001/* 002 * Copyright (C) 2009 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.Arrays.asList; 020 021import com.google.common.annotations.GwtCompatible; 022import java.util.Collection; 023import java.util.Iterator; 024import org.jspecify.annotations.Nullable; 025 026/** 027 * An implementation of {@code Iterable} which throws an exception on all invocations of the {@link 028 * #iterator()} method after the first, and whose iterator is always unmodifiable. 029 * 030 * <p>The {@code Iterable} specification does not make it absolutely clear what should happen on a 031 * second invocation, so implementors have made various choices, including: 032 * 033 * <ul> 034 * <li>returning the same iterator again 035 * <li>throwing an exception of some kind 036 * <li>or the usual, <i>robust</i> behavior, which all known {@link Collection} implementations 037 * have, of returning a new, independent iterator 038 * </ul> 039 * 040 * <p>Because of this situation, any public method accepting an iterable should invoke the {@code 041 * iterator} method only once, and should be tested using this class. Exceptions to this rule should 042 * be clearly documented. 043 * 044 * <p>Note that although your APIs should be liberal in what they accept, your methods which 045 * <i>return</i> iterables should make every attempt to return ones of the robust variety. 046 * 047 * <p>This testing utility is not thread-safe. 048 * 049 * @author Kevin Bourrillion 050 */ 051@GwtCompatible 052public final class MinimalIterable<E extends @Nullable Object> implements Iterable<E> { 053 /** Returns an iterable whose iterator returns the given elements in order. */ 054 public static <E extends @Nullable Object> MinimalIterable<E> of(E... elements) { 055 // Make sure to get an unmodifiable iterator 056 return new MinimalIterable<>(asList(elements).iterator()); 057 } 058 059 /** 060 * Returns an iterable whose iterator returns the given elements in order. The elements are copied 061 * out of the source collection at the time this method is called. 062 */ 063 @SuppressWarnings("unchecked") // Es come in, Es go out 064 public static <E extends @Nullable Object> MinimalIterable<E> from(Collection<E> elements) { 065 return (MinimalIterable) of(elements.toArray()); 066 } 067 068 private @Nullable Iterator<E> iterator; 069 070 private MinimalIterable(Iterator<E> iterator) { 071 this.iterator = iterator; 072 } 073 074 @Override 075 public Iterator<E> iterator() { 076 if (iterator == null) { 077 // TODO: throw something else? Do we worry that people's code and tests 078 // might be relying on this particular type of exception? 079 throw new IllegalStateException(); 080 } 081 try { 082 return iterator; 083 } finally { 084 iterator = null; 085 } 086 } 087}