1 /*
2 * Copyright (c) 2012, 2015, 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(java.lang.Object) orElse()}
40 * (returns a default value if no value is present) and
41 * {@link #ifPresent(java.util.function.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 * @param <T> the type of value
50 * @since 1.8
51 */
52 public final class Optional<T> {
53 /**
54 * Common instance for {@code empty()}.
55 */
56 private static final Optional<?> EMPTY = new Optional<>();
57
58 /**
59 * If non-null, the value; if null, indicates no value is present
60 */
61 private final T value;
62
63 /**
64 * Constructs an empty instance.
65 *
66 * @implNote Generally only one empty instance, {@link Optional#EMPTY},
67 * should exist per VM.
68 */
69 private Optional() {
70 this.value = null;
71 }
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 ==} against instances returned by
80 * {@code Optional.empty()}. There is no guarantee that it is a singleton.
81 * Instead, use {@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 non-{@code null} value to describe
96 * @throws NullPointerException if value is {@code null}
97 */
98 private Optional(T value) {
99 this.value = Objects.requireNonNull(value);
100 }
101
102 /**
103 * Returns an {@code Optional} describing the given non-{@code null}
104 * value.
105 *
106 * @param value the value to describe, which must be non-{@code null}
107 * @param <T> the type of the value
108 * @return an {@code Optional} with the value present
109 * @throws NullPointerException if value is {@code null}
110 */
111 public static <T> Optional<T> of(T value) {
112 return new Optional<>(value);
113 }
114
115 /**
116 * Returns an {@code Optional} describing the given value, if
117 * non-{@code null}, otherwise returns an empty {@code Optional}.
118 *
119 * @param value the possibly-{@code null} value to describe
120 * @param <T> the type of the value
121 * @return an {@code Optional} with a present value if the specified value
122 * is non-{@code null}, otherwise an empty {@code Optional}
123 */
124 public static <T> Optional<T> ofNullable(T value) {
125 return value == null ? empty() : of(value);
126 }
127
128 /**
129 * Equivalent to {@link #getWhenPresent()}.
130 * If a value is present, returns the value, otherwise throws
131 * {@code NoSuchElementException}.
132 *
133 * @deprecated
134 * This method's simple name {@code get} makes it the obvious method to
135 * call to retrieve the value from this {@code Optional}. However, it has
136 * no mechanism to avoid an exception if this {@code Optional} is empty.
137 * This tends to lead to code that mishandles empty {@code Optional}
138 * values. Consider using other methods that handle the case where
139 * the {@code Optional} might be empty, such as
140 * {@link #filter(java.util.function.Predicate) filter()},
141 * {@link #map(java.util.function.Function) map()},
142 * {@link #ifPresent(java.util.function.Consumer) ifPresent()}
143 * and related methods, and
144 * {@link #orElse(java.lang.Object) orElse()} and related methods.
145 * Use {@link getWhenPresent()} when it is known that a value is
146 * always present.
147 *
148 * @return the non-{@code null} value described by this {@code Optional}
149 * @throws NoSuchElementException if no value is present
150 * @see Optional#isPresent()
151 */
152 @Deprecated(since="9")
153 public T get() {
154 return getWhenPresent();
155 }
156
157 /**
158 * If a value is present, returns the value, otherwise throws
159 * {@code NoSuchElementException}.
160 *
161 * @apiNote
162 * Use this method only when it is known that a value is always present.
163 *
164 * @return the non-{@code null} value described by this {@code Optional}
165 * @throws NoSuchElementException if no value is present
166 * @see Optional#isPresent()
167 */
168 public T getWhenPresent() {
169 if (value == null) {
170 throw new NoSuchElementException("No value present");
171 }
172 return value;
173 }
174
175 /**
176 * If a value is present, returns {@code true}, otherwise {@code false}.
177 *
178 * @return {@code true} if a value is present, otherwise {@code false}
179 */
180 public boolean isPresent() {
181 return value != null;
182 }
183
184 /**
185 * If a value is present, performs the given action with the value,
186 * otherwise does nothing.
187 *
188 * @param action the action to be performed, if a value is present
189 * @throws NullPointerException if value is present and the given action is
190 * {@code null}
191 */
192 public void ifPresent(Consumer<? super T> action) {
193 if (value != null) {
194 action.accept(value);
195 }
196 }
197
198 /**
199 * If a value is present, performs the given action with the value,
200 * otherwise performs the given empty-based action.
201 *
202 * @param action the action to be performed, if a value is present
203 * @param emptyAction the empty-based action to be performed, if no value is
204 * present
205 * @throws NullPointerException if a value is present and the given action
206 * is {@code null}, or no value is present and the given empty-based
207 * action is {@code null}.
208 * @since 9
209 */
210 public void ifPresentOrElse(Consumer<? super T> action, Runnable emptyAction) {
211 if (value != null) {
212 action.accept(value);
213 } else {
214 emptyAction.run();
215 }
216 }
217
218 /**
219 * If a value is present, and the value matches the given predicate,
220 * returns an {@code Optional} describing the value, otherwise returns an
221 * empty {@code Optional}.
222 *
223 * @param predicate the predicate to apply to a value, if present
224 * @return an {@code Optional} describing the value of this
225 * {@code Optional}, if a value is present and the value matches the
226 * given predicate, otherwise an empty {@code Optional}
227 * @throws NullPointerException if the predicate is {@code null}
228 */
229 public Optional<T> filter(Predicate<? super T> predicate) {
230 Objects.requireNonNull(predicate);
231 if (!isPresent()) {
232 return this;
233 } else {
234 return predicate.test(value) ? this : empty();
235 }
236 }
237
238 /**
239 * If a value is present, returns an {@code Optional} describing (as if by
240 * {@link #ofNullable}) the result of applying the given mapping function to
241 * the value, otherwise returns an empty {@code Optional}.
242 *
243 * <p>If the mapping function returns a {@code null} result then this method
244 * returns an empty {@code Optional}.
245 *
246 * @apiNote
247 * This method supports post-processing on {@code Optional} values, without
248 * the need to explicitly check for a return status. For example, the
249 * following code traverses a stream of file names, selects one that has not
250 * yet been processed, and then opens that file, returning an
251 * {@code Optional<FileInputStream>}:
252 *
253 * <pre>{@code
254 * Optional<FileInputStream> fis =
255 * names.stream().filter(name -> !isProcessedYet(name))
256 * .findFirst()
257 * .map(name -> new FileInputStream(name));
258 * }</pre>
259 *
260 * Here, {@code findFirst} returns an {@code Optional<String>}, and then
261 * {@code map} returns an {@code Optional<FileInputStream>} for the desired
262 * file if one exists.
263 *
264 * @param mapper the mapping function to apply to a value, if present
265 * @param <U> The type of the value returned from the mapping function
266 * @return an {@code Optional} describing the result of applying a mapping
267 * function to the value of this {@code Optional}, if a value is
268 * present, otherwise an empty {@code Optional}
269 * @throws NullPointerException if the mapping function is {@code null}
270 */
271 public<U> Optional<U> map(Function<? super T, ? extends U> mapper) {
272 Objects.requireNonNull(mapper);
273 if (!isPresent()) {
274 return empty();
275 } else {
276 return Optional.ofNullable(mapper.apply(value));
277 }
278 }
279
280 /**
281 * If a value is present, returns the result of applying the given
282 * {@code Optional}-bearing mapping function to the value, otherwise returns
283 * an empty {@code Optional}.
284 *
285 * <p>This method is similar to {@link #map(Function)}, but the mapping
286 * function is one whose result is already an {@code Optional}, and if
287 * invoked, {@code flatMap} does not wrap it within an additional
288 * {@code Optional}.
289 *
290 * @param <U> The type of value of the {@code Optional} returned by the
291 * mapping function
292 * @param mapper the mapping function to apply to a value, if present
293 * @return the result of applying an {@code Optional}-bearing mapping
294 * function to the value of this {@code Optional}, if a value is
295 * present, otherwise an empty {@code Optional}
296 * @throws NullPointerException if the mapping function is {@code null} or
297 * returns a {@code null} result
298 */
299 public<U> Optional<U> flatMap(Function<? super T, Optional<U>> mapper) {
300 Objects.requireNonNull(mapper);
301 if (!isPresent()) {
302 return empty();
303 } else {
304 return Objects.requireNonNull(mapper.apply(value));
305 }
306 }
307
308 /**
309 * If a value is present, returns an {@code Optional} describing the value,
310 * otherwise returns an {@code Optional} produced by the supplying function.
311 *
312 * @param supplier the supplying function that produces an {@code Optional}
313 * to be returned
314 * @return returns an {@code Optional} describing the value of this
315 * {@code Optional}, if a value is present, otherwise an
316 * {@code Optional} produced by the supplying function.
317 * @throws NullPointerException if the supplying function is {@code null} or
318 * produces a {@code null} result
319 * @since 9
320 */
321 public Optional<T> or(Supplier<Optional<T>> supplier) {
322 Objects.requireNonNull(supplier);
323 if (isPresent()) {
324 return this;
325 } else {
326 return Objects.requireNonNull(supplier.get());
327 }
328 }
329
330 /**
331 * If a value is present, returns a sequential {@link Stream} containing
332 * only that value, otherwise returns an empty {@code Stream}.
333 *
334 * @apiNote
335 * This method can be used to transform a {@code Stream} of optional
336 * elements to a {@code Stream} of present value elements:
337 * <pre>{@code
338 * Stream<Optional<T>> os = ..
339 * Stream<T> s = os.flatMap(Optional::stream)
340 * }</pre>
341 *
342 * @return the optional value as a {@code Stream}
343 * @since 9
344 */
345 public Stream<T> stream() {
346 if (!isPresent()) {
347 return Stream.empty();
348 } else {
349 return Stream.of(value);
350 }
351 }
352
353 /**
354 * If a value is present, returns the value, otherwise returns
355 * {@code other}.
356 *
357 * @param other the value to be returned, if no value is present.
358 * May be {@code null}.
359 * @return the value, if present, otherwise {@code other}
360 */
361 public T orElse(T other) {
362 return value != null ? value : other;
363 }
364
365 /**
366 * If a value is present, returns the value, otherwise returns the result
367 * produced by the supplying function.
368 *
369 * @param supplier the supplying function that produces a value to be returned
370 * @return the value, if present, otherwise the result produced by the
371 * supplying function
372 * @throws NullPointerException if no value is present and the supplying
373 * function is {@code null}
374 */
375 public T orElseGet(Supplier<? extends T> supplier) {
376 return value != null ? value : supplier.get();
377 }
378
379 /**
380 * If a value is present, returns the value, otherwise throws an exception
381 * produced by the exception supplying function.
382 *
383 * @apiNote
384 * A method reference to the exception constructor with an empty argument
385 * list can be used as the supplier. For example,
386 * {@code IllegalStateException::new}
387 *
388 * @param <X> Type of the exception to be thrown
389 * @param exceptionSupplier the supplying function that produces an
390 * exception to be thrown
391 * @return the value, if present
392 * @throws X if no value is present
393 * @throws NullPointerException if no value is present and the exception
394 * supplying function is {@code null}
395 */
396 public <X extends Throwable> T orElseThrow(Supplier<? extends X> exceptionSupplier) throws X {
397 if (value != null) {
398 return value;
399 } else {
400 throw exceptionSupplier.get();
401 }
402 }
403
404 /**
405 * Indicates whether some other object is "equal to" this {@code Optional}.
406 * The other object is considered equal if:
407 * <ul>
408 * <li>it is also an {@code Optional} and;
409 * <li>both instances have no value present or;
410 * <li>the present values are "equal to" each other via {@code equals()}.
411 * </ul>
412 *
413 * @param obj an object to be tested for equality
414 * @return {@code true} if the other object is "equal to" this object
415 * otherwise {@code false}
416 */
417 @Override
418 public boolean equals(Object obj) {
419 if (this == obj) {
420 return true;
421 }
422
423 if (!(obj instanceof Optional)) {
424 return false;
425 }
426
427 Optional<?> other = (Optional<?>) obj;
428 return 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 ? String.format("Optional[%s]", value)
459 : "Optional.empty";
460 }
461 }