001/*
002 * Copyright (C) 2011 The Guava Authors
003 *
004 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
005 * in compliance with the License. You may obtain a copy of the License at
006 *
007 * http://www.apache.org/licenses/LICENSE-2.0
008 *
009 * Unless required by applicable law or agreed to in writing, software distributed under the License
010 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
011 * or implied. See the License for the specific language governing permissions and limitations under
012 * the License.
013 */
014
015package com.google.common.base;
016
017import static com.google.common.base.Preconditions.checkNotNull;
018import com.google.errorprone.annotations.DoNotMock;
019
020import com.google.common.annotations.Beta;
021import com.google.common.annotations.GwtCompatible;
022import java.io.Serializable;
023import java.util.Iterator;
024import java.util.Set;
025import org.checkerframework.checker.nullness.qual.Nullable;
026
027/**
028 * An immutable object that may contain a non-null reference to another object. Each instance of
029 * this type either contains a non-null reference, or contains nothing (in which case we say that
030 * the reference is "absent"); it is never said to "contain {@code null}".
031 *
032 * <p>A non-null {@code Optional<T>} reference can be used as a replacement for a nullable {@code T}
033 * reference. It allows you to represent "a {@code T} that must be present" and a "a {@code T} that
034 * might be absent" as two distinct types in your program, which can aid clarity.
035 *
036 * <p>Some uses of this class include
037 *
038 * <ul>
039 *   <li>As a method return type, as an alternative to returning {@code null} to indicate that no
040 *       value was available
041 *   <li>To distinguish between "unknown" (for example, not present in a map) and "known to have no
042 *       value" (present in the map, with value {@code Optional.absent()})
043 *   <li>To wrap nullable references for storage in a collection that does not support {@code null}
044 *       (though there are <a
045 *       href="https://github.com/google/guava/wiki/LivingWithNullHostileCollections">several other
046 *       approaches to this</a> that should be considered first)
047 * </ul>
048 *
049 * <p>A common alternative to using this class is to find or create a suitable <a
050 * href="http://en.wikipedia.org/wiki/Null_Object_pattern">null object</a> for the type in question.
051 *
052 * <p>This class is not intended as a direct analogue of any existing "option" or "maybe" construct
053 * from other programming environments, though it may bear some similarities.
054 *
055 * <p><b>Comparison to {@code java.util.Optional} (JDK 8 and higher):</b> A new {@code Optional}
056 * class was added for Java 8. The two classes are extremely similar, but incompatible (they cannot
057 * share a common supertype). <i>All</i> known differences are listed either here or with the
058 * relevant methods below.
059 *
060 * <ul>
061 *   <li>This class is serializable; {@code java.util.Optional} is not.
062 *   <li>{@code java.util.Optional} has the additional methods {@code ifPresent}, {@code filter},
063 *       {@code flatMap}, and {@code orElseThrow}.
064 *   <li>{@code java.util} offers the primitive-specialized versions {@code OptionalInt}, {@code
065 *       OptionalLong} and {@code OptionalDouble}, the use of which is recommended; Guava does not
066 *       have these.
067 * </ul>
068 *
069 * <p><b>There are no plans to deprecate this class in the foreseeable future.</b> However, we do
070 * gently recommend that you prefer the new, standard Java class whenever possible.
071 *
072 * <p>See the Guava User Guide article on <a
073 * href="https://github.com/google/guava/wiki/UsingAndAvoidingNullExplained#optional">using {@code
074 * Optional}</a>.
075 *
076 * @param <T> the type of instance that can be contained. {@code Optional} is naturally covariant on
077 *     this type, so it is safe to cast an {@code Optional<T>} to {@code Optional<S>} for any
078 *     supertype {@code S} of {@code T}.
079 * @author Kurt Alfred Kluever
080 * @author Kevin Bourrillion
081 * @since 10.0
082 */
083@DoNotMock("Use Optional.of(value) or Optional.absent()")
084@GwtCompatible(serializable = true)
085public abstract class Optional<T> implements Serializable {
086  /**
087   * Returns an {@code Optional} instance with no contained reference.
088   *
089   * <p><b>Comparison to {@code java.util.Optional}:</b> this method is equivalent to Java 8's
090   * {@code Optional.empty}.
091   */
092  public static <T> Optional<T> absent() {
093    return Absent.withType();
094  }
095
096  /**
097   * Returns an {@code Optional} instance containing the given non-null reference. To have {@code
098   * null} treated as {@link #absent}, use {@link #fromNullable} instead.
099   *
100   * <p><b>Comparison to {@code java.util.Optional}:</b> no differences.
101   *
102   * @throws NullPointerException if {@code reference} is null
103   */
104  public static <T> Optional<T> of(T reference) {
105    return new Present<T>(checkNotNull(reference));
106  }
107
108  /**
109   * If {@code nullableReference} is non-null, returns an {@code Optional} instance containing that
110   * reference; otherwise returns {@link Optional#absent}.
111   *
112   * <p><b>Comparison to {@code java.util.Optional}:</b> this method is equivalent to Java 8's
113   * {@code Optional.ofNullable}.
114   */
115  public static <T> Optional<T> fromNullable(@Nullable T nullableReference) {
116    return (nullableReference == null) ? Optional.<T>absent() : new Present<T>(nullableReference);
117  }
118
119  /**
120   * Returns the equivalent {@code com.google.common.base.Optional} value to the given {@code
121   * java.util.Optional}, or {@code null} if the argument is null.
122   *
123   * @since 21.0
124   */
125  public static <T> @Nullable Optional<T> fromJavaUtil(
126      java.util.@Nullable Optional<T> javaUtilOptional) {
127    return (javaUtilOptional == null) ? null : fromNullable(javaUtilOptional.orElse(null));
128  }
129
130  /**
131   * Returns the equivalent {@code java.util.Optional} value to the given {@code
132   * com.google.common.base.Optional}, or {@code null} if the argument is null.
133   *
134   * <p>If {@code googleOptional} is known to be non-null, use {@code googleOptional.toJavaUtil()}
135   * instead.
136   *
137   * <p>Unfortunately, the method reference {@code Optional::toJavaUtil} will not work, because it
138   * could refer to either the static or instance version of this method. Write out the lambda
139   * expression {@code o -> Optional.toJavaUtil(o)} instead.
140   *
141   * @since 21.0
142   */
143  public static <T> java.util.@Nullable Optional<T> toJavaUtil(
144      @Nullable Optional<T> googleOptional) {
145    return googleOptional == null ? null : googleOptional.toJavaUtil();
146  }
147
148  /**
149   * Returns the equivalent {@code java.util.Optional} value to this optional.
150   *
151   * <p>Unfortunately, the method reference {@code Optional::toJavaUtil} will not work, because it
152   * could refer to either the static or instance version of this method. Write out the lambda
153   * expression {@code o -> o.toJavaUtil()} instead.
154   *
155   * @since 21.0
156   */
157  public java.util.Optional<T> toJavaUtil() {
158    return java.util.Optional.ofNullable(orNull());
159  }
160
161  Optional() {}
162
163  /**
164   * Returns {@code true} if this holder contains a (non-null) instance.
165   *
166   * <p><b>Comparison to {@code java.util.Optional}:</b> no differences.
167   */
168  public abstract boolean isPresent();
169
170  /**
171   * Returns the contained instance, which must be present. If the instance might be absent, use
172   * {@link #or(Object)} or {@link #orNull} instead.
173   *
174   * <p><b>Comparison to {@code java.util.Optional}:</b> when the value is absent, this method
175   * throws {@link IllegalStateException}, whereas the Java 8 counterpart throws {@link
176   * java.util.NoSuchElementException NoSuchElementException}.
177   *
178   * @throws IllegalStateException if the instance is absent ({@link #isPresent} returns {@code
179   *     false}); depending on this <i>specific</i> exception type (over the more general {@link
180   *     RuntimeException}) is discouraged
181   */
182  public abstract T get();
183
184  /**
185   * Returns the contained instance if it is present; {@code defaultValue} otherwise. If no default
186   * value should be required because the instance is known to be present, use {@link #get()}
187   * instead. For a default value of {@code null}, use {@link #orNull}.
188   *
189   * <p>Note about generics: The signature {@code public T or(T defaultValue)} is overly
190   * restrictive. However, the ideal signature, {@code public <S super T> S or(S)}, is not legal
191   * Java. As a result, some sensible operations involving subtypes are compile errors:
192   *
193   * <pre>{@code
194   * Optional<Integer> optionalInt = getSomeOptionalInt();
195   * Number value = optionalInt.or(0.5); // error
196   *
197   * FluentIterable<? extends Number> numbers = getSomeNumbers();
198   * Optional<? extends Number> first = numbers.first();
199   * Number value = first.or(0.5); // error
200   * }</pre>
201   *
202   * <p>As a workaround, it is always safe to cast an {@code Optional<? extends T>} to {@code
203   * Optional<T>}. Casting either of the above example {@code Optional} instances to {@code
204   * Optional<Number>} (where {@code Number} is the desired output type) solves the problem:
205   *
206   * <pre>{@code
207   * Optional<Number> optionalInt = (Optional) getSomeOptionalInt();
208   * Number value = optionalInt.or(0.5); // fine
209   *
210   * FluentIterable<? extends Number> numbers = getSomeNumbers();
211   * Optional<Number> first = (Optional) numbers.first();
212   * Number value = first.or(0.5); // fine
213   * }</pre>
214   *
215   * <p><b>Comparison to {@code java.util.Optional}:</b> this method is similar to Java 8's {@code
216   * Optional.orElse}, but will not accept {@code null} as a {@code defaultValue} ({@link #orNull}
217   * must be used instead). As a result, the value returned by this method is guaranteed non-null,
218   * which is not the case for the {@code java.util} equivalent.
219   */
220  public abstract T or(T defaultValue);
221
222  /**
223   * Returns this {@code Optional} if it has a value present; {@code secondChoice} otherwise.
224   *
225   * <p><b>Comparison to {@code java.util.Optional}:</b> this method has no equivalent in Java 8's
226   * {@code Optional} class; write {@code thisOptional.isPresent() ? thisOptional : secondChoice}
227   * instead.
228   */
229  public abstract Optional<T> or(Optional<? extends T> secondChoice);
230
231  /**
232   * Returns the contained instance if it is present; {@code supplier.get()} otherwise.
233   *
234   * <p><b>Comparison to {@code java.util.Optional}:</b> this method is similar to Java 8's {@code
235   * Optional.orElseGet}, except when {@code supplier} returns {@code null}. In this case this
236   * method throws an exception, whereas the Java 8 method returns the {@code null} to the caller.
237   *
238   * @throws NullPointerException if this optional's value is absent and the supplier returns {@code
239   *     null}
240   */
241  @Beta
242  public abstract T or(Supplier<? extends T> supplier);
243
244  /**
245   * Returns the contained instance if it is present; {@code null} otherwise. If the instance is
246   * known to be present, use {@link #get()} instead.
247   *
248   * <p><b>Comparison to {@code java.util.Optional}:</b> this method is equivalent to Java 8's
249   * {@code Optional.orElse(null)}.
250   */
251  public abstract @Nullable T orNull();
252
253  /**
254   * Returns an immutable singleton {@link Set} whose only element is the contained instance if it
255   * is present; an empty immutable {@link Set} otherwise.
256   *
257   * <p><b>Comparison to {@code java.util.Optional}:</b> this method has no equivalent in Java 8's
258   * {@code Optional} class. However, this common usage:
259   *
260   * <pre>{@code
261   * for (Foo foo : possibleFoo.asSet()) {
262   *   doSomethingWith(foo);
263   * }
264   * }</pre>
265   *
266   * ... can be replaced with:
267   *
268   * <pre>{@code
269   * possibleFoo.ifPresent(foo -> doSomethingWith(foo));
270   * }</pre>
271   *
272   * <p><b>Java 9 users:</b> some use cases can be written with calls to {@code optional.stream()}.
273   *
274   * @since 11.0
275   */
276  public abstract Set<T> asSet();
277
278  /**
279   * If the instance is present, it is transformed with the given {@link Function}; otherwise,
280   * {@link Optional#absent} is returned.
281   *
282   * <p><b>Comparison to {@code java.util.Optional}:</b> this method is similar to Java 8's {@code
283   * Optional.map}, except when {@code function} returns {@code null}. In this case this method
284   * throws an exception, whereas the Java 8 method returns {@code Optional.absent()}.
285   *
286   * @throws NullPointerException if the function returns {@code null}
287   * @since 12.0
288   */
289  public abstract <V> Optional<V> transform(Function<? super T, V> function);
290
291  /**
292   * Returns {@code true} if {@code object} is an {@code Optional} instance, and either the
293   * contained references are {@linkplain Object#equals equal} to each other or both are absent.
294   * Note that {@code Optional} instances of differing parameterized types can be equal.
295   *
296   * <p><b>Comparison to {@code java.util.Optional}:</b> no differences.
297   */
298  @Override
299  public abstract boolean equals(@Nullable Object object);
300
301  /**
302   * Returns a hash code for this instance.
303   *
304   * <p><b>Comparison to {@code java.util.Optional}:</b> this class leaves the specific choice of
305   * hash code unspecified, unlike the Java 8 equivalent.
306   */
307  @Override
308  public abstract int hashCode();
309
310  /**
311   * Returns a string representation for this instance.
312   *
313   * <p><b>Comparison to {@code java.util.Optional}:</b> this class leaves the specific string
314   * representation unspecified, unlike the Java 8 equivalent.
315   */
316  @Override
317  public abstract String toString();
318
319  /**
320   * Returns the value of each present instance from the supplied {@code optionals}, in order,
321   * skipping over occurrences of {@link Optional#absent}. Iterators are unmodifiable and are
322   * evaluated lazily.
323   *
324   * <p><b>Comparison to {@code java.util.Optional}:</b> this method has no equivalent in Java 8's
325   * {@code Optional} class; use {@code
326   * optionals.stream().filter(Optional::isPresent).map(Optional::get)} instead.
327   *
328   * <p><b>Java 9 users:</b> use {@code optionals.stream().flatMap(Optional::stream)} instead.
329   *
330   * @since 11.0 (generics widened in 13.0)
331   */
332  @Beta
333  public static <T> Iterable<T> presentInstances(
334      final Iterable<? extends Optional<? extends T>> optionals) {
335    checkNotNull(optionals);
336    return new Iterable<T>() {
337      @Override
338      public Iterator<T> iterator() {
339        return new AbstractIterator<T>() {
340          private final Iterator<? extends Optional<? extends T>> iterator =
341              checkNotNull(optionals.iterator());
342
343          @Override
344          protected T computeNext() {
345            while (iterator.hasNext()) {
346              Optional<? extends T> optional = iterator.next();
347              if (optional.isPresent()) {
348                return optional.get();
349              }
350            }
351            return endOfData();
352          }
353        };
354      }
355    };
356  }
357
358  private static final long serialVersionUID = 0;
359}