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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 
EmptyImmutableList()40   private EmptyImmutableList() {}
41 
size()42   public int size() {
43     return 0;
44   }
45 
isEmpty()46   @Override public boolean isEmpty() {
47     return true;
48   }
49 
contains(Object target)50   @Override public boolean contains(Object target) {
51     return false;
52   }
53 
iterator()54   @Override public UnmodifiableIterator<Object> iterator() {
55     return Iterators.emptyIterator();
56   }
57 
58   private static final Object[] EMPTY_ARRAY = new Object[0];
59 
toArray()60   @Override public Object[] toArray() {
61     return EMPTY_ARRAY;
62   }
63 
toArray(T[] a)64   @Override public <T> T[] toArray(T[] a) {
65     if (a.length > 0) {
66       a[0] = null;
67     }
68     return a;
69   }
70 
get(int index)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 
indexOf(Object target)77   @Override public int indexOf(Object target) {
78     return -1;
79   }
80 
lastIndexOf(Object target)81   @Override public int lastIndexOf(Object target) {
82     return -1;
83   }
84 
subList(int fromIndex, int toIndex)85   @Override public ImmutableList<Object> subList(int fromIndex, int toIndex) {
86     checkPositionIndexes(fromIndex, toIndex, 0);
87     return this;
88   }
89 
listIterator()90   public ListIterator<Object> listIterator() {
91     return Collections.emptyList().listIterator();
92   }
93 
listIterator(int start)94   public ListIterator<Object> listIterator(int start) {
95     checkPositionIndex(start, 0);
96     return Collections.emptyList().listIterator();
97   }
98 
containsAll(Collection<?> targets)99   @Override public boolean containsAll(Collection<?> targets) {
100     return targets.isEmpty();
101   }
102 
equals(@ullable Object object)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 
hashCode()111   @Override public int hashCode() {
112     return 1;
113   }
114 
toString()115   @Override public String toString() {
116     return "[]";
117   }
118 
readResolve()119   Object readResolve() {
120     return INSTANCE; // preserve singleton property
121   }
122 
123   private static final long serialVersionUID = 0;
124 }
125