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