1 /* 2 * Copyright (c) 2012, 2020, 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 22 * or visit www.oracle.com if you need additional information or have any 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="https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/lang/doc-files/ValueBased.html">value-based</a> 46 * class; programmers should treat instances that are 47 * {@linkplain #equals(Object) equal} as interchangeable and should not 48 * use instances for synchronization, or unpredictable behavior may 49 * occur. For example, in a future release, synchronization may fail. 50 * 51 * @apiNote 52 * {@code Optional} is primarily intended for use as a method return type where 53 * there is a clear need to represent "no result," and where using {@code null} 54 * is likely to cause errors. A variable whose type is {@code Optional} should 55 * never itself be {@code null}; it should always point to an {@code Optional} 56 * instance. 57 * 58 * @param <T> the type of value 59 * @since 1.8 60 */ 61 @jdk.internal.ValueBased 62 public final class Optional<T> { 63 /** 64 * Common instance for {@code empty()}. 65 */ 66 private static final Optional<?> EMPTY = new Optional<>(null); 67 68 /** 69 * If non-null, the value; if null, indicates no value is present 70 */ 71 private final T value; 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 ==} or {@code !=} against instances returned by 80 * {@code Optional.empty()}. There is no guarantee that it is a singleton. 81 * Instead, use {@link #isEmpty()} or {@link #isPresent()}. 82 * 83 * @param <T> The type of the non-existent value 84 * @return an empty {@code Optional} 85 */ empty()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 value to describe; it's the caller's responsibility to 96 * ensure the value is non-{@code null} unless creating the singleton 97 * instance returned by {@code empty()}. 98 */ Optional(T value)99 private Optional(T value) { 100 this.value = value; 101 } 102 103 /** 104 * Returns an {@code Optional} describing the given non-{@code null} 105 * value. 106 * 107 * @param value the value to describe, which must be non-{@code null} 108 * @param <T> the type of the value 109 * @return an {@code Optional} with the value present 110 * @throws NullPointerException if value is {@code null} 111 */ of(T value)112 public static <T> Optional<T> of(T value) { 113 return new Optional<>(Objects.requireNonNull(value)); 114 } 115 116 /** 117 * Returns an {@code Optional} describing the given value, if 118 * non-{@code null}, otherwise returns an empty {@code Optional}. 119 * 120 * @param value the possibly-{@code null} value to describe 121 * @param <T> the type of the value 122 * @return an {@code Optional} with a present value if the specified value 123 * is non-{@code null}, otherwise an empty {@code Optional} 124 */ 125 @SuppressWarnings("unchecked") ofNullable(T value)126 public static <T> Optional<T> ofNullable(T value) { 127 return value == null ? (Optional<T>) EMPTY 128 : new Optional<>(value); 129 } 130 131 /** 132 * If a value is present, returns the value, otherwise throws 133 * {@code NoSuchElementException}. 134 * 135 * @apiNote 136 * The preferred alternative to this method is {@link #orElseThrow()}. 137 * 138 * @return the non-{@code null} value described by this {@code Optional} 139 * @throws NoSuchElementException if no value is present 140 */ get()141 public T get() { 142 if (value == null) { 143 throw new NoSuchElementException("No value present"); 144 } 145 return value; 146 } 147 148 /** 149 * If a value is present, returns {@code true}, otherwise {@code false}. 150 * 151 * @return {@code true} if a value is present, otherwise {@code false} 152 */ isPresent()153 public boolean isPresent() { 154 return value != null; 155 } 156 157 /** 158 * If a value is not present, returns {@code true}, otherwise 159 * {@code false}. 160 * 161 * @return {@code true} if a value is not present, otherwise {@code false} 162 * @since 11 163 */ isEmpty()164 public boolean isEmpty() { 165 return value == null; 166 } 167 168 /** 169 * If a value is present, performs the given action with the value, 170 * otherwise does nothing. 171 * 172 * @param action the action to be performed, if a value is present 173 * @throws NullPointerException if value is present and the given action is 174 * {@code null} 175 */ ifPresent(Consumer<? super T> action)176 public void ifPresent(Consumer<? super T> action) { 177 if (value != null) { 178 action.accept(value); 179 } 180 } 181 182 /** 183 * If a value is present, performs the given action with the value, 184 * otherwise performs the given empty-based action. 185 * 186 * @param action the action to be performed, if a value is present 187 * @param emptyAction the empty-based action to be performed, if no value is 188 * present 189 * @throws NullPointerException if a value is present and the given action 190 * is {@code null}, or no value is present and the given empty-based 191 * action is {@code null}. 192 * @since 9 193 */ ifPresentOrElse(Consumer<? super T> action, Runnable emptyAction)194 public void ifPresentOrElse(Consumer<? super T> action, Runnable emptyAction) { 195 if (value != null) { 196 action.accept(value); 197 } else { 198 emptyAction.run(); 199 } 200 } 201 202 /** 203 * If a value is present, and the value matches the given predicate, 204 * returns an {@code Optional} describing the value, otherwise returns an 205 * empty {@code Optional}. 206 * 207 * @param predicate the predicate to apply to a value, if present 208 * @return an {@code Optional} describing the value of this 209 * {@code Optional}, if a value is present and the value matches the 210 * given predicate, otherwise an empty {@code Optional} 211 * @throws NullPointerException if the predicate is {@code null} 212 */ filter(Predicate<? super T> predicate)213 public Optional<T> filter(Predicate<? super T> predicate) { 214 Objects.requireNonNull(predicate); 215 if (!isPresent()) { 216 return this; 217 } else { 218 return predicate.test(value) ? this : empty(); 219 } 220 } 221 222 /** 223 * If a value is present, returns an {@code Optional} describing (as if by 224 * {@link #ofNullable}) the result of applying the given mapping function to 225 * the value, otherwise returns an empty {@code Optional}. 226 * 227 * <p>If the mapping function returns a {@code null} result then this method 228 * returns an empty {@code Optional}. 229 * 230 * @apiNote 231 * This method supports post-processing on {@code Optional} values, without 232 * the need to explicitly check for a return status. For example, the 233 * following code traverses a stream of URIs, selects one that has not 234 * yet been processed, and creates a path from that URI, returning 235 * an {@code Optional<Path>}: 236 * 237 * <pre>{@code 238 * Optional<Path> p = 239 * uris.stream().filter(uri -> !isProcessedYet(uri)) 240 * .findFirst() 241 * .map(Paths::get); 242 * }</pre> 243 * 244 * Here, {@code findFirst} returns an {@code Optional<URI>}, and then 245 * {@code map} returns an {@code Optional<Path>} for the desired 246 * URI if one exists. 247 * 248 * @param mapper the mapping function to apply to a value, if present 249 * @param <U> The type of the value returned from the mapping function 250 * @return an {@code Optional} describing the result of applying a mapping 251 * function to the value of this {@code Optional}, if a value is 252 * present, otherwise an empty {@code Optional} 253 * @throws NullPointerException if the mapping function is {@code null} 254 */ map(Function<? super T, ? extends U> mapper)255 public <U> Optional<U> map(Function<? super T, ? extends U> mapper) { 256 Objects.requireNonNull(mapper); 257 if (!isPresent()) { 258 return empty(); 259 } else { 260 return Optional.ofNullable(mapper.apply(value)); 261 } 262 } 263 264 /** 265 * If a value is present, returns the result of applying the given 266 * {@code Optional}-bearing mapping function to the value, otherwise returns 267 * an empty {@code Optional}. 268 * 269 * <p>This method is similar to {@link #map(Function)}, but the mapping 270 * function is one whose result is already an {@code Optional}, and if 271 * invoked, {@code flatMap} does not wrap it within an additional 272 * {@code Optional}. 273 * 274 * @param <U> The type of value of the {@code Optional} returned by the 275 * mapping function 276 * @param mapper the mapping function to apply to a value, if present 277 * @return the result of applying an {@code Optional}-bearing mapping 278 * function to the value of this {@code Optional}, if a value is 279 * present, otherwise an empty {@code Optional} 280 * @throws NullPointerException if the mapping function is {@code null} or 281 * returns a {@code null} result 282 */ flatMap(Function<? super T, ? extends Optional<? extends U>> mapper)283 public <U> Optional<U> flatMap(Function<? super T, ? extends Optional<? extends U>> mapper) { 284 Objects.requireNonNull(mapper); 285 if (!isPresent()) { 286 return empty(); 287 } else { 288 @SuppressWarnings("unchecked") 289 Optional<U> r = (Optional<U>) mapper.apply(value); 290 return Objects.requireNonNull(r); 291 } 292 } 293 294 /** 295 * If a value is present, returns an {@code Optional} describing the value, 296 * otherwise returns an {@code Optional} produced by the supplying function. 297 * 298 * @param supplier the supplying function that produces an {@code Optional} 299 * to be returned 300 * @return returns an {@code Optional} describing the value of this 301 * {@code Optional}, if a value is present, otherwise an 302 * {@code Optional} produced by the supplying function. 303 * @throws NullPointerException if the supplying function is {@code null} or 304 * produces a {@code null} result 305 * @since 9 306 */ or(Supplier<? extends Optional<? extends T>> supplier)307 public Optional<T> or(Supplier<? extends Optional<? extends T>> supplier) { 308 Objects.requireNonNull(supplier); 309 if (isPresent()) { 310 return this; 311 } else { 312 @SuppressWarnings("unchecked") 313 Optional<T> r = (Optional<T>) supplier.get(); 314 return Objects.requireNonNull(r); 315 } 316 } 317 318 /** 319 * If a value is present, returns a sequential {@link Stream} containing 320 * only that value, otherwise returns an empty {@code Stream}. 321 * 322 * @apiNote 323 * This method can be used to transform a {@code Stream} of optional 324 * elements to a {@code Stream} of present value elements: 325 * <pre>{@code 326 * Stream<Optional<T>> os = .. 327 * Stream<T> s = os.flatMap(Optional::stream) 328 * }</pre> 329 * 330 * @return the optional value as a {@code Stream} 331 * @since 9 332 */ stream()333 public Stream<T> stream() { 334 if (!isPresent()) { 335 return Stream.empty(); 336 } else { 337 return Stream.of(value); 338 } 339 } 340 341 /** 342 * If a value is present, returns the value, otherwise returns 343 * {@code other}. 344 * 345 * @param other the value to be returned, if no value is present. 346 * May be {@code null}. 347 * @return the value, if present, otherwise {@code other} 348 */ orElse(T other)349 public T orElse(T other) { 350 return value != null ? value : other; 351 } 352 353 /** 354 * If a value is present, returns the value, otherwise returns the result 355 * produced by the supplying function. 356 * 357 * @param supplier the supplying function that produces a value to be returned 358 * @return the value, if present, otherwise the result produced by the 359 * supplying function 360 * @throws NullPointerException if no value is present and the supplying 361 * function is {@code null} 362 */ orElseGet(Supplier<? extends T> supplier)363 public T orElseGet(Supplier<? extends T> supplier) { 364 return value != null ? value : supplier.get(); 365 } 366 367 /** 368 * If a value is present, returns the value, otherwise throws 369 * {@code NoSuchElementException}. 370 * 371 * @return the non-{@code null} value described by this {@code Optional} 372 * @throws NoSuchElementException if no value is present 373 * @since 10 374 */ orElseThrow()375 public T orElseThrow() { 376 if (value == null) { 377 throw new NoSuchElementException("No value present"); 378 } 379 return value; 380 } 381 382 /** 383 * If a value is present, returns the value, otherwise throws an exception 384 * produced by the exception supplying function. 385 * 386 * @apiNote 387 * A method reference to the exception constructor with an empty argument 388 * list can be used as the supplier. For example, 389 * {@code IllegalStateException::new} 390 * 391 * @param <X> Type of the exception to be thrown 392 * @param exceptionSupplier the supplying function that produces an 393 * exception to be thrown 394 * @return the value, if present 395 * @throws X if no value is present 396 * @throws NullPointerException if no value is present and the exception 397 * supplying function is {@code null} 398 */ orElseThrow(Supplier<? extends X> exceptionSupplier)399 public <X extends Throwable> T orElseThrow(Supplier<? extends X> exceptionSupplier) throws X { 400 if (value != null) { 401 return value; 402 } else { 403 throw exceptionSupplier.get(); 404 } 405 } 406 407 /** 408 * Indicates whether some other object is "equal to" this {@code Optional}. 409 * The other object is considered equal if: 410 * <ul> 411 * <li>it is also an {@code Optional} and; 412 * <li>both instances have no value present or; 413 * <li>the present values are "equal to" each other via {@code equals()}. 414 * </ul> 415 * 416 * @param obj an object to be tested for equality 417 * @return {@code true} if the other object is "equal to" this object 418 * otherwise {@code false} 419 */ 420 @Override equals(Object obj)421 public boolean equals(Object obj) { 422 if (this == obj) { 423 return true; 424 } 425 426 if (!(obj instanceof Optional)) { 427 return false; 428 } 429 430 Optional<?> other = (Optional<?>) obj; 431 return Objects.equals(value, other.value); 432 } 433 434 /** 435 * Returns the hash code of the value, if present, otherwise {@code 0} 436 * (zero) if no value is present. 437 * 438 * @return hash code value of the present value or {@code 0} if no value is 439 * present 440 */ 441 @Override hashCode()442 public int hashCode() { 443 return Objects.hashCode(value); 444 } 445 446 /** 447 * Returns a non-empty string representation of this {@code Optional} 448 * suitable for debugging. The exact presentation format is unspecified and 449 * may vary between implementations and versions. 450 * 451 * @implSpec 452 * If a value is present the result must include its string representation 453 * in the result. Empty and present {@code Optional}s must be unambiguously 454 * differentiable. 455 * 456 * @return the string representation of this instance 457 */ 458 @Override toString()459 public String toString() { 460 return value != null 461 ? ("Optional[" + value + "]") 462 : "Optional.empty"; 463 } 464 } 465