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 */ Optional()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 */ empty()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 * @throws NullPointerException if value is null 89 */ Optional(T value)90 private Optional(T value) { 91 this.value = Objects.requireNonNull(value); 92 } 93 94 /** 95 * Returns an {@code Optional} with the specified present non-null value. 96 * 97 * @param <T> the class of the value 98 * @param value the value to be present, which must be non-null 99 * @return an {@code Optional} with the value present 100 * @throws NullPointerException if value is null 101 */ of(T value)102 public static <T> Optional<T> of(T value) { 103 return new Optional<>(value); 104 } 105 106 /** 107 * Returns an {@code Optional} describing the specified value, if non-null, 108 * otherwise returns an empty {@code Optional}. 109 * 110 * @param <T> the class of the value 111 * @param value the possibly-null value to describe 112 * @return an {@code Optional} with a present value if the specified value 113 * is non-null, otherwise an empty {@code Optional} 114 */ ofNullable(T value)115 public static <T> Optional<T> ofNullable(T value) { 116 return value == null ? empty() : of(value); 117 } 118 119 /** 120 * If a value is present in this {@code Optional}, returns the value, 121 * otherwise throws {@code NoSuchElementException}. 122 * 123 * @return the non-null value held by this {@code Optional} 124 * @throws NoSuchElementException if there is no value present 125 * 126 * @see Optional#isPresent() 127 */ get()128 public T get() { 129 if (value == null) { 130 throw new NoSuchElementException("No value present"); 131 } 132 return value; 133 } 134 135 /** 136 * Return {@code true} if there is a value present, otherwise {@code false}. 137 * 138 * @return {@code true} if there is a value present, otherwise {@code false} 139 */ isPresent()140 public boolean isPresent() { 141 return value != null; 142 } 143 144 /** 145 * If a value is present, invoke the specified consumer with the value, 146 * otherwise do nothing. 147 * 148 * @param consumer block to be executed if a value is present 149 * @throws NullPointerException if value is present and {@code consumer} is 150 * null 151 */ ifPresent(Consumer<? super T> consumer)152 public void ifPresent(Consumer<? super T> consumer) { 153 if (value != null) 154 consumer.accept(value); 155 } 156 157 /** 158 * If a value is present, and the value matches the given predicate, 159 * return an {@code Optional} describing the value, otherwise return an 160 * empty {@code Optional}. 161 * 162 * @param predicate a predicate to apply to the value, if present 163 * @return an {@code Optional} describing the value of this {@code Optional} 164 * if a value is present and the value matches the given predicate, 165 * otherwise an empty {@code Optional} 166 * @throws NullPointerException if the predicate is null 167 */ filter(Predicate<? super T> predicate)168 public Optional<T> filter(Predicate<? super T> predicate) { 169 Objects.requireNonNull(predicate); 170 if (!isPresent()) 171 return this; 172 else 173 return predicate.test(value) ? this : empty(); 174 } 175 176 /** 177 * If a value is present, apply the provided mapping function to it, 178 * and if the result is non-null, return an {@code Optional} describing the 179 * result. Otherwise return an empty {@code Optional}. 180 * 181 * @apiNote This method supports post-processing on optional values, without 182 * the need to explicitly check for a return status. For example, the 183 * following code traverses a stream of file names, selects one that has 184 * not yet been processed, and then opens that file, returning an 185 * {@code Optional<FileInputStream>}: 186 * 187 * <pre>{@code 188 * Optional<FileInputStream> fis = 189 * names.stream().filter(name -> !isProcessedYet(name)) 190 * .findFirst() 191 * .map(name -> new FileInputStream(name)); 192 * }</pre> 193 * 194 * Here, {@code findFirst} returns an {@code Optional<String>}, and then 195 * {@code map} returns an {@code Optional<FileInputStream>} for the desired 196 * file if one exists. 197 * 198 * @param <U> The type of the result of the mapping function 199 * @param mapper a mapping function to apply to the value, if present 200 * @return an {@code Optional} describing the result of applying a mapping 201 * function to the value of this {@code Optional}, if a value is present, 202 * otherwise an empty {@code Optional} 203 * @throws NullPointerException if the mapping function is null 204 */ map(Function<? super T, ? extends U> mapper)205 public<U> Optional<U> map(Function<? super T, ? extends U> mapper) { 206 Objects.requireNonNull(mapper); 207 if (!isPresent()) 208 return empty(); 209 else { 210 return Optional.ofNullable(mapper.apply(value)); 211 } 212 } 213 214 /** 215 * If a value is present, apply the provided {@code Optional}-bearing 216 * mapping function to it, return that result, otherwise return an empty 217 * {@code Optional}. This method is similar to {@link #map(Function)}, 218 * but the provided mapper is one whose result is already an {@code Optional}, 219 * and if invoked, {@code flatMap} does not wrap it with an additional 220 * {@code Optional}. 221 * 222 * @param <U> The type parameter to the {@code Optional} returned by 223 * @param mapper a mapping function to apply to the value, if present 224 * the mapping function 225 * @return the result of applying an {@code Optional}-bearing mapping 226 * function to the value of this {@code Optional}, if a value is present, 227 * otherwise an empty {@code Optional} 228 * @throws NullPointerException if the mapping function is null or returns 229 * a null result 230 */ flatMap(Function<? super T, Optional<U>> mapper)231 public<U> Optional<U> flatMap(Function<? super T, Optional<U>> mapper) { 232 Objects.requireNonNull(mapper); 233 if (!isPresent()) 234 return empty(); 235 else { 236 return Objects.requireNonNull(mapper.apply(value)); 237 } 238 } 239 240 /** 241 * Return the value if present, otherwise return {@code other}. 242 * 243 * @param other the value to be returned if there is no value present, may 244 * be null 245 * @return the value, if present, otherwise {@code other} 246 */ orElse(T other)247 public T orElse(T other) { 248 return value != null ? value : other; 249 } 250 251 /** 252 * Return the value if present, otherwise invoke {@code other} and return 253 * the result of that invocation. 254 * 255 * @param other a {@code Supplier} whose result is returned if no value 256 * is present 257 * @return the value if present otherwise the result of {@code other.get()} 258 * @throws NullPointerException if value is not present and {@code other} is 259 * null 260 */ orElseGet(Supplier<? extends T> other)261 public T orElseGet(Supplier<? extends T> other) { 262 return value != null ? value : other.get(); 263 } 264 265 /** 266 * Return the contained value, if present, otherwise throw an exception 267 * to be created by the provided supplier. 268 * 269 * @apiNote A method reference to the exception constructor with an empty 270 * argument list can be used as the supplier. For example, 271 * {@code IllegalStateException::new} 272 * 273 * @param <X> Type of the exception to be thrown 274 * @param exceptionSupplier The supplier which will return the exception to 275 * be thrown 276 * @return the present value 277 * @throws X if there is no value present 278 * @throws NullPointerException if no value is present and 279 * {@code exceptionSupplier} is null 280 */ orElseThrow(Supplier<? extends X> exceptionSupplier)281 public <X extends Throwable> T orElseThrow(Supplier<? extends X> exceptionSupplier) throws X { 282 if (value != null) { 283 return value; 284 } else { 285 throw exceptionSupplier.get(); 286 } 287 } 288 289 /** 290 * Indicates whether some other object is "equal to" this Optional. The 291 * other object is considered equal if: 292 * <ul> 293 * <li>it is also an {@code Optional} and; 294 * <li>both instances have no value present or; 295 * <li>the present values are "equal to" each other via {@code equals()}. 296 * </ul> 297 * 298 * @param obj an object to be tested for equality 299 * @return {code true} if the other object is "equal to" this object 300 * otherwise {@code false} 301 */ 302 @Override equals(Object obj)303 public boolean equals(Object obj) { 304 if (this == obj) { 305 return true; 306 } 307 308 if (!(obj instanceof Optional)) { 309 return false; 310 } 311 312 Optional<?> other = (Optional<?>) obj; 313 return Objects.equals(value, other.value); 314 } 315 316 /** 317 * Returns the hash code value of the present value, if any, or 0 (zero) if 318 * no value is present. 319 * 320 * @return hash code value of the present value or 0 if no value is present 321 */ 322 @Override hashCode()323 public int hashCode() { 324 return Objects.hashCode(value); 325 } 326 327 /** 328 * Returns a non-empty string representation of this Optional suitable for 329 * debugging. The exact presentation format is unspecified and may vary 330 * between implementations and versions. 331 * 332 * @implSpec If a value is present the result must include its string 333 * representation in the result. Empty and present Optionals must be 334 * unambiguously differentiable. 335 * 336 * @return the string representation of this instance 337 */ 338 @Override toString()339 public String toString() { 340 return value != null 341 ? String.format("Optional[%s]", value) 342 : "Optional.empty"; 343 } 344 } 345