001 /*
002 * Copyright (C) 2011 The Guava Authors
003 *
004 * Licensed under the Apache License, Version 2.0 (the "License");
005 * you may not use this file except in compliance with the License.
006 * You may obtain a copy of the License at
007 *
008 * http://www.apache.org/licenses/LICENSE-2.0
009 *
010 * Unless required by applicable law or agreed to in writing, software
011 * distributed under the License is distributed on an "AS IS" BASIS,
012 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
013 * See the License for the specific language governing permissions and
014 * limitations under the License.
015 */
016
017 package com.google.common.base;
018
019 import static com.google.common.base.Preconditions.checkNotNull;
020
021 import com.google.common.annotations.Beta;
022 import com.google.common.annotations.GwtCompatible;
023
024 import java.io.Serializable;
025 import java.util.Iterator;
026 import java.util.Set;
027
028 import javax.annotation.Nullable;
029
030 /**
031 * An immutable object that may contain a non-null reference to another object. Each
032 * instance of this type either contains a non-null reference, or contains nothing (in
033 * which case we say that the reference is "absent"); it is never said to "contain {@code
034 * null}".
035 *
036 * <p>A non-null {@code Optional<T>} reference can be used as a replacement for a nullable
037 * {@code T} reference. It allows you to represent "a {@code T} that must be present" and
038 * a "a {@code T} that might be absent" as two distinct types in your program, which can
039 * aid clarity.
040 *
041 * <p>Some uses of this class include
042 *
043 * <ul>
044 * <li>As a method return type, as an alternative to returning {@code null} to indicate
045 * that no value was available
046 * <li>To distinguish between "unknown" (for example, not present in a map) and "known to
047 * have no value" (present in the map, with value {@code Optional.absent()})
048 * <li>To wrap nullable references for storage in a collection that does not support
049 * {@code null} (though there are
050 * <a href="http://code.google.com/p/guava-libraries/wiki/LivingWithNullHostileCollections">
051 * several other approaches to this</a> that should be considered first)
052 * </ul>
053 *
054 * <p>A common alternative to using this class is to find or create a suitable
055 * <a href="http://en.wikipedia.org/wiki/Null_Object_pattern">null object</a> for the
056 * type in question.
057 *
058 * <p>This class is not intended as a direct analogue of any existing "option" or "maybe"
059 * construct from other programming environments, though it may bear some similarities.
060 *
061 * <p>See the Guava User Guide article on <a
062 * href="http://code.google.com/p/guava-libraries/wiki/UsingAndAvoidingNullExplained#Optional">
063 * using {@code Optional}</a>.
064 *
065 * @param <T> the type of instance that can be contained. {@code Optional} is naturally
066 * covariant on this type, so it is safe to cast an {@code Optional<T>} to {@code
067 * Optional<S>} for any supertype {@code S} of {@code T}.
068 * @author Kurt Alfred Kluever
069 * @author Kevin Bourrillion
070 * @since 10.0
071 */
072 @Beta
073 @GwtCompatible(serializable = true)
074 public abstract class Optional<T> implements Serializable {
075 /**
076 * Returns an {@code Optional} instance with no contained reference.
077 */
078 @SuppressWarnings("unchecked")
079 public static <T> Optional<T> absent() {
080 return (Optional<T>) Absent.INSTANCE;
081 }
082
083 /**
084 * Returns an {@code Optional} instance containing the given non-null reference.
085 */
086 public static <T> Optional<T> of(T reference) {
087 return new Present<T>(checkNotNull(reference));
088 }
089
090 /**
091 * If {@code nullableReference} is non-null, returns an {@code Optional} instance containing that
092 * reference; otherwise returns {@link Optional#absent}.
093 */
094 public static <T> Optional<T> fromNullable(@Nullable T nullableReference) {
095 return (nullableReference == null)
096 ? Optional.<T>absent()
097 : new Present<T>(nullableReference);
098 }
099
100 Optional() {}
101
102 /**
103 * Returns {@code true} if this holder contains a (non-null) instance.
104 */
105 public abstract boolean isPresent();
106
107 /**
108 * Returns the contained instance, which must be present. If the instance might be
109 * absent, use {@link #or(Object)} or {@link #orNull} instead.
110 *
111 * @throws IllegalStateException if the instance is absent ({@link #isPresent} returns
112 * {@code false})
113 */
114 public abstract T get();
115
116 /**
117 * Returns the contained instance if it is present; {@code defaultValue} otherwise. If
118 * no default value should be required because the instance is known to be present, use
119 * {@link #get()} instead. For a default value of {@code null}, use {@link #orNull}.
120 *
121 * <p>Note about generics: The signature {@code public T or(T defaultValue)} is overly
122 * restrictive. However, the ideal signature, {@code public <S super T> S or(S)}, is not legal
123 * Java. As a result, some sensible operations involving subtypes are compile errors:
124 * <pre> {@code
125 *
126 * Optional<Integer> optionalInt = getSomeOptionalInt();
127 * Number value = optionalInt.or(0.5); // error
128 *
129 * FluentIterable<? extends Number> numbers = getSomeNumbers();
130 * Optional<? extends Number> first = numbers.first();
131 * Number value = first.or(0.5); // error}</pre>
132 *
133 * As a workaround, it is always safe to cast an {@code Optional<? extends T>} to {@code
134 * Optional<T>}. Casting either of the above example {@code Optional} instances to {@code
135 * Optional<Number>} (where {@code Number} is the desired output type) solves the problem:
136 * <pre> {@code
137 *
138 * Optional<Number> optionalInt = (Optional) getSomeOptionalInt();
139 * Number value = optionalInt.or(0.5); // fine
140 *
141 * FluentIterable<? extends Number> numbers = getSomeNumbers();
142 * Optional<Number> first = (Optional) numbers.first();
143 * Number value = first.or(0.5); // fine}</pre>
144 */
145 public abstract T or(T defaultValue);
146
147 /**
148 * Returns this {@code Optional} if it has a value present; {@code secondChoice}
149 * otherwise.
150 */
151 public abstract Optional<T> or(Optional<? extends T> secondChoice);
152
153 /**
154 * Returns the contained instance if it is present; {@code supplier.get()} otherwise. If the
155 * supplier returns {@code null}, a {@link NullPointerException} is thrown.
156 *
157 * @throws NullPointerException if the supplier returns {@code null}
158 */
159 public abstract T or(Supplier<? extends T> supplier);
160
161 /**
162 * Returns the contained instance if it is present; {@code null} otherwise. If the
163 * instance is known to be present, use {@link #get()} instead.
164 */
165 @Nullable public abstract T orNull();
166
167 /**
168 * Returns an immutable singleton {@link Set} whose only element is the contained instance
169 * if it is present; an empty immutable {@link Set} otherwise.
170 *
171 * @since 11.0
172 */
173 public abstract Set<T> asSet();
174
175 /**
176 * If the instance is present, it is transformed with the given {@link Function}; otherwise,
177 * {@link Optional#absent} is returned. If the function returns {@code null}, a
178 * {@link NullPointerException} is thrown.
179 *
180 * @throws NullPointerException if the function returns {@code null}
181 *
182 * @since 12.0
183 */
184 public abstract <V> Optional<V> transform(Function<? super T, V> function);
185
186 /**
187 * Returns {@code true} if {@code object} is an {@code Optional} instance, and either
188 * the contained references are {@linkplain Object#equals equal} to each other or both
189 * are absent. Note that {@code Optional} instances of differing parameterized types can
190 * be equal.
191 */
192 @Override public abstract boolean equals(@Nullable Object object);
193
194 /**
195 * Returns a hash code for this instance.
196 */
197 @Override public abstract int hashCode();
198
199 /**
200 * Returns a string representation for this instance. The form of this string
201 * representation is unspecified.
202 */
203 @Override public abstract String toString();
204
205 /**
206 * Returns the value of each present instance from the supplied {@code optionals}, in order,
207 * skipping over occurrences of {@link Optional#absent}. Iterators are unmodifiable and are
208 * evaluated lazily.
209 *
210 * @since 11.0
211 */
212 public static <T> Iterable<T> presentInstances(final Iterable<Optional<T>> optionals) {
213 checkNotNull(optionals);
214 return new Iterable<T>() {
215 @Override public Iterator<T> iterator() {
216 return new AbstractIterator<T>() {
217 private final Iterator<Optional<T>> iterator = checkNotNull(optionals.iterator());
218
219 @Override protected T computeNext() {
220 while (iterator.hasNext()) {
221 Optional<T> optional = iterator.next();
222 if (optional.isPresent()) {
223 return optional.get();
224 }
225 }
226 return endOfData();
227 }
228 };
229 };
230 };
231 }
232
233 private static final long serialVersionUID = 0;
234 }