1 /*
2 * Copyright (c) 2012, 2018, 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="../lang/doc-files/ValueBased.html">value-based</a>
46 * class; use of identity-sensitive operations (including reference equality
47 * ({@code ==}), identity hash code, or synchronization) on instances of
48 * {@code Optional} may have unpredictable results and should be avoided.
49 *
50 * @apiNote
51 * {@code Optional} is primarily intended for use as a method return type where
52 * there is a clear need to represent "no result," and where using {@code null}
53 * is likely to cause errors. A variable whose type is {@code Optional} should
54 * never itself be {@code null}; it should always point to an {@code Optional}
55 * instance.
56 *
57 * @param <T> the type of value
58 * @since 1.8
59 */
60 public final class Optional<T> {
61 /**
62 * Common instance for {@code empty()}.
63 */
64 private static final Optional<?> EMPTY = new Optional<>();
65
66 /**
67 * If non-null, the value; if null, indicates no value is present
68 */
69 private final T value;
70
71 /**
72 * Constructs an empty instance.
73 *
74 * @implNote Generally only one empty instance, {@link Optional#EMPTY},
75 * should exist per VM.
76 */
77 private Optional() {
78 this.value = null;
79 }
80
81 /**
82 * Returns an empty {@code Optional} instance. No value is present for this
83 * {@code Optional}.
84 *
85 * @apiNote
86 * Though it may be tempting to do so, avoid testing if an object is empty
87 * by comparing with {@code ==} against instances returned by
88 * {@code Optional.empty()}. There is no guarantee that it is a singleton.
89 * Instead, use {@link #isPresent()}.
90 *
91 * @param <T> The type of the non-existent value
92 * @return an empty {@code Optional}
93 */
94 public static<T> Optional<T> empty() {
95 @SuppressWarnings("unchecked")
96 Optional<T> t = (Optional<T>) EMPTY;
97 return t;
98 }
99
100 /**
101 * Constructs an instance with the described value.
102 *
103 * @param value the non-{@code null} value to describe
104 * @throws NullPointerException if value is {@code null}
105 */
106 private Optional(T value) {
107 this.value = Objects.requireNonNull(value);
108 }
109
110 /**
111 * Returns an {@code Optional} describing the given non-{@code null}
112 * value.
113 *
114 * @param value the value to describe, which must be non-{@code null}
115 * @param <T> the type of the value
116 * @return an {@code Optional} with the value present
117 * @throws NullPointerException if value is {@code null}
118 */
119 public static <T> Optional<T> of(T value) {
120 return new Optional<>(value);
121 }
122
123 /**
124 * Returns an {@code Optional} describing the given value, if
125 * non-{@code null}, otherwise returns an empty {@code Optional}.
126 *
127 * @param value the possibly-{@code null} value to describe
128 * @param <T> the type of the value
129 * @return an {@code Optional} with a present value if the specified value
130 * is non-{@code null}, otherwise an empty {@code Optional}
131 */
132 public static <T> Optional<T> ofNullable(T value) {
133 return value == null ? empty() : of(value);
134 }
135
136 /**
137 * If a value is present, returns the value, otherwise throws
138 * {@code NoSuchElementException}.
139 *
140 * @apiNote
141 * The preferred alternative to this method is {@link #orElseThrow()}.
142 *
143 * @return the non-{@code null} value described by this {@code Optional}
144 * @throws NoSuchElementException if no value is present
145 */
146 public T get() {
147 if (value == null) {
148 throw new NoSuchElementException("No value present");
149 }
150 return value;
151 }
152
153 /**
154 * If a value is present, returns {@code true}, otherwise {@code false}.
155 *
156 * @return {@code true} if a value is present, otherwise {@code false}
157 */
158 public boolean isPresent() {
159 return value != null;
160 }
161
162 /**
163 * If a value is not present, returns {@code true}, otherwise
164 * {@code false}.
165 *
166 * @return {@code true} if a value is not present, otherwise {@code false}
167 */
168 public boolean isEmpty() {
169 return value == null;
170 }
171
172 /**
173 * If a value is present, performs the given action with the value,
174 * otherwise does nothing.
175 *
176 * @param action the action to be performed, if a value is present
177 * @throws NullPointerException if value is present and the given action is
178 * {@code null}
179 */
180 public void ifPresent(Consumer<? super T> action) {
181 if (value != null) {
182 action.accept(value);
183 }
184 }
185
186 /**
187 * If a value is present, performs the given action with the value,
188 * otherwise performs the given empty-based action.
189 *
190 * @param action the action to be performed, if a value is present
191 * @param emptyAction the empty-based action to be performed, if no value is
192 * present
193 * @throws NullPointerException if a value is present and the given action
194 * is {@code null}, or no value is present and the given empty-based
195 * action is {@code null}.
196 * @since 9
197 */
198 public void ifPresentOrElse(Consumer<? super T> action, Runnable emptyAction) {
199 if (value != null) {
200 action.accept(value);
201 } else {
202 emptyAction.run();
203 }
204 }
205
206 /**
207 * If a value is present, and the value matches the given predicate,
208 * returns an {@code Optional} describing the value, otherwise returns an
209 * empty {@code Optional}.
210 *
211 * @param predicate the predicate to apply to a value, if present
212 * @return an {@code Optional} describing the value of this
213 * {@code Optional}, if a value is present and the value matches the
214 * given predicate, otherwise an empty {@code Optional}
215 * @throws NullPointerException if the predicate is {@code null}
216 */
217 public Optional<T> filter(Predicate<? super T> predicate) {
218 Objects.requireNonNull(predicate);
219 if (!isPresent()) {
220 return this;
221 } else {
222 return predicate.test(value) ? this : empty();
223 }
224 }
225
226 /**
227 * If a value is present, returns an {@code Optional} describing (as if by
228 * {@link #ofNullable}) the result of applying the given mapping function to
229 * the value, otherwise returns an empty {@code Optional}.
230 *
231 * <p>If the mapping function returns a {@code null} result then this method
232 * returns an empty {@code Optional}.
233 *
234 * @apiNote
235 * This method supports post-processing on {@code Optional} values, without
236 * the need to explicitly check for a return status. For example, the
237 * following code traverses a stream of URIs, selects one that has not
238 * yet been processed, and creates a path from that URI, returning
239 * an {@code Optional<Path>}:
240 *
241 * <pre>{@code
242 * Optional<Path> p =
243 * uris.stream().filter(uri -> !isProcessedYet(uri))
244 * .findFirst()
245 * .map(Paths::get);
246 * }</pre>
247 *
248 * Here, {@code findFirst} returns an {@code Optional<URI>}, and then
249 * {@code map} returns an {@code Optional<Path>} for the desired
250 * URI if one exists.
251 *
252 * @param mapper the mapping function to apply to a value, if present
253 * @param <U> The type of the value returned from the mapping function
254 * @return an {@code Optional} describing the result of applying a mapping
255 * function to the value of this {@code Optional}, if a value is
256 * present, otherwise an empty {@code Optional}
257 * @throws NullPointerException if the mapping function is {@code null}
258 */
259 public <U> Optional<U> map(Function<? super T, ? extends U> mapper) {
260 Objects.requireNonNull(mapper);
261 if (!isPresent()) {
262 return empty();
263 } else {
264 return Optional.ofNullable(mapper.apply(value));
265 }
266 }
267
268 /**
269 * If a value is present, returns the result of applying the given
270 * {@code Optional}-bearing mapping function to the value, otherwise returns
271 * an empty {@code Optional}.
272 *
273 * <p>This method is similar to {@link #map(Function)}, but the mapping
274 * function is one whose result is already an {@code Optional}, and if
275 * invoked, {@code flatMap} does not wrap it within an additional
276 * {@code Optional}.
277 *
278 * @param <U> The type of value of the {@code Optional} returned by the
279 * mapping function
280 * @param mapper the mapping function to apply to a value, if present
281 * @return the result of applying an {@code Optional}-bearing mapping
282 * function to the value of this {@code Optional}, if a value is
283 * present, otherwise an empty {@code Optional}
284 * @throws NullPointerException if the mapping function is {@code null} or
285 * returns a {@code null} result
286 */
287 public <U> Optional<U> flatMap(Function<? super T, ? extends Optional<? extends U>> mapper) {
288 Objects.requireNonNull(mapper);
289 if (!isPresent()) {
290 return empty();
291 } else {
292 @SuppressWarnings("unchecked")
293 Optional<U> r = (Optional<U>) mapper.apply(value);
294 return Objects.requireNonNull(r);
295 }
296 }
297
298 /**
299 * If a value is present, returns an {@code Optional} describing the value,
300 * otherwise returns an {@code Optional} produced by the supplying function.
301 *
302 * @param supplier the supplying function that produces an {@code Optional}
303 * to be returned
304 * @return returns an {@code Optional} describing the value of this
305 * {@code Optional}, if a value is present, otherwise an
306 * {@code Optional} produced by the supplying function.
307 * @throws NullPointerException if the supplying function is {@code null} or
308 * produces a {@code null} result
309 * @since 9
310 */
311 public Optional<T> or(Supplier<? extends Optional<? extends T>> supplier) {
312 Objects.requireNonNull(supplier);
313 if (isPresent()) {
314 return this;
315 } else {
316 @SuppressWarnings("unchecked")
317 Optional<T> r = (Optional<T>) supplier.get();
318 return Objects.requireNonNull(r);
319 }
320 }
321
322 /**
323 * If a value is present, returns a sequential {@link Stream} containing
324 * only that value, otherwise returns an empty {@code Stream}.
325 *
326 * @apiNote
327 * This method can be used to transform a {@code Stream} of optional
328 * elements to a {@code Stream} of present value elements:
329 * <pre>{@code
330 * Stream<Optional<T>> os = ..
331 * Stream<T> s = os.flatMap(Optional::stream)
332 * }</pre>
333 *
334 * @return the optional value as a {@code Stream}
335 * @since 9
336 */
337 public Stream<T> stream() {
338 if (!isPresent()) {
339 return Stream.empty();
340 } else {
341 return Stream.of(value);
342 }
343 }
344
345 /**
346 * If a value is present, returns the value, otherwise returns
347 * {@code other}.
348 *
349 * @param other the value to be returned, if no value is present.
350 * May be {@code null}.
351 * @return the value, if present, otherwise {@code other}
352 */
353 public T orElse(T other) {
354 return value != null ? value : other;
355 }
356
357 /**
358 * If a value is present, returns the value, otherwise returns the result
359 * produced by the supplying function.
360 *
361 * @param supplier the supplying function that produces a value to be returned
362 * @return the value, if present, otherwise the result produced by the
363 * supplying function
364 * @throws NullPointerException if no value is present and the supplying
365 * function is {@code null}
366 */
367 public T orElseGet(Supplier<? extends T> supplier) {
368 return value != null ? value : supplier.get();
369 }
370
371 /**
372 * If a value is present, returns the value, otherwise throws
373 * {@code NoSuchElementException}.
374 *
375 * @return the non-{@code null} value described by this {@code Optional}
376 * @throws NoSuchElementException if no value is present
377 * @since 10
378 */
379 public T orElseThrow() {
380 if (value == null) {
381 throw new NoSuchElementException("No value present");
382 }
383 return value;
384 }
385
386 /**
387 * If a value is present, returns the value, otherwise throws an exception
388 * produced by the exception supplying function.
389 *
390 * @apiNote
391 * A method reference to the exception constructor with an empty argument
392 * list can be used as the supplier. For example,
393 * {@code IllegalStateException::new}
394 *
395 * @param <X> Type of the exception to be thrown
396 * @param exceptionSupplier the supplying function that produces an
397 * exception to be thrown
398 * @return the value, if present
399 * @throws X if no value is present
400 * @throws NullPointerException if no value is present and the exception
401 * supplying function is {@code null}
402 */
403 public <X extends Throwable> T orElseThrow(Supplier<? extends X> exceptionSupplier) throws X {
404 if (value != null) {
405 return value;
406 } else {
407 throw exceptionSupplier.get();
408 }
409 }
410
411 /**
412 * Indicates whether some other object is "equal to" this {@code Optional}.
413 * The other object is considered equal if:
414 * <ul>
415 * <li>it is also an {@code Optional} and;
416 * <li>both instances have no value present or;
417 * <li>the present values are "equal to" each other via {@code equals()}.
418 * </ul>
419 *
420 * @param obj an object to be tested for equality
421 * @return {@code true} if the other object is "equal to" this object
422 * otherwise {@code false}
423 */
424 @Override
425 public boolean equals(Object obj) {
426 if (this == obj) {
427 return true;
428 }
429
430 if (!(obj instanceof Optional)) {
431 return false;
432 }
433
434 Optional<?> other = (Optional<?>) obj;
435 return Objects.equals(value, other.value);
436 }
437
438 /**
439 * Returns the hash code of the value, if present, otherwise {@code 0}
440 * (zero) if no value is present.
441 *
442 * @return hash code value of the present value or {@code 0} if no value is
443 * present
444 */
445 @Override
446 public int hashCode() {
447 return Objects.hashCode(value);
448 }
449
450 /**
451 * Returns a non-empty string representation of this {@code Optional}
452 * suitable for debugging. The exact presentation format is unspecified and
453 * may vary between implementations and versions.
454 *
455 * @implSpec
456 * If a value is present the result must include its string representation
457 * in the result. Empty and present {@code Optional}s must be unambiguously
458 * differentiable.
459 *
460 * @return the string representation of this instance
461 */
462 @Override
463 public String toString() {
464 return value != null
465 ? String.format("Optional[%s]", value)
466 : "Optional.empty";
467 }
468 }