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 import org.checkerframework.checker.nullness.qual.Nullable; 24 25 /** 26 * An implementation of {@code Iterable} which throws an exception on all invocations of the {@link 27 * #iterator()} method after the first, and whose iterator is always unmodifiable. 28 * 29 * <p>The {@code Iterable} specification does not make it absolutely clear what should happen on a 30 * second invocation, so implementors have made various choices, including: 31 * 32 * <ul> 33 * <li>returning the same iterator again 34 * <li>throwing an exception of some kind 35 * <li>or the usual, <i>robust</i> behavior, which all known {@link Collection} implementations 36 * have, of returning a new, independent iterator 37 * </ul> 38 * 39 * <p>Because of this situation, any public method accepting an iterable should invoke the {@code 40 * iterator} method only once, and should be tested using this class. Exceptions to this rule should 41 * be clearly documented. 42 * 43 * <p>Note that although your APIs should be liberal in what they accept, your methods which 44 * <i>return</i> iterables should make every attempt to return ones of the robust variety. 45 * 46 * <p>This testing utility is not thread-safe. 47 * 48 * @author Kevin Bourrillion 49 */ 50 @GwtCompatible 51 public final class MinimalIterable<E extends @Nullable Object> implements Iterable<E> { 52 /** Returns an iterable whose iterator returns the given elements in order. */ of(E... elements)53 public static <E extends @Nullable Object> MinimalIterable<E> of(E... elements) { 54 // Make sure to get an unmodifiable iterator 55 return new MinimalIterable<>(Arrays.asList(elements).iterator()); 56 } 57 58 /** 59 * Returns an iterable whose iterator returns the given elements in order. The elements are copied 60 * out of the source collection at the time this method is called. 61 */ 62 @SuppressWarnings("unchecked") // Es come in, Es go out from(Collection<E> elements)63 public static <E extends @Nullable Object> MinimalIterable<E> from(Collection<E> elements) { 64 return (MinimalIterable) of(elements.toArray()); 65 } 66 67 private @Nullable Iterator<E> iterator; 68 MinimalIterable(Iterator<E> iterator)69 private MinimalIterable(Iterator<E> iterator) { 70 this.iterator = iterator; 71 } 72 73 @Override iterator()74 public Iterator<E> iterator() { 75 if (iterator == null) { 76 // TODO: throw something else? Do we worry that people's code and tests 77 // might be relying on this particular type of exception? 78 throw new IllegalStateException(); 79 } 80 try { 81 return iterator; 82 } finally { 83 iterator = null; 84 } 85 } 86 } 87