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