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