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;
018
019import com.google.common.annotations.Beta;
020import com.google.common.annotations.GwtCompatible;
021import java.io.Serializable;
022import java.util.Iterator;
023import java.util.Set;
024import javax.annotation.Nullable;
025
026/**
027 * An immutable object that may contain a non-null reference to another object. Each instance of
028 * this type either contains a non-null reference, or contains nothing (in which case we say that
029 * the reference is "absent"); it is never said to "contain {@code
030 * 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
034 * "a {@code T} that 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
045 *     <a 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
050 * <a href="http://en.wikipedia.org/wiki/Null_Object_pattern">null object</a> for the type in
051 * question.
052 *
053 * <p>This class is not intended as a direct analogue of any existing "option" or "maybe" construct
054 * from other programming environments, though it may bear some similarities.
055 *
056 * <p><b>Comparison to {@code java.util.Optional} (JDK 8 and higher):</b> A new {@code Optional}
057 * class was added for Java 8. The two classes are extremely similar, but incompatible (they cannot
058 * share a common supertype). <i>All</i> known differences are listed either here or with the
059 * relevant methods below.
060 *
061 * <ul>
062 * <li>This class is serializable; {@code java.util.Optional} is not.
063 * <li>{@code java.util.Optional} has the additional methods {@code ifPresent}, {@code filter},
064 *     {@code flatMap}, and {@code orElseThrow}.
065 * <li>{@code java.util} offers the primitive-specialized versions {@code OptionalInt}, {@code
066 *     OptionalLong} and {@code OptionalDouble}, the use of which is recommended; Guava does not
067 *     have these.
068 * </ul>
069 *
070 * <p><b>There are no plans to deprecate this class in the foreseeable future.</b> However, we do
071 * gently recommend that you prefer the new, standard Java class whenever possible.
072 *
073 * <p>See the Guava User Guide article on
074 * <a href="https://github.com/google/guava/wiki/UsingAndAvoidingNullExplained#optional">using
075 * {@code Optional}</a>.
076 *
077 * @param <T> the type of instance that can be contained. {@code Optional} is naturally covariant on
078 *     this type, so it is safe to cast an {@code Optional<T>} to {@code
079 *     Optional<S>} for any supertype {@code S} of {@code T}.
080 * @author Kurt Alfred Kluever
081 * @author Kevin Bourrillion
082 * @since 10.0
083 */
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)
117        ? Optional.<T>absent()
118        : new Present<T>(nullableReference);
119  }
120
121  /**
122   * Returns the equivalent {@code com.google.common.base.Optional} value to the given {@code
123   * java.util.Optional}, or {@code null} if the argument is null.
124   *
125   * @since 21.0
126   */
127  @Nullable
128  public static <T> Optional<T> fromJavaUtil(@Nullable java.util.Optional<T> javaUtilOptional) {
129    return (javaUtilOptional == null) ? null : fromNullable(javaUtilOptional.orElse(null));
130  }
131
132  /**
133   * Returns the equivalent {@code java.util.Optional} value to the given {@code
134   * com.google.common.base.Optional}, or {@code null} if the argument is null.
135   *
136   * <p>If {@code googleOptional} is known to be non-null, use {@code googleOptional.toJavaUtil()}
137   * instead.
138   *
139   * <p>Unfortunately, the method reference {@code Optional::toJavaUtil} will not work, because it
140   * could refer to either the static or instance version of this method. Write out the lambda
141   * expression {@code o -> Optional.toJavaUtil(o)} instead.
142   *
143   * @since 21.0
144   */
145  @Nullable
146  public static <T> java.util.Optional<T> toJavaUtil(@Nullable Optional<T> googleOptional) {
147    return googleOptional == null ? null : googleOptional.toJavaUtil();
148  }
149
150  Optional() {}
151
152  /**
153   * Returns {@code true} if this holder contains a (non-null) instance.
154   *
155   * <p><b>Comparison to {@code java.util.Optional}:</b> no differences.
156   */
157  public abstract boolean isPresent();
158
159  /**
160   * Returns the contained instance, which must be present. If the instance might be absent, use
161   * {@link #or(Object)} or {@link #orNull} instead.
162   *
163   * <p><b>Comparison to {@code java.util.Optional}:</b> when the value is absent, this method
164   * throws {@link IllegalStateException}, whereas the Java 8 counterpart throws
165   * {@link NoSuchElementException}.
166   *
167   * @throws IllegalStateException if the instance is absent ({@link #isPresent} returns
168   *     {@code false}); depending on this <i>specific</i> exception type (over the more general
169   *     {@link RuntimeException}) is discouraged
170   */
171  public abstract T get();
172
173  /**
174   * Returns the contained instance if it is present; {@code defaultValue} otherwise. If no default
175   * value should be required because the instance is known to be present, use {@link #get()}
176   * instead. For a default value of {@code null}, use {@link #orNull}.
177   *
178   * <p>Note about generics: The signature {@code public T or(T defaultValue)} is overly
179   * restrictive. However, the ideal signature, {@code public <S super T> S or(S)}, is not legal
180   * Java. As a result, some sensible operations involving subtypes are compile errors:
181   * <pre>   {@code
182   *
183   *   Optional<Integer> optionalInt = getSomeOptionalInt();
184   *   Number value = optionalInt.or(0.5); // error
185   *
186   *   FluentIterable<? extends Number> numbers = getSomeNumbers();
187   *   Optional<? extends Number> first = numbers.first();
188   *   Number value = first.or(0.5); // error}</pre>
189   *
190   * <p>As a workaround, it is always safe to cast an {@code Optional<? extends T>} to {@code
191   * Optional<T>}. Casting either of the above example {@code Optional} instances to {@code
192   * Optional<Number>} (where {@code Number} is the desired output type) solves the problem:
193   * <pre>   {@code
194   *
195   *   Optional<Number> optionalInt = (Optional) getSomeOptionalInt();
196   *   Number value = optionalInt.or(0.5); // fine
197   *
198   *   FluentIterable<? extends Number> numbers = getSomeNumbers();
199   *   Optional<Number> first = (Optional) numbers.first();
200   *   Number value = first.or(0.5); // fine}</pre>
201   *
202   * <p><b>Comparison to {@code java.util.Optional}:</b> this method is similar to Java 8's
203   * {@code Optional.orElse}, but will not accept {@code null} as a {@code defaultValue}
204   * ({@link #orNull} must be used instead). As a result, the value returned by this method is
205   * guaranteed non-null, which is not the case for the {@code java.util} equivalent.
206   */
207  public abstract T or(T defaultValue);
208
209  /**
210   * Returns this {@code Optional} if it has a value present; {@code secondChoice} otherwise.
211   *
212   * <p><b>Comparison to {@code java.util.Optional}:</b> this method has no equivalent in Java 8's
213   * {@code Optional} class; write {@code thisOptional.isPresent() ? thisOptional : secondChoice}
214   * instead.
215   */
216  public abstract Optional<T> or(Optional<? extends T> secondChoice);
217
218  /**
219   * Returns the contained instance if it is present; {@code supplier.get()} otherwise.
220   *
221   * <p><b>Comparison to {@code java.util.Optional}:</b> this method is similar to Java 8's
222   * {@code Optional.orElseGet}, except when {@code supplier} returns {@code null}. In this case
223   * this method throws an exception, whereas the Java 8 method returns the {@code null} to the
224   * caller.
225   *
226   * @throws NullPointerException if this optional's value is absent and the supplier returns
227   *     {@code null}
228   */
229  @Beta
230  public abstract T or(Supplier<? extends T> supplier);
231
232  /**
233   * Returns the contained instance if it is present; {@code null} otherwise. If the instance is
234   * known to be present, use {@link #get()} instead.
235   *
236   * <p><b>Comparison to {@code java.util.Optional}:</b> this method is equivalent to Java 8's
237   * {@code Optional.orElse(null)}.
238   */
239  @Nullable
240  public abstract T orNull();
241
242  /**
243   * Returns an immutable singleton {@link Set} whose only element is the contained instance if it
244   * is present; an empty immutable {@link Set} otherwise.
245   *
246   * <p><b>Comparison to {@code java.util.Optional}:</b> this method has no equivalent in Java 8's
247   * {@code Optional} class. However, this common usage: <pre>   {@code
248   *
249   *   for (Foo foo : possibleFoo.asSet()) {
250   *     doSomethingWith(foo);
251   *   }}</pre>
252   *
253   * ... can be replaced with: <pre>   {@code
254   *
255   *   possibleFoo.ifPresent(foo -> doSomethingWith(foo));}</pre>
256   *
257   * @since 11.0
258   */
259  public abstract Set<T> asSet();
260
261  /**
262   * If the instance is present, it is transformed with the given {@link Function}; otherwise,
263   * {@link Optional#absent} is returned.
264   *
265   * <p><b>Comparison to {@code java.util.Optional}:</b> this method is similar to Java 8's
266   * {@code Optional.map}, except when {@code function} returns {@code null}. In this case this
267   * method throws an exception, whereas the Java 8 method returns {@code Optional.absent()}.
268   *
269   * @throws NullPointerException if the function returns {@code null}
270   * @since 12.0
271   */
272  public abstract <V> Optional<V> transform(Function<? super T, V> function);
273
274  /**
275   * Returns the equivalent {@code java.util.Optional} value to this optional.
276   *
277   * <p>Unfortunately, the method reference {@code Optional::toJavaUtil} will not work, because it
278   * could refer to either the static or instance version of this method. Write out the lambda
279   * expression {@code o -> o.toJavaUtil()} instead.
280   *
281   * @since 21.0
282   */
283  public java.util.Optional<T> toJavaUtil() {
284    return java.util.Optional.ofNullable(orNull());
285  }
286
287  /**
288   * Returns {@code true} if {@code object} is an {@code Optional} instance, and either the
289   * contained references are {@linkplain Object#equals equal} to each other or both are absent.
290   * Note that {@code Optional} instances of differing parameterized types can be equal.
291   *
292   * <p><b>Comparison to {@code java.util.Optional}:</b> no differences.
293   */
294  @Override
295  public abstract boolean equals(@Nullable Object object);
296
297  /**
298   * Returns a hash code for this instance.
299   *
300   * <p><b>Comparison to {@code java.util.Optional}:</b> this class leaves the specific choice of
301   * hash code unspecified, unlike the Java 8 equivalent.
302   */
303  @Override
304  public abstract int hashCode();
305
306  /**
307   * Returns a string representation for this instance.
308   *
309   * <p><b>Comparison to {@code java.util.Optional}:</b> this class leaves the specific string
310   * representation unspecified, unlike the Java 8 equivalent.
311   */
312  @Override
313  public abstract String toString();
314
315  /**
316   * Returns the value of each present instance from the supplied {@code optionals}, in order,
317   * skipping over occurrences of {@link Optional#absent}. Iterators are unmodifiable and are
318   * evaluated lazily.
319   *
320   * <p><b>Comparison to {@code java.util.Optional}:</b> this method has no equivalent in Java 8's
321   * {@code Optional} class; use
322   * {@code optionals.stream().filter(Optional::isPresent).map(Optional::get)} instead.
323   *
324   * @since 11.0 (generics widened in 13.0)
325   */
326  @Beta
327  public static <T> Iterable<T> presentInstances(
328      final Iterable<? extends Optional<? extends T>> optionals) {
329    checkNotNull(optionals);
330    return new Iterable<T>() {
331      @Override
332      public Iterator<T> iterator() {
333        return new AbstractIterator<T>() {
334          private final Iterator<? extends Optional<? extends T>> iterator =
335              checkNotNull(optionals.iterator());
336
337          @Override
338          protected T computeNext() {
339            while (iterator.hasNext()) {
340              Optional<? extends T> optional = iterator.next();
341              if (optional.isPresent()) {
342                return optional.get();
343              }
344            }
345            return endOfData();
346          }
347        };
348      }
349    };
350  }
351
352  private static final long serialVersionUID = 0;
353}