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
  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} 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 preferred alternative to this method is {@link #orElseThrow()}.
 141      *
 142      * @return the non-{@code null} value described by this {@code Optional}
 143      * @throws NoSuchElementException if no value is present
 144      */
 145     public T get() {
 146         if (value == null) {
 147             throw new NoSuchElementException("No value present");
 148         }
 149         return value;
 150     }
 151 
 152     /**
 153      * If a value is present, returns {@code true}, otherwise {@code false}.
 154      *
 155      * @return {@code true} if a value is present, otherwise {@code false}
 156      */
 157     public boolean isPresent() {
 158         return value != null;
 159     }
 160 
 161     /**
 162      * If a value is present, performs the given action with the value,
 163      * otherwise does nothing.
 164      *
 165      * @param action the action to be performed, if a value is present
 166      * @throws NullPointerException if value is present and the given action is
 167      *         {@code null}
 168      */
 169     public void ifPresent(Consumer<? super T> action) {
 170         if (value != null) {
 171             action.accept(value);
 172         }
 173     }
 174 
 175     /**
 176      * If a value is present, performs the given action with the value,
 177      * otherwise performs the given empty-based action.
 178      *
 179      * @param action the action to be performed, if a value is present
 180      * @param emptyAction the empty-based action to be performed, if no value is
 181      *        present
 182      * @throws NullPointerException if a value is present and the given action
 183      *         is {@code null}, or no value is present and the given empty-based
 184      *         action is {@code null}.
 185      * @since 9
 186      */
 187     public void ifPresentOrElse(Consumer<? super T> action, Runnable emptyAction) {
 188         if (value != null) {
 189             action.accept(value);
 190         } else {
 191             emptyAction.run();
 192         }
 193     }
 194 
 195     /**
 196      * If a value is present, and the value matches the given predicate,
 197      * returns an {@code Optional} describing the value, otherwise returns an
 198      * empty {@code Optional}.
 199      *
 200      * @param predicate the predicate to apply to a value, if present
 201      * @return an {@code Optional} describing the value of this
 202      *         {@code Optional}, if a value is present and the value matches the
 203      *         given predicate, otherwise an empty {@code Optional}
 204      * @throws NullPointerException if the predicate is {@code null}
 205      */
 206     public Optional<T> filter(Predicate<? super T> predicate) {
 207         Objects.requireNonNull(predicate);
 208         if (!isPresent()) {
 209             return this;
 210         } else {
 211             return predicate.test(value) ? this : empty();
 212         }
 213     }
 214 
 215     /**
 216      * If a value is present, returns an {@code Optional} describing (as if by
 217      * {@link #ofNullable}) the result of applying the given mapping function to
 218      * the value, otherwise returns an empty {@code Optional}.
 219      *
 220      * <p>If the mapping function returns a {@code null} result then this method
 221      * returns an empty {@code Optional}.
 222      *
 223      * @apiNote
 224      * This method supports post-processing on {@code Optional} values, without
 225      * the need to explicitly check for a return status.  For example, the
 226      * following code traverses a stream of URIs, selects one that has not
 227      * yet been processed, and creates a path from that URI, returning
 228      * an {@code Optional<Path>}:
 229      *
 230      * <pre>{@code
 231      *     Optional<Path> p =
 232      *         uris.stream().filter(uri -> !isProcessedYet(uri))
 233      *                       .findFirst()
 234      *                       .map(Paths::get);
 235      * }</pre>
 236      *
 237      * Here, {@code findFirst} returns an {@code Optional<URI>}, and then
 238      * {@code map} returns an {@code Optional<Path>} for the desired
 239      * URI if one exists.
 240      *
 241      * @param mapper the mapping function to apply to a value, if present
 242      * @param <U> The type of the value returned from the mapping function
 243      * @return an {@code Optional} describing the result of applying a mapping
 244      *         function to the value of this {@code Optional}, if a value is
 245      *         present, otherwise an empty {@code Optional}
 246      * @throws NullPointerException if the mapping function is {@code null}
 247      */
 248     public <U> Optional<U> map(Function<? super T, ? extends U> mapper) {
 249         Objects.requireNonNull(mapper);
 250         if (!isPresent()) {
 251             return empty();
 252         } else {
 253             return Optional.ofNullable(mapper.apply(value));
 254         }
 255     }
 256 
 257     /**
 258      * If a value is present, returns the result of applying the given
 259      * {@code Optional}-bearing mapping function to the value, otherwise returns
 260      * an empty {@code Optional}.
 261      *
 262      * <p>This method is similar to {@link #map(Function)}, but the mapping
 263      * function is one whose result is already an {@code Optional}, and if
 264      * invoked, {@code flatMap} does not wrap it within an additional
 265      * {@code Optional}.
 266      *
 267      * @param <U> The type of value of the {@code Optional} returned by the
 268      *            mapping function
 269      * @param mapper the mapping function to apply to a value, if present
 270      * @return the result of applying an {@code Optional}-bearing mapping
 271      *         function to the value of this {@code Optional}, if a value is
 272      *         present, otherwise an empty {@code Optional}
 273      * @throws NullPointerException if the mapping function is {@code null} or
 274      *         returns a {@code null} result
 275      */
 276     public <U> Optional<U> flatMap(Function<? super T, ? extends Optional<? extends U>> mapper) {
 277         Objects.requireNonNull(mapper);
 278         if (!isPresent()) {
 279             return empty();
 280         } else {
 281             @SuppressWarnings("unchecked")
 282             Optional<U> r = (Optional<U>) mapper.apply(value);
 283             return Objects.requireNonNull(r);
 284         }
 285     }
 286 
 287     /**
 288      * If a value is present, returns an {@code Optional} describing the value,
 289      * otherwise returns an {@code Optional} produced by the supplying function.
 290      *
 291      * @param supplier the supplying function that produces an {@code Optional}
 292      *        to be returned
 293      * @return returns an {@code Optional} describing the value of this
 294      *         {@code Optional}, if a value is present, otherwise an
 295      *         {@code Optional} produced by the supplying function.
 296      * @throws NullPointerException if the supplying function is {@code null} or
 297      *         produces a {@code null} result
 298      * @since 9
 299      */
 300     public Optional<T> or(Supplier<? extends Optional<? extends T>> supplier) {
 301         Objects.requireNonNull(supplier);
 302         if (isPresent()) {
 303             return this;
 304         } else {
 305             @SuppressWarnings("unchecked")
 306             Optional<T> r = (Optional<T>) supplier.get();
 307             return Objects.requireNonNull(r);
 308         }
 309     }
 310 
 311     /**
 312      * If a value is present, returns a sequential {@link Stream} containing
 313      * only that value, otherwise returns an empty {@code Stream}.
 314      *
 315      * @apiNote
 316      * This method can be used to transform a {@code Stream} of optional
 317      * elements to a {@code Stream} of present value elements:
 318      * <pre>{@code
 319      *     Stream<Optional<T>> os = ..
 320      *     Stream<T> s = os.flatMap(Optional::stream)
 321      * }</pre>
 322      *
 323      * @return the optional value as a {@code Stream}
 324      * @since 9
 325      */
 326     public Stream<T> stream() {
 327         if (!isPresent()) {
 328             return Stream.empty();
 329         } else {
 330             return Stream.of(value);
 331         }
 332     }
 333 
 334     /**
 335      * If a value is present, returns the value, otherwise returns
 336      * {@code other}.
 337      *
 338      * @param other the value to be returned, if no value is present.
 339      *        May be {@code null}.
 340      * @return the value, if present, otherwise {@code other}
 341      */
 342     public T orElse(T other) {
 343         return value != null ? value : other;
 344     }
 345 
 346     /**
 347      * If a value is present, returns the value, otherwise returns the result
 348      * produced by the supplying function.
 349      *
 350      * @param supplier the supplying function that produces a value to be returned
 351      * @return the value, if present, otherwise the result produced by the
 352      *         supplying function
 353      * @throws NullPointerException if no value is present and the supplying
 354      *         function is {@code null}
 355      */
 356     public T orElseGet(Supplier<? extends T> supplier) {
 357         return value != null ? value : supplier.get();
 358     }
 359 
 360     /**
 361      * If a value is present, returns the value, otherwise throws
 362      * {@code NoSuchElementException}.
 363      *
 364      * @return the non-{@code null} value described by this {@code Optional}
 365      * @throws NoSuchElementException if no value is present
 366      * @since 10
 367      */
 368     public T orElseThrow() {
 369         if (value == null) {
 370             throw new NoSuchElementException("No value present");
 371         }
 372         return value;
 373     }
 374 
 375     /**
 376      * If a value is present, returns the value, otherwise throws an exception
 377      * produced by the exception supplying function.
 378      *
 379      * @apiNote
 380      * A method reference to the exception constructor with an empty argument
 381      * list can be used as the supplier. For example,
 382      * {@code IllegalStateException::new}
 383      *
 384      * @param <X> Type of the exception to be thrown
 385      * @param exceptionSupplier the supplying function that produces an
 386      *        exception to be thrown
 387      * @return the value, if present
 388      * @throws X if no value is present
 389      * @throws NullPointerException if no value is present and the exception
 390      *          supplying function is {@code null}
 391      */
 392     public <X extends Throwable> T orElseThrow(Supplier<? extends X> exceptionSupplier) throws X {
 393         if (value != null) {
 394             return value;
 395         } else {
 396             throw exceptionSupplier.get();
 397         }
 398     }
 399 
 400     /**
 401      * Indicates whether some other object is "equal to" this {@code Optional}.
 402      * The other object is considered equal if:
 403      * <ul>
 404      * <li>it is also an {@code Optional} and;
 405      * <li>both instances have no value present or;
 406      * <li>the present values are "equal to" each other via {@code equals()}.
 407      * </ul>
 408      *
 409      * @param obj an object to be tested for equality
 410      * @return {@code true} if the other object is "equal to" this object
 411      *         otherwise {@code false}
 412      */
 413     @Override
 414     public boolean equals(Object obj) {
 415         if (this == obj) {
 416             return true;
 417         }
 418 
 419         if (!(obj instanceof Optional)) {
 420             return false;
 421         }
 422 
 423         Optional<?> other = (Optional<?>) obj;
 424         return Objects.equals(value, other.value);
 425     }
 426 
 427     /**
 428      * Returns the hash code of the value, if present, otherwise {@code 0}
 429      * (zero) if no value is present.
 430      *
 431      * @return hash code value of the present value or {@code 0} if no value is
 432      *         present
 433      */
 434     @Override
 435     public int hashCode() {
 436         return Objects.hashCode(value);
 437     }
 438 
 439     /**
 440      * Returns a non-empty string representation of this {@code Optional}
 441      * suitable for debugging.  The exact presentation format is unspecified and
 442      * may vary between implementations and versions.
 443      *
 444      * @implSpec
 445      * If a value is present the result must include its string representation
 446      * in the result.  Empty and present {@code Optional}s must be unambiguously
 447      * differentiable.
 448      *
 449      * @return the string representation of this instance
 450      */
 451     @Override
 452     public String toString() {
 453         return value != null
 454             ? String.format("Optional[%s]", value)
 455             : "Optional.empty";
 456     }
 457 }