guava

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/*
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 * Copyright (C) 2009 The Guava Authors
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 *
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 * Licensed under the Apache License, Version 2.0 (the "License");
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 * you may not use this file except in compliance with the License.
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 * You may obtain a copy of the License at
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 *
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 * http://www.apache.org/licenses/LICENSE-2.0
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 *
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 * Unless required by applicable law or agreed to in writing, software
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 * distributed under the License is distributed on an "AS IS" BASIS,
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 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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 * See the License for the specific language governing permissions and
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 * limitations under the License.
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 */
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package com.google.common.collect.testing;
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import com.google.common.annotations.GwtCompatible;
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import java.util.Arrays;
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import java.util.Collection;
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import java.util.Iterator;
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import org.checkerframework.checker.nullness.qual.Nullable;
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/**
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 * An implementation of {@code Iterable} which throws an exception on all invocations of the {@link
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 * #iterator()} method after the first, and whose iterator is always unmodifiable.
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 *
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 * <p>The {@code Iterable} specification does not make it absolutely clear what should happen on a
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 * second invocation, so implementors have made various choices, including:
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 *
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 * <ul>
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 *   <li>returning the same iterator again
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 *   <li>throwing an exception of some kind
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 *   <li>or the usual, <i>robust</i> behavior, which all known {@link Collection} implementations
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 *       have, of returning a new, independent iterator
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 * </ul>
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 *
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 * <p>Because of this situation, any public method accepting an iterable should invoke the {@code
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 * iterator} method only once, and should be tested using this class. Exceptions to this rule should
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 * be clearly documented.
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 *
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 * <p>Note that although your APIs should be liberal in what they accept, your methods which
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 * <i>return</i> iterables should make every attempt to return ones of the robust variety.
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 *
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 * <p>This testing utility is not thread-safe.
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 *
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 * @author Kevin Bourrillion
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 */
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@GwtCompatible
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public final class MinimalIterable<E extends @Nullable Object> implements Iterable<E> {
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  /** Returns an iterable whose iterator returns the given elements in order. */
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  public static <E extends @Nullable Object> MinimalIterable<E> of(E... elements) {
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    // Make sure to get an unmodifiable iterator
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    return new MinimalIterable<>(Arrays.asList(elements).iterator());
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  }
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  /**
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   * Returns an iterable whose iterator returns the given elements in order. The elements are copied
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   * out of the source collection at the time this method is called.
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   */
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  @SuppressWarnings("unchecked") // Es come in, Es go out
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  public static <E extends @Nullable Object> MinimalIterable<E> from(Collection<E> elements) {
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    return (MinimalIterable) of(elements.toArray());
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  }
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  private @Nullable Iterator<E> iterator;
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  private MinimalIterable(Iterator<E> iterator) {
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    this.iterator = iterator;
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  }
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  @Override
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  public Iterator<E> iterator() {
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    if (iterator == null) {
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      // TODO: throw something else? Do we worry that people's code and tests
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      // might be relying on this particular type of exception?
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      throw new IllegalStateException();
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    }
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    try {
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      return iterator;
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    } finally {
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      iterator = null;
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    }
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  }
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}
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