1 /* 2 * Copyright (c) 2012, 2013, 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 32 /** 33 * A container object which may or may not contain a non-null value. 34 * If a value is present, {@code isPresent()} will return {@code true} and 35 * {@code get()} will return the value. 36 * 37 * <p>Additional methods that depend on the presence or absence of a contained 38 * value are provided, such as {@link #orElse(java.lang.Object) orElse()} 39 * (return a default value if value not present) and 40 * {@link #ifPresent(java.util.function.Consumer) ifPresent()} (execute a block 41 * of code if the value is present). 42 * 43 * <p>This is a <a href="../lang/doc-files/ValueBased.html">value-based</a> 44 * class; use of identity-sensitive operations (including reference equality 45 * ({@code ==}), identity hash code, or synchronization) on instances of 46 * {@code Optional} may have unpredictable results and should be avoided. 47 * 48 * @since 1.8 49 */ 50 public final class Optional<T> { 51 /** 52 * Common instance for {@code empty()}. 53 */ 54 private static final Optional<?> EMPTY = new Optional<>(); 55 56 /** 57 * If non-null, the value; if null, indicates no value is present 58 */ 59 private final T value; 60 61 /** 62 * Constructs an empty instance. 63 * 64 * @implNote Generally only one empty instance, {@link Optional#EMPTY}, 65 * should exist per VM. 66 */ Optional()67 private Optional() { 68 this.value = null; 69 } 70 71 /** 72 * Returns an empty {@code Optional} instance. No value is present for this 73 * Optional. 74 * 75 * @apiNote Though it may be tempting to do so, avoid testing if an object 76 * is empty by comparing with {@code ==} against instances returned by 77 * {@code Option.empty()}. There is no guarantee that it is a singleton. 78 * Instead, use {@link #isPresent()}. 79 * 80 * @param <T> Type of the non-existent value 81 * @return an empty {@code Optional} 82 */ empty()83 public static<T> Optional<T> empty() { 84 @SuppressWarnings("unchecked") 85 Optional<T> t = (Optional<T>) EMPTY; 86 return t; 87 } 88 89 /** 90 * Constructs an instance with the value present. 91 * 92 * @param value the non-null value to be present 93 * @throws NullPointerException if value is null 94 */ Optional(T value)95 private Optional(T value) { 96 this.value = Objects.requireNonNull(value); 97 } 98 99 /** 100 * Returns an {@code Optional} with the specified present non-null value. 101 * 102 * @param <T> the class of the value 103 * @param value the value to be present, which must be non-null 104 * @return an {@code Optional} with the value present 105 * @throws NullPointerException if value is null 106 */ of(T value)107 public static <T> Optional<T> of(T value) { 108 return new Optional<>(value); 109 } 110 111 /** 112 * Returns an {@code Optional} describing the specified value, if non-null, 113 * otherwise returns an empty {@code Optional}. 114 * 115 * @param <T> the class of the value 116 * @param value the possibly-null value to describe 117 * @return an {@code Optional} with a present value if the specified value 118 * is non-null, otherwise an empty {@code Optional} 119 */ ofNullable(T value)120 public static <T> Optional<T> ofNullable(T value) { 121 return value == null ? empty() : of(value); 122 } 123 124 /** 125 * If a value is present in this {@code Optional}, returns the value, 126 * otherwise throws {@code NoSuchElementException}. 127 * 128 * @return the non-null value held by this {@code Optional} 129 * @throws NoSuchElementException if there is no value present 130 * 131 * @see Optional#isPresent() 132 */ get()133 public T get() { 134 if (value == null) { 135 throw new NoSuchElementException("No value present"); 136 } 137 return value; 138 } 139 140 /** 141 * Return {@code true} if there is a value present, otherwise {@code false}. 142 * 143 * @return {@code true} if there is a value present, otherwise {@code false} 144 */ isPresent()145 public boolean isPresent() { 146 return value != null; 147 } 148 149 /** 150 * If a value is present, invoke the specified consumer with the value, 151 * otherwise do nothing. 152 * 153 * @param consumer block to be executed if a value is present 154 * @throws NullPointerException if value is present and {@code consumer} is 155 * null 156 */ ifPresent(Consumer<? super T> consumer)157 public void ifPresent(Consumer<? super T> consumer) { 158 if (value != null) 159 consumer.accept(value); 160 } 161 162 /** 163 * If a value is present, and the value matches the given predicate, 164 * return an {@code Optional} describing the value, otherwise return an 165 * empty {@code Optional}. 166 * 167 * @param predicate a predicate to apply to the value, if present 168 * @return an {@code Optional} describing the value of this {@code Optional} 169 * if a value is present and the value matches the given predicate, 170 * otherwise an empty {@code Optional} 171 * @throws NullPointerException if the predicate is null 172 */ filter(Predicate<? super T> predicate)173 public Optional<T> filter(Predicate<? super T> predicate) { 174 Objects.requireNonNull(predicate); 175 if (!isPresent()) 176 return this; 177 else 178 return predicate.test(value) ? this : empty(); 179 } 180 181 /** 182 * If a value is present, apply the provided mapping function to it, 183 * and if the result is non-null, return an {@code Optional} describing the 184 * result. Otherwise return an empty {@code Optional}. 185 * 186 * @apiNote This method supports post-processing on optional values, without 187 * the need to explicitly check for a return status. For example, the 188 * following code traverses a stream of file names, selects one that has 189 * not yet been processed, and then opens that file, returning an 190 * {@code Optional<FileInputStream>}: 191 * 192 * <pre>{@code 193 * Optional<FileInputStream> fis = 194 * names.stream().filter(name -> !isProcessedYet(name)) 195 * .findFirst() 196 * .map(name -> new FileInputStream(name)); 197 * }</pre> 198 * 199 * Here, {@code findFirst} returns an {@code Optional<String>}, and then 200 * {@code map} returns an {@code Optional<FileInputStream>} for the desired 201 * file if one exists. 202 * 203 * @param <U> The type of the result of the mapping function 204 * @param mapper a mapping function to apply to the value, if present 205 * @return an {@code Optional} describing the result of applying a mapping 206 * function to the value of this {@code Optional}, if a value is present, 207 * otherwise an empty {@code Optional} 208 * @throws NullPointerException if the mapping function is null 209 */ map(Function<? super T, ? extends U> mapper)210 public<U> Optional<U> map(Function<? super T, ? extends U> mapper) { 211 Objects.requireNonNull(mapper); 212 if (!isPresent()) 213 return empty(); 214 else { 215 return Optional.ofNullable(mapper.apply(value)); 216 } 217 } 218 219 /** 220 * If a value is present, apply the provided {@code Optional}-bearing 221 * mapping function to it, return that result, otherwise return an empty 222 * {@code Optional}. This method is similar to {@link #map(Function)}, 223 * but the provided mapper is one whose result is already an {@code Optional}, 224 * and if invoked, {@code flatMap} does not wrap it with an additional 225 * {@code Optional}. 226 * 227 * @param <U> The type parameter to the {@code Optional} returned by 228 * @param mapper a mapping function to apply to the value, if present 229 * the mapping function 230 * @return the result of applying an {@code Optional}-bearing mapping 231 * function to the value of this {@code Optional}, if a value is present, 232 * otherwise an empty {@code Optional} 233 * @throws NullPointerException if the mapping function is null or returns 234 * a null result 235 */ flatMap(Function<? super T, Optional<U>> mapper)236 public<U> Optional<U> flatMap(Function<? super T, Optional<U>> mapper) { 237 Objects.requireNonNull(mapper); 238 if (!isPresent()) 239 return empty(); 240 else { 241 return Objects.requireNonNull(mapper.apply(value)); 242 } 243 } 244 245 /** 246 * Return the value if present, otherwise return {@code other}. 247 * 248 * @param other the value to be returned if there is no value present, may 249 * be null 250 * @return the value, if present, otherwise {@code other} 251 */ orElse(T other)252 public T orElse(T other) { 253 return value != null ? value : other; 254 } 255 256 /** 257 * Return the value if present, otherwise invoke {@code other} and return 258 * the result of that invocation. 259 * 260 * @param other a {@code Supplier} whose result is returned if no value 261 * is present 262 * @return the value if present otherwise the result of {@code other.get()} 263 * @throws NullPointerException if value is not present and {@code other} is 264 * null 265 */ orElseGet(Supplier<? extends T> other)266 public T orElseGet(Supplier<? extends T> other) { 267 return value != null ? value : other.get(); 268 } 269 270 /** 271 * Return the contained value, if present, otherwise throw an exception 272 * to be created by the provided supplier. 273 * 274 * @apiNote A method reference to the exception constructor with an empty 275 * argument list can be used as the supplier. For example, 276 * {@code IllegalStateException::new} 277 * 278 * @param <X> Type of the exception to be thrown 279 * @param exceptionSupplier The supplier which will return the exception to 280 * be thrown 281 * @return the present value 282 * @throws X if there is no value present 283 * @throws NullPointerException if no value is present and 284 * {@code exceptionSupplier} is null 285 */ orElseThrow(Supplier<? extends X> exceptionSupplier)286 public <X extends Throwable> T orElseThrow(Supplier<? extends X> exceptionSupplier) throws X { 287 if (value != null) { 288 return value; 289 } else { 290 throw exceptionSupplier.get(); 291 } 292 } 293 294 /** 295 * Indicates whether some other object is "equal to" this Optional. The 296 * other object is considered equal if: 297 * <ul> 298 * <li>it is also an {@code Optional} and; 299 * <li>both instances have no value present or; 300 * <li>the present values are "equal to" each other via {@code equals()}. 301 * </ul> 302 * 303 * @param obj an object to be tested for equality 304 * @return {code true} if the other object is "equal to" this object 305 * otherwise {@code false} 306 */ 307 @Override equals(Object obj)308 public boolean equals(Object obj) { 309 if (this == obj) { 310 return true; 311 } 312 313 if (!(obj instanceof Optional)) { 314 return false; 315 } 316 317 Optional<?> other = (Optional<?>) obj; 318 return Objects.equals(value, other.value); 319 } 320 321 /** 322 * Returns the hash code value of the present value, if any, or 0 (zero) if 323 * no value is present. 324 * 325 * @return hash code value of the present value or 0 if no value is present 326 */ 327 @Override hashCode()328 public int hashCode() { 329 return Objects.hashCode(value); 330 } 331 332 /** 333 * Returns a non-empty string representation of this Optional suitable for 334 * debugging. The exact presentation format is unspecified and may vary 335 * between implementations and versions. 336 * 337 * @implSpec If a value is present the result must include its string 338 * representation in the result. Empty and present Optionals must be 339 * unambiguously differentiable. 340 * 341 * @return the string representation of this instance 342 */ 343 @Override toString()344 public String toString() { 345 return value != null 346 ? String.format("Optional[%s]", value) 347 : "Optional.empty"; 348 } 349 } 350