001/*
002 * Copyright (C) 2007 The Guava Authors
003 *
004 * Licensed under the Apache License, Version 2.0 (the "License");
005 * you may not use this file except in compliance with the License.
006 * You may obtain a copy of the License at
007 *
008 * http://www.apache.org/licenses/LICENSE-2.0
009 *
010 * Unless required by applicable law or agreed to in writing, software
011 * distributed under the License is distributed on an "AS IS" BASIS,
012 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
013 * See the License for the specific language governing permissions and
014 * limitations under the License.
015 */
016
017package com.google.common.collect.testing;
018
019import static java.util.Collections.singleton;
020
021import com.google.common.annotations.GwtCompatible;
022import java.util.Iterator;
023import org.jspecify.annotations.NullMarked;
024import org.jspecify.annotations.Nullable;
025
026/**
027 * A utility for testing an Iterator implementation by comparing its behavior to that of a "known
028 * good" reference implementation. In order to accomplish this, it's important to test a great
029 * variety of sequences of the {@link Iterator#next}, {@link Iterator#hasNext} and {@link
030 * Iterator#remove} operations. This utility takes the brute-force approach of trying <i>all</i>
031 * possible sequences of these operations, up to a given number of steps. So, if the caller
032 * specifies to use <i>n</i> steps, a total of <i>3^n</i> tests are actually performed.
033 *
034 * <p>For instance, if <i>steps</i> is 5, one example sequence that will be tested is:
035 *
036 * <ol>
037 *   <li>remove();
038 *   <li>hasNext()
039 *   <li>hasNext();
040 *   <li>remove();
041 *   <li>next();
042 * </ol>
043 *
044 * <p>This particular order of operations may be unrealistic, and testing all 3^5 of them may be
045 * thought of as overkill; however, it's difficult to determine which proper subset of this massive
046 * set would be sufficient to expose any possible bug. Brute force is simpler.
047 *
048 * <p>To use this class the concrete subclass must implement the {@link
049 * IteratorTester#newTargetIterator()} method. This is because it's impossible to test an Iterator
050 * without changing its state, so the tester needs a steady supply of fresh Iterators.
051 *
052 * <p>If your iterator supports modification through {@code remove()}, you may wish to override the
053 * verify() method, which is called <em>after</em> each sequence and is guaranteed to be called
054 * using the latest values obtained from {@link IteratorTester#newTargetIterator()}.
055 *
056 * <p>The value you pass to the parameter {@code steps} should be greater than the length of your
057 * iterator, so that this class can check that your iterator behaves correctly when it is exhausted.
058 *
059 * <p>For example, to test {@link java.util.Collections#unmodifiableList(java.util.List)
060 * Collections.unmodifiableList}'s iterator:
061 *
062 * {@snippet :
063 * List<String> expectedElements = Arrays.asList("a", "b", "c", "d", "e");
064 * List<String> actualElements =
065 *     Collections.unmodifiableList(Arrays.asList("a", "b", "c", "d", "e"));
066 * IteratorTester<String> iteratorTester =
067 *     new IteratorTester<String>(
068 *         6,
069 *         IteratorFeature.UNMODIFIABLE,
070 *         expectedElements,
071 *         IteratorTester.KnownOrder.KNOWN_ORDER) {
072 *       @Override
073 *       protected Iterator<String> newTargetIterator() {
074 *         return actualElements.iterator();
075 *       }
076 *     };
077 * iteratorTester.test();
078 * iteratorTester.testForEachRemaining();
079 * }
080 *
081 * <p><b>Note</b>: It is necessary to use {@code IteratorTester.KnownOrder} as shown above, rather
082 * than {@code KnownOrder} directly, because otherwise the code cannot be compiled.
083 *
084 * @author Kevin Bourrillion
085 * @author Chris Povirk
086 */
087@GwtCompatible
088@NullMarked
089public abstract class IteratorTester<E extends @Nullable Object>
090    extends AbstractIteratorTester<E, Iterator<E>> {
091  /**
092   * Creates an IteratorTester.
093   *
094   * @param steps how many operations to test for each tested pair of iterators
095   * @param features the features supported by the iterator
096   */
097  protected IteratorTester(
098      int steps,
099      Iterable<? extends IteratorFeature> features,
100      Iterable<E> expectedElements,
101      KnownOrder knownOrder) {
102    super(steps, singleton(null), features, expectedElements, knownOrder, 0);
103  }
104
105  @Override
106  protected final Iterable<Stimulus<E, Iterator<E>>> getStimulusValues() {
107    return iteratorStimuli();
108  }
109}