1 /* 2 * Copyright (C) 2007 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; 18 19 import static com.google.common.base.Preconditions.checkElementIndex; 20 import static com.google.common.base.Preconditions.checkPositionIndex; 21 import static com.google.common.base.Preconditions.checkPositionIndexes; 22 23 import com.google.common.annotations.GwtCompatible; 24 25 import java.util.Collection; 26 import java.util.List; 27 import java.util.NoSuchElementException; 28 29 import javax.annotation.Nullable; 30 31 /** 32 * An empty immutable list. 33 * 34 * @author Kevin Bourrillion 35 */ 36 @GwtCompatible(serializable = true, emulated = true) 37 final class EmptyImmutableList extends ImmutableList<Object> { 38 static final EmptyImmutableList INSTANCE = new EmptyImmutableList(); 39 static final UnmodifiableListIterator<Object> ITERATOR = 40 new UnmodifiableListIterator<Object>() { 41 42 @Override public boolean hasNext() { 43 return false; 44 } 45 46 @Override public boolean hasPrevious() { 47 return false; 48 } 49 50 @Override public Object next() { 51 throw new NoSuchElementException(); 52 } 53 54 @Override public int nextIndex() { 55 return 0; 56 } 57 58 @Override public Object previous() { 59 throw new NoSuchElementException(); 60 } 61 62 @Override public int previousIndex() { 63 return -1; 64 } 65 }; 66 67 private EmptyImmutableList() {} 68 69 @Override 70 public int size() { 71 return 0; 72 } 73 74 @Override public boolean isEmpty() { 75 return true; 76 } 77 78 @Override boolean isPartialView() { 79 return false; 80 } 81 82 @Override public boolean contains(Object target) { 83 return false; 84 } 85 86 @Override public UnmodifiableIterator<Object> iterator() { 87 return Iterators.emptyIterator(); 88 } 89 90 private static final Object[] EMPTY_ARRAY = new Object[0]; 91 92 @Override public Object[] toArray() { 93 return EMPTY_ARRAY; 94 } 95 96 @Override public <T> T[] toArray(T[] a) { 97 if (a.length > 0) { 98 a[0] = null; 99 } 100 return a; 101 } 102 103 @Override 104 public Object get(int index) { 105 // guaranteed to fail, but at least we get a consistent message 106 checkElementIndex(index, 0); 107 throw new AssertionError("unreachable"); 108 } 109 110 @Override public int indexOf(@Nullable Object target) { 111 return -1; 112 } 113 114 @Override public int lastIndexOf(@Nullable Object target) { 115 return -1; 116 } 117 118 @Override public ImmutableList<Object> subList(int fromIndex, int toIndex) { 119 checkPositionIndexes(fromIndex, toIndex, 0); 120 return this; 121 } 122 123 @Override public ImmutableList<Object> reverse() { 124 return this; 125 } 126 127 @Override public UnmodifiableListIterator<Object> listIterator(){ 128 return ITERATOR; 129 } 130 131 @Override public UnmodifiableListIterator<Object> listIterator(int start) { 132 checkPositionIndex(start, 0); 133 return ITERATOR; 134 } 135 136 @Override public boolean containsAll(Collection<?> targets) { 137 return targets.isEmpty(); 138 } 139 140 @Override public boolean equals(@Nullable Object object) { 141 if (object instanceof List) { 142 List<?> that = (List<?>) object; 143 return that.isEmpty(); 144 } 145 return false; 146 } 147 148 @Override public int hashCode() { 149 return 1; 150 } 151 152 @Override public String toString() { 153 return "[]"; 154 } 155 156 Object readResolve() { 157 return INSTANCE; // preserve singleton property 158 } 159 160 private static final long serialVersionUID = 0; 161 } 162
