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. */
of(E... elements)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
from(final Collection<E> elements)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 
MinimalIterable(Iterator<E> iterator)68   private MinimalIterable(Iterator<E> iterator) {
69     this.iterator = iterator;
70   }
71 
72   @Override
iterator()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 }
86