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 * If a value is present, returns the value, otherwise throws
130 * {@code NoSuchElementException}.
131 *
132 * @return the non-{@code null} value described by this {@code Optional}
133 * @throws NoSuchElementException if no value is present
134 * @see Optional#isPresent()
135 */
136 public T get() {
137 if (value == null) {
138 throw new NoSuchElementException("No value present");
139 }
140 return value;
141 }
142
143 /**
144 * If a value is present, returns {@code true}, otherwise {@code false}.
145 *
146 * @return {@code true} if a value is present, otherwise {@code false}
147 */
148 public boolean isPresent() {
149 return value != null;
150 }
151
152 /**
153 * If a value is present, performs the given action with the value,
154 * otherwise does nothing.
155 *
156 * @param action the action to be performed, if a value is present
157 * @throws NullPointerException if value is present and the given action is
158 * {@code null}
159 */
160 public void ifPresent(Consumer<? super T> action) {
161 if (value != null) {
162 action.accept(value);
163 }
164 }
165
166 /**
167 * If a value is present, performs the given action with the value,
168 * otherwise performs the given empty-based action.
169 *
170 * @param action the action to be performed, if a value is present
171 * @param emptyAction the empty-based action to be performed, if no value is
172 * present
173 * @throws NullPointerException if a value is present and the given action
174 * is {@code null}, or no value is present and the given empty-based
175 * action is {@code null}.
176 * @since 9
177 */
178 public void ifPresentOrElse(Consumer<? super T> action, Runnable emptyAction) {
179 if (value != null) {
180 action.accept(value);
181 } else {
182 emptyAction.run();
183 }
184 }
185
186 /**
187 * If a value is present, and the value matches the given predicate,
188 * returns an {@code Optional} describing the value, otherwise returns an
189 * empty {@code Optional}.
190 *
191 * @param predicate the predicate to apply to a value, if present
192 * @return an {@code Optional} describing the value of this
193 * {@code Optional}, if a value is present and the value matches the
194 * given predicate, otherwise an empty {@code Optional}
195 * @throws NullPointerException if the predicate is {@code null}
196 */
197 public Optional<T> filter(Predicate<? super T> predicate) {
198 Objects.requireNonNull(predicate);
199 if (!isPresent()) {
200 return this;
201 } else {
202 return predicate.test(value) ? this : empty();
203 }
204 }
205
206 /**
207 * If a value is present, returns an {@code Optional} describing (as if by
208 * {@link #ofNullable}) the result of applying the given mapping function to
209 * the value, otherwise returns an empty {@code Optional}.
210 *
211 * <p>If the mapping function returns a {@code null} result then this method
212 * returns an empty {@code Optional}.
213 *
214 * @apiNote
215 * This method supports post-processing on {@code Optional} values, without
216 * the need to explicitly check for a return status. For example, the
217 * following code traverses a stream of file names, selects one that has not
218 * yet been processed, and then opens that file, returning an
219 * {@code Optional<FileInputStream>}:
220 *
221 * <pre>{@code
222 * Optional<FileInputStream> fis =
223 * names.stream().filter(name -> !isProcessedYet(name))
224 * .findFirst()
225 * .map(name -> new FileInputStream(name));
226 * }</pre>
227 *
228 * Here, {@code findFirst} returns an {@code Optional<String>}, and then
229 * {@code map} returns an {@code Optional<FileInputStream>} for the desired
230 * file if one exists.
231 *
232 * @param mapper the mapping function to apply to a value, if present
233 * @param <U> The type of the value returned from the mapping function
234 * @return an {@code Optional} describing the result of applying a mapping
235 * function to the value of this {@code Optional}, if a value is
236 * present, otherwise an empty {@code Optional}
237 * @throws NullPointerException if the mapping function is {@code null}
238 */
239 public<U> Optional<U> map(Function<? super T, ? extends U> mapper) {
240 Objects.requireNonNull(mapper);
241 if (!isPresent()) {
242 return empty();
243 } else {
244 return Optional.ofNullable(mapper.apply(value));
245 }
246 }
247
248 /**
249 * If a value is present, returns the result of applying the given
250 * {@code Optional}-bearing mapping function to the value, otherwise returns
251 * an empty {@code Optional}.
252 *
253 * <p>This method is similar to {@link #map(Function)}, but the mapping
254 * function is one whose result is already an {@code Optional}, and if
255 * invoked, {@code flatMap} does not wrap it within an additional
256 * {@code Optional}.
257 *
258 * @param <U> The type of value of the {@code Optional} returned by the
259 * mapping function
260 * @param mapper the mapping function to apply to a value, if present
261 * @return the result of applying an {@code Optional}-bearing mapping
262 * function to the value of this {@code Optional}, if a value is
263 * present, otherwise an empty {@code Optional}
264 * @throws NullPointerException if the mapping function is {@code null} or
265 * returns a {@code null} result
266 */
267 public<U> Optional<U> flatMap(Function<? super T, Optional<U>> mapper) {
268 Objects.requireNonNull(mapper);
269 if (!isPresent()) {
270 return empty();
271 } else {
272 return Objects.requireNonNull(mapper.apply(value));
273 }
274 }
275
276 /**
277 * If a value is present, returns an {@code Optional} describing the value,
278 * otherwise returns an {@code Optional} produced by the supplying function.
279 *
280 * @param supplier the supplying function that produces an {@code Optional}
281 * to be returned
282 * @return returns an {@code Optional} describing the value of this
283 * {@code Optional}, if a value is present, otherwise an
284 * {@code Optional} produced by the supplying function.
285 * @throws NullPointerException if the supplying function is {@code null} or
286 * produces a {@code null} result
287 * @since 9
288 */
289 public Optional<T> or(Supplier<Optional<T>> supplier) {
290 Objects.requireNonNull(supplier);
291 if (isPresent()) {
292 return this;
293 } else {
294 return Objects.requireNonNull(supplier.get());
295 }
296 }
297
298 /**
299 * If a value is present, returns a sequential {@link Stream} containing
300 * only that value, otherwise returns an empty {@code Stream}.
301 *
302 * @apiNote
303 * This method can be used to transform a {@code Stream} of optional
304 * elements to a {@code Stream} of present value elements:
305 * <pre>{@code
306 * Stream<Optional<T>> os = ..
307 * Stream<T> s = os.flatMap(Optional::stream)
308 * }</pre>
309 *
310 * @return the optional value as a {@code Stream}
311 * @since 9
312 */
313 public Stream<T> stream() {
314 if (!isPresent()) {
315 return Stream.empty();
316 } else {
317 return Stream.of(value);
318 }
319 }
320
321 /**
322 * If a value is present, returns the value, otherwise returns
323 * {@code other}.
324 *
325 * @param other the value to be returned, if no value is present.
326 * May be {@code null}.
327 * @return the value, if present, otherwise {@code other}
328 */
329 public T orElse(T other) {
330 return value != null ? value : other;
331 }
332
333 /**
334 * If a value is present, returns the value, otherwise returns the result
335 * produced by the supplying function.
336 *
337 * @param supplier the supplying function that produces a value to be returned
338 * @return the value, if present, otherwise the result produced by the
339 * supplying function
340 * @throws NullPointerException if no value is present and the supplying
341 * function is {@code null}
342 */
343 public T orElseGet(Supplier<? extends T> supplier) {
344 return value != null ? value : supplier.get();
345 }
346
347 /**
348 * If a value is present, returns the value, otherwise throws an exception
349 * produced by the exception supplying function.
350 *
351 * @apiNote
352 * A method reference to the exception constructor with an empty argument
353 * list can be used as the supplier. For example,
354 * {@code IllegalStateException::new}
355 *
356 * @param <X> Type of the exception to be thrown
357 * @param exceptionSupplier the supplying function that produces an
358 * exception to be thrown
359 * @return the value, if present
360 * @throws X if no value is present
361 * @throws NullPointerException if no value is present and the exception
362 * supplying function is {@code null}
363 */
364 public <X extends Throwable> T orElseThrow(Supplier<? extends X> exceptionSupplier) throws X {
365 if (value != null) {
366 return value;
367 } else {
368 throw exceptionSupplier.get();
369 }
370 }
371
372 /**
373 * Indicates whether some other object is "equal to" this {@code Optional}.
374 * The other object is considered equal if:
375 * <ul>
376 * <li>it is also an {@code Optional} and;
377 * <li>both instances have no value present or;
378 * <li>the present values are "equal to" each other via {@code equals()}.
379 * </ul>
380 *
381 * @param obj an object to be tested for equality
382 * @return {@code true} if the other object is "equal to" this object
383 * otherwise {@code false}
384 */
385 @Override
386 public boolean equals(Object obj) {
387 if (this == obj) {
388 return true;
389 }
390
391 if (!(obj instanceof Optional)) {
392 return false;
393 }
394
395 Optional<?> other = (Optional<?>) obj;
396 return Objects.equals(value, other.value);
397 }
398
399 /**
400 * Returns the hash code of the value, if present, otherwise {@code 0}
401 * (zero) if no value is present.
402 *
403 * @return hash code value of the present value or {@code 0} if no value is
404 * present
405 */
406 @Override
407 public int hashCode() {
408 return Objects.hashCode(value);
409 }
410
411 /**
412 * Returns a non-empty string representation of this {@code Optional}
413 * suitable for debugging. The exact presentation format is unspecified and
414 * may vary between implementations and versions.
415 *
416 * @implSpec
417 * If a value is present the result must include its string representation
418 * in the result. Empty and present {@code Optional}s must be unambiguously
419 * differentiable.
420 *
421 * @return the string representation of this instance
422 */
423 @Override
424 public String toString() {
425 return value != null
426 ? String.format("Optional[%s]", value)
427 : "Optional.empty";
428 }
429 }