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