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
17package com.google.common.collect;
18
19import com.google.common.annotations.GwtCompatible;
20
21import java.util.Collection;
22import java.util.Set;
23
24import javax.annotation.Nullable;
25
26/**
27 * An empty immutable set.
28 *
29 * @author Kevin Bourrillion
30 */
31@GwtCompatible(serializable = true, emulated = true)
32final class EmptyImmutableSet extends ImmutableSet<Object> {
33  static final EmptyImmutableSet INSTANCE = new EmptyImmutableSet();
34
35  private EmptyImmutableSet() {}
36
37  @Override
38  public int size() {
39    return 0;
40  }
41
42  @Override public boolean isEmpty() {
43    return true;
44  }
45
46  @Override public boolean contains(Object target) {
47    return false;
48  }
49
50  @Override public UnmodifiableIterator<Object> iterator() {
51    return Iterators.emptyIterator();
52  }
53
54  @Override boolean isPartialView() {
55    return false;
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  @Override public boolean containsAll(Collection<?> targets) {
72    return targets.isEmpty();
73  }
74
75  @Override public boolean equals(@Nullable Object object) {
76    if (object instanceof Set) {
77      Set<?> that = (Set<?>) object;
78      return that.isEmpty();
79    }
80    return false;
81  }
82
83  @Override public final int hashCode() {
84    return 0;
85  }
86
87  @Override boolean isHashCodeFast() {
88    return true;
89  }
90
91  @Override public String toString() {
92    return "[]";
93  }
94
95  Object readResolve() {
96    return INSTANCE; // preserve singleton property
97  }
98
99  private static final long serialVersionUID = 0;
100}
101