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} and
36 * {@code get()} returns the value.
37 *
38 * <p>Additional methods that depend on the presence or absence of a contained
39 * value are provided, such as {@link #orElse(Object) orElse()}
40 * (returns a default value if no value is present) and
41 * {@link #ifPresent(Consumer) ifPresent()} (performs an
42 * action if a value is present).
43 *
44 * <p>This is a <a href="../lang/doc-files/ValueBased.html">value-based</a>
45 * class; use of identity-sensitive operations (including reference equality
46 * ({@code ==}), identity hash code, or synchronization) on instances of
47 * {@code Optional} may have unpredictable results and should be avoided.
48 *
49 * @apiNote
50 * {@code Optional} is primarily intended for use as a method return type where
51 * there is a clear need to represent "no result," and where using {@code null}
52 * is likely to cause errors. A variable whose type is {@code Optional} should
53 * never itself be {@code null}; it should always point to an {@code Optional}
54 * instance.
55 *
56 * @param <T> the type of value
57 * @since 1.8
58 */
59 public final class Optional<T> {
60 /**
61 * Common instance for {@code empty()}.
62 */
63 private static final Optional<?> EMPTY = new Optional<>();
64
65 /**
66 * If non-null, the value; if null, indicates no value is present
67 */
68 private final T value;
69
70 /**
71 * Constructs an empty instance.
72 *
73 * @implNote Generally only one empty instance, {@link Optional#EMPTY},
74 * should exist per VM.
75 */
76 private Optional() {
77 this.value = null;
78 }
79
80 /**
81 * Returns an empty {@code Optional} instance. No value is present for this
82 * {@code Optional}.
83 *
84 * @apiNote
85 * Though it may be tempting to do so, avoid testing if an object is empty
86 * by comparing with {@code ==} against instances returned by
87 * {@code Optional.empty()}. There is no guarantee that it is a singleton.
88 * Instead, use {@link #isPresent()}.
89 *
90 * @param <T> The type of the non-existent value
91 * @return an empty {@code Optional}
92 */
93 public static<T> Optional<T> empty() {
94 @SuppressWarnings("unchecked")
95 Optional<T> t = (Optional<T>) EMPTY;
96 return t;
97 }
98
99 /**
100 * Constructs an instance with the described value.
101 *
102 * @param value the non-{@code null} value to describe
103 * @throws NullPointerException if value is {@code null}
104 */
105 private Optional(T value) {
106 this.value = Objects.requireNonNull(value);
107 }
108
109 /**
110 * Returns an {@code Optional} describing the given non-{@code null}
111 * value.
112 *
113 * @param value the value to describe, which must be non-{@code null}
114 * @param <T> the type of the value
115 * @return an {@code Optional} with the value present
116 * @throws NullPointerException if value is {@code null}
117 */
118 public static <T> Optional<T> of(T value) {
119 return new Optional<>(value);
120 }
121
122 /**
123 * Returns an {@code Optional} describing the given value, if
124 * non-{@code null}, otherwise returns an empty {@code Optional}.
125 *
126 * @param value the possibly-{@code null} value to describe
127 * @param <T> the type of the value
128 * @return an {@code Optional} with a present value if the specified value
129 * is non-{@code null}, otherwise an empty {@code Optional}
130 */
131 public static <T> Optional<T> ofNullable(T value) {
132 return value == null ? empty() : of(value);
133 }
134
135 /**
136 * If a value is present, returns the value, otherwise throws
137 * {@code NoSuchElementException}.
138 *
139 * @apiNote
140 * The methods {@link #orElse(Object) orElse} and
141 * {@link #orElseGet(Supplier) orElseGet}
142 * are generally preferable to this method, as they return a substitute
143 * value if the value is absent, instead of throwing an exception.
144 *
145 * @return the non-{@code null} value described by this {@code Optional}
146 * @throws NoSuchElementException if no value is present
147 * @see Optional#isPresent()
148 */
149 public T get() {
150 if (value == null) {
151 throw new NoSuchElementException("No value present");
152 }
153 return value;
154 }
155
156 /**
157 * If a value is present, returns {@code true}, otherwise {@code false}.
158 *
159 * @return {@code true} if a value is present, otherwise {@code false}
160 */
161 public boolean isPresent() {
162 return value != null;
163 }
164
165 /**
166 * If a value is present, performs the given action with the value,
167 * otherwise does nothing.
168 *
169 * @param action the action to be performed, if a value is present
170 * @throws NullPointerException if value is present and the given action is
171 * {@code null}
172 */
173 public void ifPresent(Consumer<? super T> action) {
174 if (value != null) {
175 action.accept(value);
176 }
177 }
178
179 /**
180 * If a value is present, performs the given action with the value,
181 * otherwise performs the given empty-based action.
182 *
183 * @param action the action to be performed, if a value is present
184 * @param emptyAction the empty-based action to be performed, if no value is
185 * present
186 * @throws NullPointerException if a value is present and the given action
187 * is {@code null}, or no value is present and the given empty-based
188 * action is {@code null}.
189 * @since 9
190 */
191 public void ifPresentOrElse(Consumer<? super T> action, Runnable emptyAction) {
192 if (value != null) {
193 action.accept(value);
194 } else {
195 emptyAction.run();
196 }
197 }
198
199 /**
200 * If a value is present, and the value matches the given predicate,
201 * returns an {@code Optional} describing the value, otherwise returns an
202 * empty {@code Optional}.
203 *
204 * @param predicate the predicate to apply to a value, if present
205 * @return an {@code Optional} describing the value of this
206 * {@code Optional}, if a value is present and the value matches the
207 * given predicate, otherwise an empty {@code Optional}
208 * @throws NullPointerException if the predicate is {@code null}
209 */
210 public Optional<T> filter(Predicate<? super T> predicate) {
211 Objects.requireNonNull(predicate);
212 if (!isPresent()) {
213 return this;
214 } else {
215 return predicate.test(value) ? this : empty();
216 }
217 }
218
219 /**
220 * If a value is present, returns an {@code Optional} describing (as if by
221 * {@link #ofNullable}) the result of applying the given mapping function to
222 * the value, otherwise returns an empty {@code Optional}.
223 *
224 * <p>If the mapping function returns a {@code null} result then this method
225 * returns an empty {@code Optional}.
226 *
227 * @apiNote
228 * This method supports post-processing on {@code Optional} values, without
229 * the need to explicitly check for a return status. For example, the
230 * following code traverses a stream of URIs, selects one that has not
231 * yet been processed, and creates a path from that URI, returning
232 * an {@code Optional<Path>}:
233 *
234 * <pre>{@code
235 * Optional<Path> p =
236 * uris.stream().filter(uri -> !isProcessedYet(uri))
237 * .findFirst()
238 * .map(Paths::get);
239 * }</pre>
240 *
241 * Here, {@code findFirst} returns an {@code Optional<URI>}, and then
242 * {@code map} returns an {@code Optional<Path>} for the desired
243 * URI if one exists.
244 *
245 * @param mapper the mapping function to apply to a value, if present
246 * @param <U> The type of the value returned from the mapping function
247 * @return an {@code Optional} describing the result of applying a mapping
248 * function to the value of this {@code Optional}, if a value is
249 * present, otherwise an empty {@code Optional}
250 * @throws NullPointerException if the mapping function is {@code null}
251 */
252 public <U> Optional<U> map(Function<? super T, ? extends U> mapper) {
253 Objects.requireNonNull(mapper);
254 if (!isPresent()) {
255 return empty();
256 } else {
257 return Optional.ofNullable(mapper.apply(value));
258 }
259 }
260
261 /**
262 * If a value is present, returns the result of applying the given
263 * {@code Optional}-bearing mapping function to the value, otherwise returns
264 * an empty {@code Optional}.
265 *
266 * <p>This method is similar to {@link #map(Function)}, but the mapping
267 * function is one whose result is already an {@code Optional}, and if
268 * invoked, {@code flatMap} does not wrap it within an additional
269 * {@code Optional}.
270 *
271 * @param <U> The type of value of the {@code Optional} returned by the
272 * mapping function
273 * @param mapper the mapping function to apply to a value, if present
274 * @return the result of applying an {@code Optional}-bearing mapping
275 * function to the value of this {@code Optional}, if a value is
276 * present, otherwise an empty {@code Optional}
277 * @throws NullPointerException if the mapping function is {@code null} or
278 * returns a {@code null} result
279 */
280 public <U> Optional<U> flatMap(Function<? super T, ? extends Optional<? extends U>> mapper) {
281 Objects.requireNonNull(mapper);
282 if (!isPresent()) {
283 return empty();
284 } else {
285 @SuppressWarnings("unchecked")
286 Optional<U> r = (Optional<U>) mapper.apply(value);
287 return Objects.requireNonNull(r);
288 }
289 }
290
291 /**
292 * If a value is present, returns an {@code Optional} describing the value,
293 * otherwise returns an {@code Optional} produced by the supplying function.
294 *
295 * @param supplier the supplying function that produces an {@code Optional}
296 * to be returned
297 * @return returns an {@code Optional} describing the value of this
298 * {@code Optional}, if a value is present, otherwise an
299 * {@code Optional} produced by the supplying function.
300 * @throws NullPointerException if the supplying function is {@code null} or
301 * produces a {@code null} result
302 * @since 9
303 */
304 public Optional<T> or(Supplier<? extends Optional<? extends T>> supplier) {
305 Objects.requireNonNull(supplier);
306 if (isPresent()) {
307 return this;
308 } else {
309 @SuppressWarnings("unchecked")
310 Optional<T> r = (Optional<T>) supplier.get();
311 return Objects.requireNonNull(r);
312 }
313 }
314
315 /**
316 * If a value is present, returns a sequential {@link Stream} containing
317 * only that value, otherwise returns an empty {@code Stream}.
318 *
319 * @apiNote
320 * This method can be used to transform a {@code Stream} of optional
321 * elements to a {@code Stream} of present value elements:
322 * <pre>{@code
323 * Stream<Optional<T>> os = ..
324 * Stream<T> s = os.flatMap(Optional::stream)
325 * }</pre>
326 *
327 * @return the optional value as a {@code Stream}
328 * @since 9
329 */
330 public Stream<T> stream() {
331 if (!isPresent()) {
332 return Stream.empty();
333 } else {
334 return Stream.of(value);
335 }
336 }
337
338 /**
339 * If a value is present, returns the value, otherwise returns
340 * {@code other}.
341 *
342 * @param other the value to be returned, if no value is present.
343 * May be {@code null}.
344 * @return the value, if present, otherwise {@code other}
345 */
346 public T orElse(T other) {
347 return value != null ? value : other;
348 }
349
350 /**
351 * If a value is present, returns the value, otherwise returns the result
352 * produced by the supplying function.
353 *
354 * @param supplier the supplying function that produces a value to be returned
355 * @return the value, if present, otherwise the result produced by the
356 * supplying function
357 * @throws NullPointerException if no value is present and the supplying
358 * function is {@code null}
359 */
360 public T orElseGet(Supplier<? extends T> supplier) {
361 return value != null ? value : supplier.get();
362 }
363
364 /**
365 * If a value is present, returns the value, otherwise throws an exception
366 * produced by the exception supplying function.
367 *
368 * @apiNote
369 * A method reference to the exception constructor with an empty argument
370 * list can be used as the supplier. For example,
371 * {@code IllegalStateException::new}
372 *
373 * @param <X> Type of the exception to be thrown
374 * @param exceptionSupplier the supplying function that produces an
375 * exception to be thrown
376 * @return the value, if present
377 * @throws X if no value is present
378 * @throws NullPointerException if no value is present and the exception
379 * supplying function is {@code null}
380 */
381 public <X extends Throwable> T orElseThrow(Supplier<? extends X> exceptionSupplier) throws X {
382 if (value != null) {
383 return value;
384 } else {
385 throw exceptionSupplier.get();
386 }
387 }
388
389 /**
390 * Indicates whether some other object is "equal to" this {@code Optional}.
391 * The other object is considered equal if:
392 * <ul>
393 * <li>it is also an {@code Optional} and;
394 * <li>both instances have no value present or;
395 * <li>the present values are "equal to" each other via {@code equals()}.
396 * </ul>
397 *
398 * @param obj an object to be tested for equality
399 * @return {@code true} if the other object is "equal to" this object
400 * otherwise {@code false}
401 */
402 @Override
403 public boolean equals(Object obj) {
404 if (this == obj) {
405 return true;
406 }
407
408 if (!(obj instanceof Optional)) {
409 return false;
410 }
411
412 Optional<?> other = (Optional<?>) obj;
413 return Objects.equals(value, other.value);
414 }
415
416 /**
417 * Returns the hash code of the value, if present, otherwise {@code 0}
418 * (zero) if no value is present.
419 *
420 * @return hash code value of the present value or {@code 0} if no value is
421 * present
422 */
423 @Override
424 public int hashCode() {
425 return Objects.hashCode(value);
426 }
427
428 /**
429 * Returns a non-empty string representation of this {@code Optional}
430 * suitable for debugging. The exact presentation format is unspecified and
431 * may vary between implementations and versions.
432 *
433 * @implSpec
434 * If a value is present the result must include its string representation
435 * in the result. Empty and present {@code Optional}s must be unambiguously
436 * differentiable.
437 *
438 * @return the string representation of this instance
439 */
440 @Override
441 public String toString() {
442 return value != null
443 ? String.format("Optional[%s]", value)
444 : "Optional.empty";
445 }
446 }