1 /*
2 * Copyright (c) 2012, 2022, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation. Oracle designates this
8 * particular file as subject to the "Classpath" exception as provided
9 * by Oracle in the LICENSE file that accompanied this code.
10 *
11 * This code is distributed in the hope that it will be useful, but WITHOUT
12 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
13 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
14 * version 2 for more details (a copy is included in the LICENSE file that
15 * accompanied this code).
16 *
17 * You should have received a copy of the GNU General Public License version
18 * 2 along with this work; if not, write to the Free Software Foundation,
19 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
20 *
21 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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23 * questions.
24 */
25 package java.util;
26
27 import java.util.function.Consumer;
28 import java.util.function.Function;
29 import java.util.function.Predicate;
30 import java.util.function.Supplier;
31 import java.util.stream.Stream;
32
33 /**
34 * A container object which may or may not contain a non-{@code null} value.
35 * If a value is present, {@code isPresent()} returns {@code true}. If no
36 * value is present, the object is considered <i>empty</i> and
37 * {@code isPresent()} returns {@code false}.
38 *
39 * <p>Additional methods that depend on the presence or absence of a contained
40 * value are provided, such as {@link #orElse(Object) orElse()}
41 * (returns a default value if no value is present) and
42 * {@link #ifPresent(Consumer) ifPresent()} (performs an
43 * action if a value is present).
44 *
45 * <p>This is a <a href="{@docRoot}/java.base/java/lang/doc-files/ValueBased.html">value-based</a>
46 * class; programmers should treat instances that are {@linkplain #equals(Object) equal}
47 * as interchangeable and should not use instances for synchronization, mutexes, or
48 * with {@linkplain java.lang.ref.Reference object references}.
49 *
50 * <div class="preview-block">
51 * <div class="preview-comment">
52 * When preview features are enabled, {@code Optional} is a {@linkplain Class#isValue value class}.
53 * Use of value class instances for synchronization, mutexes, or with
54 * {@linkplain java.lang.ref.Reference object references} result in
55 * {@link IdentityException}.
56 * </div>
57 * </div>
58 *
59 * @apiNote
60 * {@code Optional} is primarily intended for use as a method return type where
61 * there is a clear need to represent "no result," and where using {@code null}
62 * is likely to cause errors. A variable whose type is {@code Optional} should
63 * never itself be {@code null}; it should always point to an {@code Optional}
64 * instance.
65 *
66 * @param <T> the type of value
67 * @since 1.8
68 */
69 @jdk.internal.MigratedValueClass
70 @jdk.internal.ValueBased
71 public final class Optional<T> {
72 /**
73 * Common instance for {@code empty()}.
74 */
75 private static final Optional<?> EMPTY = new Optional<>(null);
76
77 /**
78 * If non-null, the value; if null, indicates no value is present
79 */
80 private final T value;
81
82 /**
83 * Returns an empty {@code Optional} instance. No value is present for this
84 * {@code Optional}.
85 *
86 * @apiNote
87 * Though it may be tempting to do so, avoid testing if an object is empty
88 * by comparing with {@code ==} or {@code !=} against instances returned by
89 * {@code Optional.empty()}. There is no guarantee that it is a singleton.
90 * Instead, use {@link #isEmpty()} or {@link #isPresent()}.
91 *
92 * @param <T> The type of the non-existent value
93 * @return an empty {@code Optional}
94 */
95 public static<T> Optional<T> empty() {
96 @SuppressWarnings("unchecked")
97 Optional<T> t = (Optional<T>) EMPTY;
98 return t;
99 }
100
101 /**
102 * Constructs an instance with the described value.
103 *
104 * @param value the value to describe; it's the caller's responsibility to
105 * ensure the value is non-{@code null} unless creating the singleton
106 * instance returned by {@code empty()}.
107 */
108 private Optional(T value) {
109 this.value = value;
110 }
111
112 /**
113 * Returns an {@code Optional} describing the given non-{@code null}
114 * value.
115 *
116 * @param value the value to describe, which must be non-{@code null}
117 * @param <T> the type of the value
118 * @return an {@code Optional} with the value present
119 * @throws NullPointerException if value is {@code null}
120 */
121 public static <T> Optional<T> of(T value) {
122 return new Optional<>(Objects.requireNonNull(value));
123 }
124
125 /**
126 * Returns an {@code Optional} describing the given value, if
127 * non-{@code null}, otherwise returns an empty {@code Optional}.
128 *
129 * @param value the possibly-{@code null} value to describe
130 * @param <T> the type of the value
131 * @return an {@code Optional} with a present value if the specified value
132 * is non-{@code null}, otherwise an empty {@code Optional}
133 */
134 @SuppressWarnings("unchecked")
135 public static <T> Optional<T> ofNullable(T value) {
136 return value == null ? (Optional<T>) EMPTY
137 : new Optional<>(value);
138 }
139
140 /**
141 * If a value is present, returns the value, otherwise throws
142 * {@code NoSuchElementException}.
143 *
144 * @apiNote
145 * The preferred alternative to this method is {@link #orElseThrow()}.
146 *
147 * @return the non-{@code null} value described by this {@code Optional}
148 * @throws NoSuchElementException if no value is present
149 */
150 public T get() {
151 if (value == null) {
152 throw new NoSuchElementException("No value present");
153 }
154 return value;
155 }
156
157 /**
158 * If a value is present, returns {@code true}, otherwise {@code false}.
159 *
160 * @return {@code true} if a value is present, otherwise {@code false}
161 */
162 public boolean isPresent() {
163 return value != null;
164 }
165
166 /**
167 * If a value is not present, returns {@code true}, otherwise
168 * {@code false}.
169 *
170 * @return {@code true} if a value is not present, otherwise {@code false}
171 * @since 11
172 */
173 public boolean isEmpty() {
174 return value == null;
175 }
176
177 /**
178 * If a value is present, performs the given action with the value,
179 * otherwise does nothing.
180 *
181 * @param action the action to be performed, if a value is present
182 * @throws NullPointerException if value is present and the given action is
183 * {@code null}
184 */
185 public void ifPresent(Consumer<? super T> action) {
186 if (value != null) {
187 action.accept(value);
188 }
189 }
190
191 /**
192 * If a value is present, performs the given action with the value,
193 * otherwise performs the given empty-based action.
194 *
195 * @param action the action to be performed, if a value is present
196 * @param emptyAction the empty-based action to be performed, if no value is
197 * present
198 * @throws NullPointerException if a value is present and the given action
199 * is {@code null}, or no value is present and the given empty-based
200 * action is {@code null}.
201 * @since 9
202 */
203 public void ifPresentOrElse(Consumer<? super T> action, Runnable emptyAction) {
204 if (value != null) {
205 action.accept(value);
206 } else {
207 emptyAction.run();
208 }
209 }
210
211 /**
212 * If a value is present, and the value matches the given predicate,
213 * returns an {@code Optional} describing the value, otherwise returns an
214 * empty {@code Optional}.
215 *
216 * @param predicate the predicate to apply to a value, if present
217 * @return an {@code Optional} describing the value of this
218 * {@code Optional}, if a value is present and the value matches the
219 * given predicate, otherwise an empty {@code Optional}
220 * @throws NullPointerException if the predicate is {@code null}
221 */
222 public Optional<T> filter(Predicate<? super T> predicate) {
223 Objects.requireNonNull(predicate);
224 if (isEmpty()) {
225 return this;
226 } else {
227 return predicate.test(value) ? this : empty();
228 }
229 }
230
231 /**
232 * If a value is present, returns an {@code Optional} describing (as if by
233 * {@link #ofNullable}) the result of applying the given mapping function to
234 * the value, otherwise returns an empty {@code Optional}.
235 *
236 * <p>If the mapping function returns a {@code null} result then this method
237 * returns an empty {@code Optional}.
238 *
239 * @apiNote
240 * This method supports post-processing on {@code Optional} values, without
241 * the need to explicitly check for a return status. For example, the
242 * following code traverses a stream of URIs, selects one that has not
243 * yet been processed, and creates a path from that URI, returning
244 * an {@code Optional<Path>}:
245 *
246 * <pre>{@code
247 * Optional<Path> p =
248 * uris.stream().filter(uri -> !isProcessedYet(uri))
249 * .findFirst()
250 * .map(Paths::get);
251 * }</pre>
252 *
253 * Here, {@code findFirst} returns an {@code Optional<URI>}, and then
254 * {@code map} returns an {@code Optional<Path>} for the desired
255 * URI if one exists.
256 *
257 * @param mapper the mapping function to apply to a value, if present
258 * @param <U> The type of the value returned from the mapping function
259 * @return an {@code Optional} describing the result of applying a mapping
260 * function to the value of this {@code Optional}, if a value is
261 * present, otherwise an empty {@code Optional}
262 * @throws NullPointerException if the mapping function is {@code null}
263 */
264 public <U> Optional<U> map(Function<? super T, ? extends U> mapper) {
265 Objects.requireNonNull(mapper);
266 if (isEmpty()) {
267 return empty();
268 } else {
269 return Optional.ofNullable(mapper.apply(value));
270 }
271 }
272
273 /**
274 * If a value is present, returns the result of applying the given
275 * {@code Optional}-bearing mapping function to the value, otherwise returns
276 * an empty {@code Optional}.
277 *
278 * <p>This method is similar to {@link #map(Function)}, but the mapping
279 * function is one whose result is already an {@code Optional}, and if
280 * invoked, {@code flatMap} does not wrap it within an additional
281 * {@code Optional}.
282 *
283 * @param <U> The type of value of the {@code Optional} returned by the
284 * mapping function
285 * @param mapper the mapping function to apply to a value, if present
286 * @return the result of applying an {@code Optional}-bearing mapping
287 * function to the value of this {@code Optional}, if a value is
288 * present, otherwise an empty {@code Optional}
289 * @throws NullPointerException if the mapping function is {@code null} or
290 * returns a {@code null} result
291 */
292 public <U> Optional<U> flatMap(Function<? super T, ? extends Optional<? extends U>> mapper) {
293 Objects.requireNonNull(mapper);
294 if (isEmpty()) {
295 return empty();
296 } else {
297 @SuppressWarnings("unchecked")
298 Optional<U> r = (Optional<U>) mapper.apply(value);
299 return Objects.requireNonNull(r);
300 }
301 }
302
303 /**
304 * If a value is present, returns an {@code Optional} describing the value,
305 * otherwise returns an {@code Optional} produced by the supplying function.
306 *
307 * @param supplier the supplying function that produces an {@code Optional}
308 * to be returned
309 * @return returns an {@code Optional} describing the value of this
310 * {@code Optional}, if a value is present, otherwise an
311 * {@code Optional} produced by the supplying function.
312 * @throws NullPointerException if the supplying function is {@code null} or
313 * produces a {@code null} result
314 * @since 9
315 */
316 public Optional<T> or(Supplier<? extends Optional<? extends T>> supplier) {
317 Objects.requireNonNull(supplier);
318 if (isPresent()) {
319 return this;
320 } else {
321 @SuppressWarnings("unchecked")
322 Optional<T> r = (Optional<T>) supplier.get();
323 return Objects.requireNonNull(r);
324 }
325 }
326
327 /**
328 * If a value is present, returns a sequential {@link Stream} containing
329 * only that value, otherwise returns an empty {@code Stream}.
330 *
331 * @apiNote
332 * This method can be used to transform a {@code Stream} of optional
333 * elements to a {@code Stream} of present value elements:
334 * <pre>{@code
335 * Stream<Optional<T>> os = ..
336 * Stream<T> s = os.flatMap(Optional::stream)
337 * }</pre>
338 *
339 * @return the optional value as a {@code Stream}
340 * @since 9
341 */
342 public Stream<T> stream() {
343 if (isEmpty()) {
344 return Stream.empty();
345 } else {
346 return Stream.of(value);
347 }
348 }
349
350 /**
351 * If a value is present, returns the value, otherwise returns
352 * {@code other}.
353 *
354 * @param other the value to be returned, if no value is present.
355 * May be {@code null}.
356 * @return the value, if present, otherwise {@code other}
357 */
358 public T orElse(T other) {
359 return value != null ? value : other;
360 }
361
362 /**
363 * If a value is present, returns the value, otherwise returns the result
364 * produced by the supplying function.
365 *
366 * @param supplier the supplying function that produces a value to be returned
367 * @return the value, if present, otherwise the result produced by the
368 * supplying function
369 * @throws NullPointerException if no value is present and the supplying
370 * function is {@code null}
371 */
372 public T orElseGet(Supplier<? extends T> supplier) {
373 return value != null ? value : supplier.get();
374 }
375
376 /**
377 * If a value is present, returns the value, otherwise throws
378 * {@code NoSuchElementException}.
379 *
380 * @return the non-{@code null} value described by this {@code Optional}
381 * @throws NoSuchElementException if no value is present
382 * @since 10
383 */
384 public T orElseThrow() {
385 if (value == null) {
386 throw new NoSuchElementException("No value present");
387 }
388 return value;
389 }
390
391 /**
392 * If a value is present, returns the value, otherwise throws an exception
393 * produced by the exception supplying function.
394 *
395 * @apiNote
396 * A method reference to the exception constructor with an empty argument
397 * list can be used as the supplier. For example,
398 * {@code IllegalStateException::new}
399 *
400 * @param <X> Type of the exception to be thrown
401 * @param exceptionSupplier the supplying function that produces an
402 * exception to be thrown
403 * @return the value, if present
404 * @throws X if no value is present
405 * @throws NullPointerException if no value is present and the exception
406 * supplying function is {@code null}
407 */
408 public <X extends Throwable> T orElseThrow(Supplier<? extends X> exceptionSupplier) throws X {
409 if (value != null) {
410 return value;
411 } else {
412 throw exceptionSupplier.get();
413 }
414 }
415
416 /**
417 * Indicates whether some other object is "equal to" this {@code Optional}.
418 * The other object is considered equal if:
419 * <ul>
420 * <li>it is also an {@code Optional} and;
421 * <li>both instances have no value present or;
422 * <li>the present values are "equal to" each other via {@code equals()}.
423 * </ul>
424 *
425 * @param obj an object to be tested for equality
426 * @return {@code true} if the other object is "equal to" this object
427 * otherwise {@code false}
428 */
429 @Override
430 public boolean equals(Object obj) {
431 if (this == obj) {
432 return true;
433 }
434
435 return obj instanceof Optional<?> other
436 && Objects.equals(value, other.value);
437 }
438
439 /**
440 * Returns the hash code of the value, if present, otherwise {@code 0}
441 * (zero) if no value is present.
442 *
443 * @return hash code value of the present value or {@code 0} if no value is
444 * present
445 */
446 @Override
447 public int hashCode() {
448 return Objects.hashCode(value);
449 }
450
451 /**
452 * Returns a non-empty string representation of this {@code Optional}
453 * suitable for debugging. The exact presentation format is unspecified and
454 * may vary between implementations and versions.
455 *
456 * @implSpec
457 * If a value is present the result must include its string representation
458 * in the result. Empty and present {@code Optional}s must be unambiguously
459 * differentiable.
460 *
461 * @return the string representation of this instance
462 */
463 @Override
464 public String toString() {
465 return value != null
466 ? ("Optional[" + value + "]")
467 : "Optional.empty";
468 }
469 }