code Iterable} specification does not make it absolutely clear what should happen on a
29 * second invocation, so implementors have made various choices, including:
30 *
31 * <ul>
32 * <li>returning the same iterator again
33 * <li>throwing an exception of some kind
34 * <li>or the usual, <i>robust</i> behavior, which all known {@link Collection} implementations
35 * have, of returning a new, independent iterator
36 * </ul>
37 *
38 * <p>Because of this situation, any public method accepting an iterable should invoke the {@code
39 * iterator} method only once, and should be tested using this class. Exceptions to this rule should
40 * be clearly documented.
41 *
42 * <p>Note that although your APIs should be liberal in what they accept, your methods which
43 * <i>return</i> iterables should make every attempt to return ones of the robust variety.
44 *
45 * <p>This testing utility is not thread-safe.
46 *
47 * @author Kevin Bourrillion
48 */
49 @GwtCompatible
50 public final class MinimalIterable<E> implements Iterable<E> {
51 /** Returns an iterable whose iterator returns the given elements in order. */
52 public static <E> MinimalIterable<E> of(E... elements) {
53 // Make sure to get an unmodifiable iterator
54 return new MinimalIterable<E>(Arrays.asList(elements).iterator());
55 }
56
57 /**
58 * Returns an iterable whose iterator returns the given elements in order. The elements are copied
59 * out of the source collection at the time this method is called.
60 */
61 @SuppressWarnings("unchecked") // Es come in, Es go out
62 public static <E> MinimalIterable<E> from(final Collection<E> elements) {
63 return (MinimalIterable) of(elements.toArray());
64 }
65
66 private Iterator<E> iterator;
67
68 private MinimalIterable(Iterator<E> iterator) {
69 this.iterator = iterator;
70 }
71
72 @Override
73 public Iterator<E> iterator() {
74 if (iterator == null) {
75 // TODO: throw something else? Do we worry that people's code and tests
76 // might be relying on this particular type of exception?
77 throw new IllegalStateException();
78 }
79 try {
80 return iterator;
81 } finally {
82 iterator = null;
83 }
84 }
85 }