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
47 * {@linkplain #equals(Object) equal} as interchangeable and should not
48 * use instances for synchronization, or unpredictable behavior may
49 * occur. For example, in a future release, synchronization may fail.
50 *
51 * @apiNote
52 * {@code Optional} is primarily intended for use as a method return type where
53 * there is a clear need to represent "no result," and where using {@code null}
54 * is likely to cause errors. A variable whose type is {@code Optional} should
55 * never itself be {@code null}; it should always point to an {@code Optional}
56 * instance.
57 *
58 * @param <T> the type of value
59 * @since 1.8
60 */
61 @jdk.internal.MigratedValueClass
62 @jdk.internal.ValueBased
63 public final class Optional<T> {
64 /**
65 * Common instance for {@code empty()}.
66 */
67 private static final Optional<?> EMPTY = new Optional<>(null);
68
69 /**
70 * If non-null, the value; if null, indicates no value is present
71 */
72 private final T value;
73
74 /**
75 * Returns an empty {@code Optional} instance. No value is present for this
76 * {@code Optional}.
77 *
78 * @apiNote
79 * Though it may be tempting to do so, avoid testing if an object is empty
80 * by comparing with {@code ==} or {@code !=} against instances returned by
81 * {@code Optional.empty()}. There is no guarantee that it is a singleton.
82 * Instead, use {@link #isEmpty()} or {@link #isPresent()}.
83 *
84 * @param <T> The type of the non-existent value
85 * @return an empty {@code Optional}
86 */
87 public static<T> Optional<T> empty() {
88 @SuppressWarnings("unchecked")
89 Optional<T> t = (Optional<T>) EMPTY;
90 return t;
91 }
92
93 /**
94 * Constructs an instance with the described value.
95 *
96 * @param value the value to describe; it's the caller's responsibility to
97 * ensure the value is non-{@code null} unless creating the singleton
98 * instance returned by {@code empty()}.
99 */
100 private Optional(T value) {
101 this.value = value;
102 }
103
104 /**
105 * Returns an {@code Optional} describing the given non-{@code null}
106 * value.
107 *
108 * @param value the value to describe, which must be non-{@code null}
109 * @param <T> the type of the value
110 * @return an {@code Optional} with the value present
111 * @throws NullPointerException if value is {@code null}
112 */
113 public static <T> Optional<T> of(T value) {
114 return new Optional<>(Objects.requireNonNull(value));
115 }
116
117 /**
118 * Returns an {@code Optional} describing the given value, if
119 * non-{@code null}, otherwise returns an empty {@code Optional}.
120 *
121 * @param value the possibly-{@code null} value to describe
122 * @param <T> the type of the value
123 * @return an {@code Optional} with a present value if the specified value
124 * is non-{@code null}, otherwise an empty {@code Optional}
125 */
126 @SuppressWarnings("unchecked")
127 public static <T> Optional<T> ofNullable(T value) {
128 return value == null ? (Optional<T>) EMPTY
129 : new Optional<>(value);
130 }
131
132 /**
133 * If a value is present, returns the value, otherwise throws
134 * {@code NoSuchElementException}.
135 *
136 * @apiNote
137 * The preferred alternative to this method is {@link #orElseThrow()}.
138 *
139 * @return the non-{@code null} value described by this {@code Optional}
140 * @throws NoSuchElementException if no value is present
141 */
142 public T get() {
143 if (value == null) {
144 throw new NoSuchElementException("No value present");
145 }
146 return value;
147 }
148
149 /**
150 * If a value is present, returns {@code true}, otherwise {@code false}.
151 *
152 * @return {@code true} if a value is present, otherwise {@code false}
153 */
154 public boolean isPresent() {
155 return value != null;
156 }
157
158 /**
159 * If a value is not present, returns {@code true}, otherwise
160 * {@code false}.
161 *
162 * @return {@code true} if a value is not present, otherwise {@code false}
163 * @since 11
164 */
165 public boolean isEmpty() {
166 return value == null;
167 }
168
169 /**
170 * If a value is present, performs the given action with the value,
171 * otherwise does nothing.
172 *
173 * @param action the action to be performed, if a value is present
174 * @throws NullPointerException if value is present and the given action is
175 * {@code null}
176 */
177 public void ifPresent(Consumer<? super T> action) {
178 if (value != null) {
179 action.accept(value);
180 }
181 }
182
183 /**
184 * If a value is present, performs the given action with the value,
185 * otherwise performs the given empty-based action.
186 *
187 * @param action the action to be performed, if a value is present
188 * @param emptyAction the empty-based action to be performed, if no value is
189 * present
190 * @throws NullPointerException if a value is present and the given action
191 * is {@code null}, or no value is present and the given empty-based
192 * action is {@code null}.
193 * @since 9
194 */
195 public void ifPresentOrElse(Consumer<? super T> action, Runnable emptyAction) {
196 if (value != null) {
197 action.accept(value);
198 } else {
199 emptyAction.run();
200 }
201 }
202
203 /**
204 * If a value is present, and the value matches the given predicate,
205 * returns an {@code Optional} describing the value, otherwise returns an
206 * empty {@code Optional}.
207 *
208 * @param predicate the predicate to apply to a value, if present
209 * @return an {@code Optional} describing the value of this
210 * {@code Optional}, if a value is present and the value matches the
211 * given predicate, otherwise an empty {@code Optional}
212 * @throws NullPointerException if the predicate is {@code null}
213 */
214 public Optional<T> filter(Predicate<? super T> predicate) {
215 Objects.requireNonNull(predicate);
216 if (isEmpty()) {
217 return this;
218 } else {
219 return predicate.test(value) ? this : empty();
220 }
221 }
222
223 /**
224 * If a value is present, returns an {@code Optional} describing (as if by
225 * {@link #ofNullable}) the result of applying the given mapping function to
226 * the value, otherwise returns an empty {@code Optional}.
227 *
228 * <p>If the mapping function returns a {@code null} result then this method
229 * returns an empty {@code Optional}.
230 *
231 * @apiNote
232 * This method supports post-processing on {@code Optional} values, without
233 * the need to explicitly check for a return status. For example, the
234 * following code traverses a stream of URIs, selects one that has not
235 * yet been processed, and creates a path from that URI, returning
236 * an {@code Optional<Path>}:
237 *
238 * <pre>{@code
239 * Optional<Path> p =
240 * uris.stream().filter(uri -> !isProcessedYet(uri))
241 * .findFirst()
242 * .map(Paths::get);
243 * }</pre>
244 *
245 * Here, {@code findFirst} returns an {@code Optional<URI>}, and then
246 * {@code map} returns an {@code Optional<Path>} for the desired
247 * URI if one exists.
248 *
249 * @param mapper the mapping function to apply to a value, if present
250 * @param <U> The type of the value returned from the mapping function
251 * @return an {@code Optional} describing the result of applying a mapping
252 * function to the value of this {@code Optional}, if a value is
253 * present, otherwise an empty {@code Optional}
254 * @throws NullPointerException if the mapping function is {@code null}
255 */
256 public <U> Optional<U> map(Function<? super T, ? extends U> mapper) {
257 Objects.requireNonNull(mapper);
258 if (isEmpty()) {
259 return empty();
260 } else {
261 return Optional.ofNullable(mapper.apply(value));
262 }
263 }
264
265 /**
266 * If a value is present, returns the result of applying the given
267 * {@code Optional}-bearing mapping function to the value, otherwise returns
268 * an empty {@code Optional}.
269 *
270 * <p>This method is similar to {@link #map(Function)}, but the mapping
271 * function is one whose result is already an {@code Optional}, and if
272 * invoked, {@code flatMap} does not wrap it within an additional
273 * {@code Optional}.
274 *
275 * @param <U> The type of value of the {@code Optional} returned by the
276 * mapping function
277 * @param mapper the mapping function to apply to a value, if present
278 * @return the result of applying an {@code Optional}-bearing mapping
279 * function to the value of this {@code Optional}, if a value is
280 * present, otherwise an empty {@code Optional}
281 * @throws NullPointerException if the mapping function is {@code null} or
282 * returns a {@code null} result
283 */
284 public <U> Optional<U> flatMap(Function<? super T, ? extends Optional<? extends U>> mapper) {
285 Objects.requireNonNull(mapper);
286 if (isEmpty()) {
287 return empty();
288 } else {
289 @SuppressWarnings("unchecked")
290 Optional<U> r = (Optional<U>) mapper.apply(value);
291 return Objects.requireNonNull(r);
292 }
293 }
294
295 /**
296 * If a value is present, returns an {@code Optional} describing the value,
297 * otherwise returns an {@code Optional} produced by the supplying function.
298 *
299 * @param supplier the supplying function that produces an {@code Optional}
300 * to be returned
301 * @return returns an {@code Optional} describing the value of this
302 * {@code Optional}, if a value is present, otherwise an
303 * {@code Optional} produced by the supplying function.
304 * @throws NullPointerException if the supplying function is {@code null} or
305 * produces a {@code null} result
306 * @since 9
307 */
308 public Optional<T> or(Supplier<? extends Optional<? extends T>> supplier) {
309 Objects.requireNonNull(supplier);
310 if (isPresent()) {
311 return this;
312 } else {
313 @SuppressWarnings("unchecked")
314 Optional<T> r = (Optional<T>) supplier.get();
315 return Objects.requireNonNull(r);
316 }
317 }
318
319 /**
320 * If a value is present, returns a sequential {@link Stream} containing
321 * only that value, otherwise returns an empty {@code Stream}.
322 *
323 * @apiNote
324 * This method can be used to transform a {@code Stream} of optional
325 * elements to a {@code Stream} of present value elements:
326 * <pre>{@code
327 * Stream<Optional<T>> os = ..
328 * Stream<T> s = os.flatMap(Optional::stream)
329 * }</pre>
330 *
331 * @return the optional value as a {@code Stream}
332 * @since 9
333 */
334 public Stream<T> stream() {
335 if (isEmpty()) {
336 return Stream.empty();
337 } else {
338 return Stream.of(value);
339 }
340 }
341
342 /**
343 * If a value is present, returns the value, otherwise returns
344 * {@code other}.
345 *
346 * @param other the value to be returned, if no value is present.
347 * May be {@code null}.
348 * @return the value, if present, otherwise {@code other}
349 */
350 public T orElse(T other) {
351 return value != null ? value : other;
352 }
353
354 /**
355 * If a value is present, returns the value, otherwise returns the result
356 * produced by the supplying function.
357 *
358 * @param supplier the supplying function that produces a value to be returned
359 * @return the value, if present, otherwise the result produced by the
360 * supplying function
361 * @throws NullPointerException if no value is present and the supplying
362 * function is {@code null}
363 */
364 public T orElseGet(Supplier<? extends T> supplier) {
365 return value != null ? value : supplier.get();
366 }
367
368 /**
369 * If a value is present, returns the value, otherwise throws
370 * {@code NoSuchElementException}.
371 *
372 * @return the non-{@code null} value described by this {@code Optional}
373 * @throws NoSuchElementException if no value is present
374 * @since 10
375 */
376 public T orElseThrow() {
377 if (value == null) {
378 throw new NoSuchElementException("No value present");
379 }
380 return value;
381 }
382
383 /**
384 * If a value is present, returns the value, otherwise throws an exception
385 * produced by the exception supplying function.
386 *
387 * @apiNote
388 * A method reference to the exception constructor with an empty argument
389 * list can be used as the supplier. For example,
390 * {@code IllegalStateException::new}
391 *
392 * @param <X> Type of the exception to be thrown
393 * @param exceptionSupplier the supplying function that produces an
394 * exception to be thrown
395 * @return the value, if present
396 * @throws X if no value is present
397 * @throws NullPointerException if no value is present and the exception
398 * supplying function is {@code null}
399 */
400 public <X extends Throwable> T orElseThrow(Supplier<? extends X> exceptionSupplier) throws X {
401 if (value != null) {
402 return value;
403 } else {
404 throw exceptionSupplier.get();
405 }
406 }
407
408 /**
409 * Indicates whether some other object is "equal to" this {@code Optional}.
410 * The other object is considered equal if:
411 * <ul>
412 * <li>it is also an {@code Optional} and;
413 * <li>both instances have no value present or;
414 * <li>the present values are "equal to" each other via {@code equals()}.
415 * </ul>
416 *
417 * @param obj an object to be tested for equality
418 * @return {@code true} if the other object is "equal to" this object
419 * otherwise {@code false}
420 */
421 @Override
422 public boolean equals(Object obj) {
423 if (this == obj) {
424 return true;
425 }
426
427 return obj instanceof Optional<?> other
428 && Objects.equals(value, other.value);
429 }
430
431 /**
432 * Returns the hash code of the value, if present, otherwise {@code 0}
433 * (zero) if no value is present.
434 *
435 * @return hash code value of the present value or {@code 0} if no value is
436 * present
437 */
438 @Override
439 public int hashCode() {
440 return Objects.hashCode(value);
441 }
442
443 /**
444 * Returns a non-empty string representation of this {@code Optional}
445 * suitable for debugging. The exact presentation format is unspecified and
446 * may vary between implementations and versions.
447 *
448 * @implSpec
449 * If a value is present the result must include its string representation
450 * in the result. Empty and present {@code Optional}s must be unambiguously
451 * differentiable.
452 *
453 * @return the string representation of this instance
454 */
455 @Override
456 public String toString() {
457 return value != null
458 ? ("Optional[" + value + "]")
459 : "Optional.empty";
460 }
461 }