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 com.google.common.annotations.GwtCompatible;
020import java.util.Collections;
021import java.util.Iterator;
022import org.jspecify.annotations.NullMarked;
023import org.jspecify.annotations.Nullable;
024
025/**
026 * A utility for testing an Iterator implementation by comparing its behavior to that of a "known
027 * good" reference implementation. In order to accomplish this, it's important to test a great
028 * variety of sequences of the {@link Iterator#next}, {@link Iterator#hasNext} and {@link
029 * Iterator#remove} operations. This utility takes the brute-force approach of trying <i>all</i>
030 * possible sequences of these operations, up to a given number of steps. So, if the caller
031 * specifies to use <i>n</i> steps, a total of <i>3^n</i> tests are actually performed.
032 *
033 * <p>For instance, if <i>steps</i> is 5, one example sequence that will be tested is:
034 *
035 * <ol>
036 *   <li>remove();
037 *   <li>hasNext()
038 *   <li>hasNext();
039 *   <li>remove();
040 *   <li>next();
041 * </ol>
042 *
043 * <p>This particular order of operations may be unrealistic, and testing all 3^5 of them may be
044 * thought of as overkill; however, it's difficult to determine which proper subset of this massive
045 * set would be sufficient to expose any possible bug. Brute force is simpler.
046 *
047 * <p>To use this class the concrete subclass must implement the {@link
048 * IteratorTester#newTargetIterator()} method. This is because it's impossible to test an Iterator
049 * without changing its state, so the tester needs a steady supply of fresh Iterators.
050 *
051 * <p>If your iterator supports modification through {@code remove()}, you may wish to override the
052 * verify() method, which is called <em>after</em> each sequence and is guaranteed to be called
053 * using the latest values obtained from {@link IteratorTester#newTargetIterator()}.
054 *
055 * <p>The value you pass to the parameter {@code steps} should be greater than the length of your
056 * iterator, so that this class can check that your iterator behaves correctly when it is exhausted.
057 *
058 * <p>For example, to test {@link java.util.Collections#unmodifiableList(java.util.List)
059 * Collections.unmodifiableList}'s iterator:
060 *
061 * {@snippet :
062 * List<String> expectedElements =
063 *     Arrays.asList("a", "b", "c", "d", "e");
064 * List<String> actualElements =
065 *     Collections.unmodifiableList(
066 *         Arrays.asList("a", "b", "c", "d", "e"));
067 * IteratorTester<String> iteratorTester =
068 *     new IteratorTester<String>(
069 *         6,
070 *         IteratorFeature.UNMODIFIABLE,
071 *         expectedElements,
072 *         IteratorTester.KnownOrder.KNOWN_ORDER) {
073 *       @Override
074 *       protected Iterator<String> newTargetIterator() {
075 *         return actualElements.iterator();
076 *       }
077 *     };
078 * iteratorTester.test();
079 * iteratorTester.testForEachRemaining();
080 * }
081 *
082 * <p><b>Note</b>: It is necessary to use {@code IteratorTester.KnownOrder} as shown above, rather
083 * than {@code KnownOrder} directly, because otherwise the code cannot be compiled.
084 *
085 * @author Kevin Bourrillion
086 * @author Chris Povirk
087 */
088@GwtCompatible
089@NullMarked
090public abstract class IteratorTester<E extends @Nullable Object>
091    extends AbstractIteratorTester<E, Iterator<E>> {
092  /**
093   * Creates an IteratorTester.
094   *
095   * @param steps how many operations to test for each tested pair of iterators
096   * @param features the features supported by the iterator
097   */
098  protected IteratorTester(
099      int steps,
100      Iterable<? extends IteratorFeature> features,
101      Iterable<E> expectedElements,
102      KnownOrder knownOrder) {
103    super(steps, Collections.<E>singleton(null), features, expectedElements, knownOrder, 0);
104  }
105
106  @Override
107  protected final Iterable<Stimulus<E, Iterator<E>>> getStimulusValues() {
108    return iteratorStimuli();
109  }
110}