1 /*
2 * Copyright (c) 2012, 2017, 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 present, performs the given action with the value,
164 * otherwise does nothing.
165 *
166 * @param action the action to be performed, if a value is present
167 * @throws NullPointerException if value is present and the given action is
168 * {@code null}
169 */
170 public void ifPresent(Consumer<? super T> action) {
171 if (value != null) {
172 action.accept(value);
173 }
174 }
175
176 /**
177 * If a value is present, performs the given action with the value,
178 * otherwise performs the given empty-based action.
179 *
180 * @param action the action to be performed, if a value is present
181 * @param emptyAction the empty-based action to be performed, if no value is
182 * present
183 * @throws NullPointerException if a value is present and the given action
184 * is {@code null}, or no value is present and the given empty-based
185 * action is {@code null}.
186 * @since 9
187 */
188 public void ifPresentOrElse(Consumer<? super T> action, Runnable emptyAction) {
189 if (value != null) {
190 action.accept(value);
191 } else {
192 emptyAction.run();
193 }
194 }
195
196 /**
197 * If a value is present, and the value matches the given predicate,
198 * returns an {@code Optional} describing the value, otherwise returns an
199 * empty {@code Optional}.
200 *
201 * @param predicate the predicate to apply to a value, if present
202 * @return an {@code Optional} describing the value of this
203 * {@code Optional}, if a value is present and the value matches the
204 * given predicate, otherwise an empty {@code Optional}
205 * @throws NullPointerException if the predicate is {@code null}
206 */
207 public Optional<T> filter(Predicate<? super T> predicate) {
208 Objects.requireNonNull(predicate);
209 if (!isPresent()) {
210 return this;
211 } else {
212 return predicate.test(value) ? this : empty();
213 }
214 }
215
216 /**
217 * If a value is present, returns an {@code Optional} describing (as if by
218 * {@link #ofNullable}) the result of applying the given mapping function to
219 * the value, otherwise returns an empty {@code Optional}.
220 *
221 * <p>If the mapping function returns a {@code null} result then this method
222 * returns an empty {@code Optional}.
223 *
224 * @apiNote
225 * This method supports post-processing on {@code Optional} values, without
226 * the need to explicitly check for a return status. For example, the
227 * following code traverses a stream of URIs, selects one that has not
228 * yet been processed, and creates a path from that URI, returning
229 * an {@code Optional<Path>}:
230 *
231 * <pre>{@code
232 * Optional<Path> p =
233 * uris.stream().filter(uri -> !isProcessedYet(uri))
234 * .findFirst()
235 * .map(Paths::get);
236 * }</pre>
237 *
238 * Here, {@code findFirst} returns an {@code Optional<URI>}, and then
239 * {@code map} returns an {@code Optional<Path>} for the desired
240 * URI if one exists.
241 *
242 * @param mapper the mapping function to apply to a value, if present
243 * @param <U> The type of the value returned from the mapping function
244 * @return an {@code Optional} describing the result of applying a mapping
245 * function to the value of this {@code Optional}, if a value is
246 * present, otherwise an empty {@code Optional}
247 * @throws NullPointerException if the mapping function is {@code null}
248 */
249 public <U> Optional<U> map(Function<? super T, ? extends U> mapper) {
250 Objects.requireNonNull(mapper);
251 if (!isPresent()) {
252 return empty();
253 } else {
254 return Optional.ofNullable(mapper.apply(value));
255 }
256 }
257
258 /**
259 * If a value is present, returns the result of applying the given
260 * {@code Optional}-bearing mapping function to the value, otherwise returns
261 * an empty {@code Optional}.
262 *
263 * <p>This method is similar to {@link #map(Function)}, but the mapping
264 * function is one whose result is already an {@code Optional}, and if
265 * invoked, {@code flatMap} does not wrap it within an additional
266 * {@code Optional}.
267 *
268 * @param <U> The type of value of the {@code Optional} returned by the
269 * mapping function
270 * @param mapper the mapping function to apply to a value, if present
271 * @return the result of applying an {@code Optional}-bearing mapping
272 * function to the value of this {@code Optional}, if a value is
273 * present, otherwise an empty {@code Optional}
274 * @throws NullPointerException if the mapping function is {@code null} or
275 * returns a {@code null} result
276 */
277 public <U> Optional<U> flatMap(Function<? super T, ? extends Optional<? extends U>> mapper) {
278 Objects.requireNonNull(mapper);
279 if (!isPresent()) {
280 return empty();
281 } else {
282 @SuppressWarnings("unchecked")
283 Optional<U> r = (Optional<U>) mapper.apply(value);
284 return Objects.requireNonNull(r);
285 }
286 }
287
288 /**
289 * If a value is present, returns an {@code Optional} describing the value,
290 * otherwise returns an {@code Optional} produced by the supplying function.
291 *
292 * @param supplier the supplying function that produces an {@code Optional}
293 * to be returned
294 * @return returns an {@code Optional} describing the value of this
295 * {@code Optional}, if a value is present, otherwise an
296 * {@code Optional} produced by the supplying function.
297 * @throws NullPointerException if the supplying function is {@code null} or
298 * produces a {@code null} result
299 * @since 9
300 */
301 public Optional<T> or(Supplier<? extends Optional<? extends T>> supplier) {
302 Objects.requireNonNull(supplier);
303 if (isPresent()) {
304 return this;
305 } else {
306 @SuppressWarnings("unchecked")
307 Optional<T> r = (Optional<T>) supplier.get();
308 return Objects.requireNonNull(r);
309 }
310 }
311
312 /**
313 * If a value is present, returns a sequential {@link Stream} containing
314 * only that value, otherwise returns an empty {@code Stream}.
315 *
316 * @apiNote
317 * This method can be used to transform a {@code Stream} of optional
318 * elements to a {@code Stream} of present value elements:
319 * <pre>{@code
320 * Stream<Optional<T>> os = ..
321 * Stream<T> s = os.flatMap(Optional::stream)
322 * }</pre>
323 *
324 * @return the optional value as a {@code Stream}
325 * @since 9
326 */
327 public Stream<T> stream() {
328 if (!isPresent()) {
329 return Stream.empty();
330 } else {
331 return Stream.of(value);
332 }
333 }
334
335 /**
336 * If a value is present, returns the value, otherwise returns
337 * {@code other}.
338 *
339 * @param other the value to be returned, if no value is present.
340 * May be {@code null}.
341 * @return the value, if present, otherwise {@code other}
342 */
343 public T orElse(T other) {
344 return value != null ? value : other;
345 }
346
347 /**
348 * If a value is present, returns the value, otherwise returns the result
349 * produced by the supplying function.
350 *
351 * @param supplier the supplying function that produces a value to be returned
352 * @return the value, if present, otherwise the result produced by the
353 * supplying function
354 * @throws NullPointerException if no value is present and the supplying
355 * function is {@code null}
356 */
357 public T orElseGet(Supplier<? extends T> supplier) {
358 return value != null ? value : supplier.get();
359 }
360
361 /**
362 * If a value is present, returns the value, otherwise throws
363 * {@code NoSuchElementException}.
364 *
365 * @return the non-{@code null} value described by this {@code Optional}
366 * @throws NoSuchElementException if no value is present
367 * @since 10
368 */
369 public T orElseThrow() {
370 if (value == null) {
371 throw new NoSuchElementException("No value present");
372 }
373 return value;
374 }
375
376 /**
377 * If a value is present, returns the value, otherwise throws an exception
378 * produced by the exception supplying function.
379 *
380 * @apiNote
381 * A method reference to the exception constructor with an empty argument
382 * list can be used as the supplier. For example,
383 * {@code IllegalStateException::new}
384 *
385 * @param <X> Type of the exception to be thrown
386 * @param exceptionSupplier the supplying function that produces an
387 * exception to be thrown
388 * @return the value, if present
389 * @throws X if no value is present
390 * @throws NullPointerException if no value is present and the exception
391 * supplying function is {@code null}
392 */
393 public <X extends Throwable> T orElseThrow(Supplier<? extends X> exceptionSupplier) throws X {
394 if (value != null) {
395 return value;
396 } else {
397 throw exceptionSupplier.get();
398 }
399 }
400
401 /**
402 * Indicates whether some other object is "equal to" this {@code Optional}.
403 * The other object is considered equal if:
404 * <ul>
405 * <li>it is also an {@code Optional} and;
406 * <li>both instances have no value present or;
407 * <li>the present values are "equal to" each other via {@code equals()}.
408 * </ul>
409 *
410 * @param obj an object to be tested for equality
411 * @return {@code true} if the other object is "equal to" this object
412 * otherwise {@code false}
413 */
414 @Override
415 public boolean equals(Object obj) {
416 if (this == obj) {
417 return true;
418 }
419
420 if (!(obj instanceof Optional)) {
421 return false;
422 }
423
424 Optional<?> other = (Optional<?>) obj;
425 return Objects.equals(value, other.value);
426 }
427
428 /**
429 * Returns the hash code of the value, if present, otherwise {@code 0}
430 * (zero) if no value is present.
431 *
432 * @return hash code value of the present value or {@code 0} if no value is
433 * present
434 */
435 @Override
436 public int hashCode() {
437 return Objects.hashCode(value);
438 }
439
440 /**
441 * Returns a non-empty string representation of this {@code Optional}
442 * suitable for debugging. The exact presentation format is unspecified and
443 * may vary between implementations and versions.
444 *
445 * @implSpec
446 * If a value is present the result must include its string representation
447 * in the result. Empty and present {@code Optional}s must be unambiguously
448 * differentiable.
449 *
450 * @return the string representation of this instance
451 */
452 @Override
453 public String toString() {
454 return value != null
455 ? String.format("Optional[%s]", value)
456 : "Optional.empty";
457 }
458 }