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.GwtCompatible;
020import com.google.errorprone.annotations.DoNotMock;
021import java.io.Serializable;
022import java.util.Iterator;
023import java.util.Set;
024import javax.annotation.CheckForNull;
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 null}".
030 *
031 * <p>A non-null {@code Optional<T>} reference can be used as a replacement for a nullable {@code T}
032 * reference. It allows you to represent "a {@code T} that must be present" and a "a {@code T} that
033 * might be absent" as two distinct types in your program, which can aid clarity.
034 *
035 * <p>Some uses of this class include
036 *
037 * <ul>
038 *   <li>As a method return type, as an alternative to returning {@code null} to indicate that no
039 *       value was available
040 *   <li>To distinguish between "unknown" (for example, not present in a map) and "known to have no
041 *       value" (present in the map, with value {@code Optional.absent()})
042 *   <li>To wrap nullable references for storage in a collection that does not support {@code null}
043 *       (though there are <a
044 *       href="https://github.com/google/guava/wiki/LivingWithNullHostileCollections">several other
045 *       approaches to this</a> that should be considered first)
046 * </ul>
047 *
048 * <p>A common alternative to using this class is to find or create a suitable <a
049 * href="http://en.wikipedia.org/wiki/Null_Object_pattern">null object</a> for the type in question.
050 *
051 * <p>This class is not intended as a direct analogue of any existing "option" or "maybe" construct
052 * from other programming environments, though it may bear some similarities.
053 *
054 * <p>An instance of this class is serializable if its reference is absent or is a serializable
055 * object.
056 *
057 * <p><b>Comparison to {@code java.util.Optional} (JDK 8 and higher):</b> A new {@code Optional}
058 * class was added for Java 8. The two classes are extremely similar, but incompatible (they cannot
059 * share a common supertype). <i>All</i> known differences are listed either here or with the
060 * relevant methods below.
061 *
062 * <ul>
063 *   <li>This class is serializable; {@code java.util.Optional} is not.
064 *   <li>{@code java.util.Optional} has the additional methods {@code ifPresent}, {@code filter},
065 *       {@code flatMap}, and {@code orElseThrow}.
066 *   <li>{@code java.util} offers the primitive-specialized versions {@code OptionalInt}, {@code
067 *       OptionalLong} and {@code OptionalDouble}, the use of which is recommended; Guava does not
068 *       have these.
069 * </ul>
070 *
071 * <p><b>There are no plans to deprecate this class in the foreseeable future.</b> However, we do
072 * gently recommend that you prefer the new, standard Java class whenever possible.
073 *
074 * <p>See the Guava User Guide article on <a
075 * href="https://github.com/google/guava/wiki/UsingAndAvoidingNullExplained#optional">using {@code
076 * Optional}</a>.
077 *
078 * @param <T> the type of instance that can be contained. {@code Optional} is naturally covariant on
079 *     this type, so it is safe to cast an {@code Optional<T>} to {@code Optional<S>} for any
080 *     supertype {@code S} of {@code T}.
081 * @author Kurt Alfred Kluever
082 * @author Kevin Bourrillion
083 * @since 10.0
084 */
085@DoNotMock("Use Optional.of(value) or Optional.absent()")
086@GwtCompatible(serializable = true)
087@ElementTypesAreNonnullByDefault
088public abstract class Optional<T> implements Serializable {
089  /**
090   * Returns an {@code Optional} instance with no contained reference.
091   *
092   * <p><b>Comparison to {@code java.util.Optional}:</b> this method is equivalent to Java 8's
093   * {@code Optional.empty}.
094   */
095  public static <T> Optional<T> absent() {
096    return Absent.withType();
097  }
098
099  /**
100   * Returns an {@code Optional} instance containing the given non-null reference. To have {@code
101   * null} treated as {@link #absent}, use {@link #fromNullable} instead.
102   *
103   * <p><b>Comparison to {@code java.util.Optional}:</b> no differences.
104   *
105   * @throws NullPointerException if {@code reference} is null
106   */
107  public static <T> Optional<T> of(T reference) {
108    return new Present<T>(checkNotNull(reference));
109  }
110
111  /**
112   * If {@code nullableReference} is non-null, returns an {@code Optional} instance containing that
113   * reference; otherwise returns {@link Optional#absent}.
114   *
115   * <p><b>Comparison to {@code java.util.Optional}:</b> this method is equivalent to Java 8's
116   * {@code Optional.ofNullable}.
117   */
118  public static <T> Optional<T> fromNullable(@CheckForNull T nullableReference) {
119    return (nullableReference == null) ? Optional.<T>absent() : new Present<T>(nullableReference);
120  }
121
122  /**
123   * Returns the equivalent {@code com.google.common.base.Optional} value to the given {@code
124   * java.util.Optional}, or {@code null} if the argument is null.
125   *
126   * @since 21.0
127   */
128  @CheckForNull
129  public static <T> Optional<T> fromJavaUtil(@CheckForNull java.util.Optional<T> javaUtilOptional) {
130    return (javaUtilOptional == null) ? null : fromNullable(javaUtilOptional.orElse(null));
131  }
132
133  /**
134   * Returns the equivalent {@code java.util.Optional} value to the given {@code
135   * com.google.common.base.Optional}, or {@code null} if the argument is null.
136   *
137   * <p>If {@code googleOptional} is known to be non-null, use {@code googleOptional.toJavaUtil()}
138   * instead.
139   *
140   * <p>Unfortunately, the method reference {@code Optional::toJavaUtil} will not work, because it
141   * could refer to either the static or instance version of this method. Write out the lambda
142   * expression {@code o -> Optional.toJavaUtil(o)} instead.
143   *
144   * @since 21.0
145   */
146  @CheckForNull
147  public static <T> java.util.Optional<T> toJavaUtil(@CheckForNull Optional<T> googleOptional) {
148    return googleOptional == null ? null : googleOptional.toJavaUtil();
149  }
150
151  /**
152   * Returns the equivalent {@code java.util.Optional} value to this optional.
153   *
154   * <p>Unfortunately, the method reference {@code Optional::toJavaUtil} will not work, because it
155   * could refer to either the static or instance version of this method. Write out the lambda
156   * expression {@code o -> o.toJavaUtil()} instead.
157   *
158   * @since 21.0
159   */
160  public java.util.Optional<T> toJavaUtil() {
161    return java.util.Optional.ofNullable(orNull());
162  }
163
164  Optional() {}
165
166  /**
167   * Returns {@code true} if this holder contains a (non-null) instance.
168   *
169   * <p><b>Comparison to {@code java.util.Optional}:</b> no differences.
170   */
171  public abstract boolean isPresent();
172
173  /**
174   * Returns the contained instance, which must be present. If the instance might be absent, use
175   * {@link #or(Object)} or {@link #orNull} instead.
176   *
177   * <p><b>Comparison to {@code java.util.Optional}:</b> when the value is absent, this method
178   * throws {@link IllegalStateException}, whereas the {@code java.util} counterpart throws {@link
179   * java.util.NoSuchElementException NoSuchElementException}.
180   *
181   * @throws IllegalStateException if the instance is absent ({@link #isPresent} returns {@code
182   *     false}); depending on this <i>specific</i> exception type (over the more general {@link
183   *     RuntimeException}) is discouraged
184   */
185  public abstract T get();
186
187  /**
188   * Returns the contained instance if it is present; {@code defaultValue} otherwise. If no default
189   * value should be required because the instance is known to be present, use {@link #get()}
190   * instead. For a default value of {@code null}, use {@link #orNull}.
191   *
192   * <p>Note about generics: The signature {@code public T or(T defaultValue)} is overly
193   * restrictive. However, the ideal signature, {@code public <S super T> S or(S)}, is not legal
194   * Java. As a result, some sensible operations involving subtypes are compile errors:
195   *
196   * <pre>{@code
197   * Optional<Integer> optionalInt = getSomeOptionalInt();
198   * Number value = optionalInt.or(0.5); // error
199   *
200   * FluentIterable<? extends Number> numbers = getSomeNumbers();
201   * Optional<? extends Number> first = numbers.first();
202   * Number value = first.or(0.5); // error
203   * }</pre>
204   *
205   * <p>As a workaround, it is always safe to cast an {@code Optional<? extends T>} to {@code
206   * Optional<T>}. Casting either of the above example {@code Optional} instances to {@code
207   * Optional<Number>} (where {@code Number} is the desired output type) solves the problem:
208   *
209   * <pre>{@code
210   * Optional<Number> optionalInt = (Optional) getSomeOptionalInt();
211   * Number value = optionalInt.or(0.5); // fine
212   *
213   * FluentIterable<? extends Number> numbers = getSomeNumbers();
214   * Optional<Number> first = (Optional) numbers.first();
215   * Number value = first.or(0.5); // fine
216   * }</pre>
217   *
218   * <p><b>Comparison to {@code java.util.Optional}:</b> this method is similar to Java 8's {@code
219   * Optional.orElse}, but will not accept {@code null} as a {@code defaultValue} ({@link #orNull}
220   * must be used instead). As a result, the value returned by this method is guaranteed non-null,
221   * which is not the case for the {@code java.util} equivalent.
222   */
223  public abstract T or(T defaultValue);
224
225  /**
226   * Returns this {@code Optional} if it has a value present; {@code secondChoice} otherwise.
227   *
228   * <p><b>Comparison to {@code java.util.Optional}:</b> this method has no equivalent in Java 8's
229   * {@code Optional} class; write {@code thisOptional.isPresent() ? thisOptional : secondChoice}
230   * instead.
231   */
232  public abstract Optional<T> or(Optional<? extends T> secondChoice);
233
234  /**
235   * Returns the contained instance if it is present; {@code supplier.get()} otherwise.
236   *
237   * <p><b>Comparison to {@code java.util.Optional}:</b> this method is similar to Java 8's {@code
238   * Optional.orElseGet}, except when {@code supplier} returns {@code null}. In this case this
239   * method throws an exception, whereas the Java 8+ method returns the {@code null} to the caller.
240   *
241   * @throws NullPointerException if this optional's value is absent and the supplier returns {@code
242   *     null}
243   */
244  public abstract T or(Supplier<? extends T> supplier);
245
246  /**
247   * Returns the contained instance if it is present; {@code null} otherwise. If the instance is
248   * known to be present, use {@link #get()} instead.
249   *
250   * <p><b>Comparison to {@code java.util.Optional}:</b> this method is equivalent to Java 8's
251   * {@code Optional.orElse(null)}.
252   */
253  @CheckForNull
254  public abstract T orNull();
255
256  /**
257   * Returns an immutable singleton {@link Set} whose only element is the contained instance if it
258   * is present; an empty immutable {@link Set} otherwise.
259   *
260   * <p><b>Comparison to {@code java.util.Optional}:</b> this method has no equivalent in Java 8's
261   * {@code Optional} class. However, this common usage:
262   *
263   * <pre>{@code
264   * for (Foo foo : possibleFoo.asSet()) {
265   *   doSomethingWith(foo);
266   * }
267   * }</pre>
268   *
269   * ... can be replaced with:
270   *
271   * <pre>{@code
272   * possibleFoo.ifPresent(foo -> doSomethingWith(foo));
273   * }</pre>
274   *
275   * <p><b>Java 9 users:</b> some use cases can be written with calls to {@code optional.stream()}.
276   *
277   * @since 11.0
278   */
279  public abstract Set<T> asSet();
280
281  /**
282   * If the instance is present, it is transformed with the given {@link Function}; otherwise,
283   * {@link Optional#absent} is returned.
284   *
285   * <p><b>Comparison to {@code java.util.Optional}:</b> this method is similar to Java 8's {@code
286   * Optional.map}, except when {@code function} returns {@code null}. In this case this method
287   * throws an exception, whereas the Java 8+ method returns {@code Optional.absent()}.
288   *
289   * @throws NullPointerException if the function returns {@code null}
290   * @since 12.0
291   */
292  public abstract <V> Optional<V> transform(Function<? super T, V> function);
293
294  /**
295   * Returns {@code true} if {@code object} is an {@code Optional} instance, and either the
296   * contained references are {@linkplain Object#equals equal} to each other or both are absent.
297   * Note that {@code Optional} instances of differing parameterized types can be equal.
298   *
299   * <p><b>Comparison to {@code java.util.Optional}:</b> no differences.
300   */
301  @Override
302  public abstract boolean equals(@CheckForNull Object object);
303
304  /**
305   * Returns a hash code for this instance.
306   *
307   * <p><b>Comparison to {@code java.util.Optional}:</b> this class leaves the specific choice of
308   * hash code unspecified, unlike the Java 8+ equivalent.
309   */
310  @Override
311  public abstract int hashCode();
312
313  /**
314   * Returns a string representation for this instance.
315   *
316   * <p><b>Comparison to {@code java.util.Optional}:</b> this class leaves the specific string
317   * representation unspecified, unlike the Java 8+ equivalent.
318   */
319  @Override
320  public abstract String toString();
321
322  /**
323   * Returns the value of each present instance from the supplied {@code optionals}, in order,
324   * skipping over occurrences of {@link Optional#absent}. Iterators are unmodifiable and are
325   * evaluated lazily.
326   *
327   * <p><b>Comparison to {@code java.util.Optional}:</b> this method has no equivalent in Java 8's
328   * {@code Optional} class; use {@code
329   * optionals.stream().filter(Optional::isPresent).map(Optional::get)} instead.
330   *
331   * <p><b>Java 9 users:</b> use {@code optionals.stream().flatMap(Optional::stream)} instead.
332   *
333   * @since 11.0 (generics widened in 13.0)
334   */
335  public static <T> Iterable<T> presentInstances(
336      final Iterable<? extends Optional<? extends T>> optionals) {
337    checkNotNull(optionals);
338    return new Iterable<T>() {
339      @Override
340      public Iterator<T> iterator() {
341        return new AbstractIterator<T>() {
342          private final Iterator<? extends Optional<? extends T>> iterator =
343              checkNotNull(optionals.iterator());
344
345          @Override
346          @CheckForNull
347          protected T computeNext() {
348            while (iterator.hasNext()) {
349              Optional<? extends T> optional = iterator.next();
350              if (optional.isPresent()) {
351                return optional.get();
352              }
353            }
354            return endOfData();
355          }
356        };
357      }
358    };
359  }
360
361  private static final long serialVersionUID = 0;
362}