1/*
2 * Copyright (C) 2011 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.base;
18
19import static com.google.common.base.Preconditions.checkNotNull;
20
21import com.google.common.annotations.GwtCompatible;
22
23import java.util.Collections;
24import java.util.Set;
25
26import javax.annotation.Nullable;
27
28/**
29 * Implementation of an {@link Optional} not containing a reference.
30 */
31@GwtCompatible
32final class Absent<T> extends Optional<T> {
33  static final Absent<Object> INSTANCE = new Absent<Object>();
34
35  @SuppressWarnings("unchecked") // implementation is "fully variant"
36  static <T> Optional<T> withType() {
37    return (Optional<T>) INSTANCE;
38  }
39
40  private Absent() {}
41
42  @Override public boolean isPresent() {
43    return false;
44  }
45
46  @Override public T get() {
47    throw new IllegalStateException("Optional.get() cannot be called on an absent value");
48  }
49
50  @Override public T or(T defaultValue) {
51    return checkNotNull(defaultValue, "use Optional.orNull() instead of Optional.or(null)");
52  }
53
54  @SuppressWarnings("unchecked") // safe covariant cast
55  @Override public Optional<T> or(Optional<? extends T> secondChoice) {
56    return (Optional<T>) checkNotNull(secondChoice);
57  }
58
59  @Override public T or(Supplier<? extends T> supplier) {
60    return checkNotNull(supplier.get(),
61        "use Optional.orNull() instead of a Supplier that returns null");
62  }
63
64  @Override @Nullable public T orNull() {
65    return null;
66  }
67
68  @Override public Set<T> asSet() {
69    return Collections.emptySet();
70  }
71
72  @Override public <V> Optional<V> transform(Function<? super T, V> function) {
73    checkNotNull(function);
74    return Optional.absent();
75  }
76
77  @Override public boolean equals(@Nullable Object object) {
78    return object == this;
79  }
80
81  @Override public int hashCode() {
82    return 0x598df91c;
83  }
84
85  @Override public String toString() {
86    return "Optional.absent()";
87  }
88
89  private Object readResolve() {
90    return INSTANCE;
91  }
92
93  private static final long serialVersionUID = 0;
94}
95