001/* 002 * Copyright (C) 2011 The Guava Authors 003 * 004 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except 005 * in compliance with the License. You may obtain a copy of the License at 006 * 007 * http://www.apache.org/licenses/LICENSE-2.0 008 * 009 * Unless required by applicable law or agreed to in writing, software distributed under the License 010 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express 011 * or implied. See the License for the specific language governing permissions and limitations under 012 * the License. 013 */ 014 015package com.google.common.base; 016 017import static com.google.common.base.Preconditions.checkNotNull; 018 019import com.google.common.annotations.Beta; 020import com.google.common.annotations.GwtCompatible; 021import com.google.errorprone.annotations.DoNotMock; 022import java.io.Serializable; 023import java.util.Iterator; 024import java.util.Set; 025import javax.annotation.CheckForNull; 026 027/** 028 * An immutable object that may contain a non-null reference to another object. Each instance of 029 * this type either contains a non-null reference, or contains nothing (in which case we say that 030 * the reference is "absent"); it is never said to "contain {@code null}". 031 * 032 * <p>A non-null {@code Optional<T>} reference can be used as a replacement for a nullable {@code T} 033 * reference. It allows you to represent "a {@code T} that must be present" and a "a {@code T} that 034 * might be absent" as two distinct types in your program, which can aid clarity. 035 * 036 * <p>Some uses of this class include 037 * 038 * <ul> 039 * <li>As a method return type, as an alternative to returning {@code null} to indicate that no 040 * value was available 041 * <li>To distinguish between "unknown" (for example, not present in a map) and "known to have no 042 * value" (present in the map, with value {@code Optional.absent()}) 043 * <li>To wrap nullable references for storage in a collection that does not support {@code null} 044 * (though there are <a 045 * href="https://github.com/google/guava/wiki/LivingWithNullHostileCollections">several other 046 * approaches to this</a> that should be considered first) 047 * </ul> 048 * 049 * <p>A common alternative to using this class is to find or create a suitable <a 050 * href="http://en.wikipedia.org/wiki/Null_Object_pattern">null object</a> for the type in question. 051 * 052 * <p>This class is not intended as a direct analogue of any existing "option" or "maybe" construct 053 * from other programming environments, though it may bear some similarities. 054 * 055 * <p><b>Comparison to {@code java.util.Optional} (JDK 8 and higher):</b> A new {@code Optional} 056 * class was added for Java 8. The two classes are extremely similar, but incompatible (they cannot 057 * share a common supertype). <i>All</i> known differences are listed either here or with the 058 * relevant methods below. 059 * 060 * <ul> 061 * <li>This class is serializable; {@code java.util.Optional} is not. 062 * <li>{@code java.util.Optional} has the additional methods {@code ifPresent}, {@code filter}, 063 * {@code flatMap}, and {@code orElseThrow}. 064 * <li>{@code java.util} offers the primitive-specialized versions {@code OptionalInt}, {@code 065 * OptionalLong} and {@code OptionalDouble}, the use of which is recommended; Guava does not 066 * have these. 067 * </ul> 068 * 069 * <p><b>There are no plans to deprecate this class in the foreseeable future.</b> However, we do 070 * gently recommend that you prefer the new, standard Java class whenever possible. 071 * 072 * <p>See the Guava User Guide article on <a 073 * href="https://github.com/google/guava/wiki/UsingAndAvoidingNullExplained#optional">using {@code 074 * Optional}</a>. 075 * 076 * @param <T> the type of instance that can be contained. {@code Optional} is naturally covariant on 077 * this type, so it is safe to cast an {@code Optional<T>} to {@code Optional<S>} for any 078 * supertype {@code S} of {@code T}. 079 * @author Kurt Alfred Kluever 080 * @author Kevin Bourrillion 081 * @since 10.0 082 */ 083@DoNotMock("Use Optional.of(value) or Optional.absent()") 084@GwtCompatible(serializable = true) 085@ElementTypesAreNonnullByDefault 086public abstract class Optional<T> implements Serializable { 087 /** 088 * Returns an {@code Optional} instance with no contained reference. 089 * 090 * <p><b>Comparison to {@code java.util.Optional}:</b> this method is equivalent to Java 8's 091 * {@code Optional.empty}. 092 */ 093 public static <T> Optional<T> absent() { 094 return Absent.withType(); 095 } 096 097 /** 098 * Returns an {@code Optional} instance containing the given non-null reference. To have {@code 099 * null} treated as {@link #absent}, use {@link #fromNullable} instead. 100 * 101 * <p><b>Comparison to {@code java.util.Optional}:</b> no differences. 102 * 103 * @throws NullPointerException if {@code reference} is null 104 */ 105 public static <T> Optional<T> of(T reference) { 106 return new Present<T>(checkNotNull(reference)); 107 } 108 109 /** 110 * If {@code nullableReference} is non-null, returns an {@code Optional} instance containing that 111 * reference; otherwise returns {@link Optional#absent}. 112 * 113 * <p><b>Comparison to {@code java.util.Optional}:</b> this method is equivalent to Java 8's 114 * {@code Optional.ofNullable}. 115 */ 116 public static <T> Optional<T> fromNullable(@CheckForNull T nullableReference) { 117 return (nullableReference == null) ? Optional.<T>absent() : new Present<T>(nullableReference); 118 } 119 120 /** 121 * Returns the equivalent {@code com.google.common.base.Optional} value to the given {@code 122 * java.util.Optional}, or {@code null} if the argument is null. 123 * 124 * @since 21.0 125 */ 126 @CheckForNull 127 public static <T> Optional<T> fromJavaUtil(@CheckForNull java.util.Optional<T> javaUtilOptional) { 128 return (javaUtilOptional == null) ? null : fromNullable(javaUtilOptional.orElse(null)); 129 } 130 131 /** 132 * Returns the equivalent {@code java.util.Optional} value to the given {@code 133 * com.google.common.base.Optional}, or {@code null} if the argument is null. 134 * 135 * <p>If {@code googleOptional} is known to be non-null, use {@code googleOptional.toJavaUtil()} 136 * instead. 137 * 138 * <p>Unfortunately, the method reference {@code Optional::toJavaUtil} will not work, because it 139 * could refer to either the static or instance version of this method. Write out the lambda 140 * expression {@code o -> Optional.toJavaUtil(o)} instead. 141 * 142 * @since 21.0 143 */ 144 @CheckForNull 145 public static <T> java.util.Optional<T> toJavaUtil(@CheckForNull Optional<T> googleOptional) { 146 return googleOptional == null ? null : googleOptional.toJavaUtil(); 147 } 148 149 /** 150 * Returns the equivalent {@code java.util.Optional} value to this optional. 151 * 152 * <p>Unfortunately, the method reference {@code Optional::toJavaUtil} will not work, because it 153 * could refer to either the static or instance version of this method. Write out the lambda 154 * expression {@code o -> o.toJavaUtil()} instead. 155 * 156 * @since 21.0 157 */ 158 public java.util.Optional<T> toJavaUtil() { 159 return java.util.Optional.ofNullable(orNull()); 160 } 161 162 Optional() {} 163 164 /** 165 * Returns {@code true} if this holder contains a (non-null) instance. 166 * 167 * <p><b>Comparison to {@code java.util.Optional}:</b> no differences. 168 */ 169 public abstract boolean isPresent(); 170 171 /** 172 * Returns the contained instance, which must be present. If the instance might be absent, use 173 * {@link #or(Object)} or {@link #orNull} instead. 174 * 175 * <p><b>Comparison to {@code java.util.Optional}:</b> when the value is absent, this method 176 * throws {@link IllegalStateException}, whereas the Java 8 counterpart throws {@link 177 * java.util.NoSuchElementException NoSuchElementException}. 178 * 179 * @throws IllegalStateException if the instance is absent ({@link #isPresent} returns {@code 180 * false}); depending on this <i>specific</i> exception type (over the more general {@link 181 * RuntimeException}) is discouraged 182 */ 183 public abstract T get(); 184 185 /** 186 * Returns the contained instance if it is present; {@code defaultValue} otherwise. If no default 187 * value should be required because the instance is known to be present, use {@link #get()} 188 * instead. For a default value of {@code null}, use {@link #orNull}. 189 * 190 * <p>Note about generics: The signature {@code public T or(T defaultValue)} is overly 191 * restrictive. However, the ideal signature, {@code public <S super T> S or(S)}, is not legal 192 * Java. As a result, some sensible operations involving subtypes are compile errors: 193 * 194 * <pre>{@code 195 * Optional<Integer> optionalInt = getSomeOptionalInt(); 196 * Number value = optionalInt.or(0.5); // error 197 * 198 * FluentIterable<? extends Number> numbers = getSomeNumbers(); 199 * Optional<? extends Number> first = numbers.first(); 200 * Number value = first.or(0.5); // error 201 * }</pre> 202 * 203 * <p>As a workaround, it is always safe to cast an {@code Optional<? extends T>} to {@code 204 * Optional<T>}. Casting either of the above example {@code Optional} instances to {@code 205 * Optional<Number>} (where {@code Number} is the desired output type) solves the problem: 206 * 207 * <pre>{@code 208 * Optional<Number> optionalInt = (Optional) getSomeOptionalInt(); 209 * Number value = optionalInt.or(0.5); // fine 210 * 211 * FluentIterable<? extends Number> numbers = getSomeNumbers(); 212 * Optional<Number> first = (Optional) numbers.first(); 213 * Number value = first.or(0.5); // fine 214 * }</pre> 215 * 216 * <p><b>Comparison to {@code java.util.Optional}:</b> this method is similar to Java 8's {@code 217 * Optional.orElse}, but will not accept {@code null} as a {@code defaultValue} ({@link #orNull} 218 * must be used instead). As a result, the value returned by this method is guaranteed non-null, 219 * which is not the case for the {@code java.util} equivalent. 220 */ 221 public abstract T or(T defaultValue); 222 223 /** 224 * Returns this {@code Optional} if it has a value present; {@code secondChoice} otherwise. 225 * 226 * <p><b>Comparison to {@code java.util.Optional}:</b> this method has no equivalent in Java 8's 227 * {@code Optional} class; write {@code thisOptional.isPresent() ? thisOptional : secondChoice} 228 * instead. 229 */ 230 public abstract Optional<T> or(Optional<? extends T> secondChoice); 231 232 /** 233 * Returns the contained instance if it is present; {@code supplier.get()} otherwise. 234 * 235 * <p><b>Comparison to {@code java.util.Optional}:</b> this method is similar to Java 8's {@code 236 * Optional.orElseGet}, except when {@code supplier} returns {@code null}. In this case this 237 * method throws an exception, whereas the Java 8 method returns the {@code null} to the caller. 238 * 239 * @throws NullPointerException if this optional's value is absent and the supplier returns {@code 240 * null} 241 */ 242 @Beta 243 public abstract T or(Supplier<? extends T> supplier); 244 245 /** 246 * Returns the contained instance if it is present; {@code null} otherwise. If the instance is 247 * known to be present, use {@link #get()} instead. 248 * 249 * <p><b>Comparison to {@code java.util.Optional}:</b> this method is equivalent to Java 8's 250 * {@code Optional.orElse(null)}. 251 */ 252 @CheckForNull 253 public abstract T orNull(); 254 255 /** 256 * Returns an immutable singleton {@link Set} whose only element is the contained instance if it 257 * is present; an empty immutable {@link Set} otherwise. 258 * 259 * <p><b>Comparison to {@code java.util.Optional}:</b> this method has no equivalent in Java 8's 260 * {@code Optional} class. However, this common usage: 261 * 262 * <pre>{@code 263 * for (Foo foo : possibleFoo.asSet()) { 264 * doSomethingWith(foo); 265 * } 266 * }</pre> 267 * 268 * ... can be replaced with: 269 * 270 * <pre>{@code 271 * possibleFoo.ifPresent(foo -> doSomethingWith(foo)); 272 * }</pre> 273 * 274 * <p><b>Java 9 users:</b> some use cases can be written with calls to {@code optional.stream()}. 275 * 276 * @since 11.0 277 */ 278 public abstract Set<T> asSet(); 279 280 /** 281 * If the instance is present, it is transformed with the given {@link Function}; otherwise, 282 * {@link Optional#absent} is returned. 283 * 284 * <p><b>Comparison to {@code java.util.Optional}:</b> this method is similar to Java 8's {@code 285 * Optional.map}, except when {@code function} returns {@code null}. In this case this method 286 * throws an exception, whereas the Java 8 method returns {@code Optional.absent()}. 287 * 288 * @throws NullPointerException if the function returns {@code null} 289 * @since 12.0 290 */ 291 public abstract <V> Optional<V> transform(Function<? super T, V> function); 292 293 /** 294 * Returns {@code true} if {@code object} is an {@code Optional} instance, and either the 295 * contained references are {@linkplain Object#equals equal} to each other or both are absent. 296 * Note that {@code Optional} instances of differing parameterized types can be equal. 297 * 298 * <p><b>Comparison to {@code java.util.Optional}:</b> no differences. 299 */ 300 @Override 301 public abstract boolean equals(@CheckForNull Object object); 302 303 /** 304 * Returns a hash code for this instance. 305 * 306 * <p><b>Comparison to {@code java.util.Optional}:</b> this class leaves the specific choice of 307 * hash code unspecified, unlike the Java 8 equivalent. 308 */ 309 @Override 310 public abstract int hashCode(); 311 312 /** 313 * Returns a string representation for this instance. 314 * 315 * <p><b>Comparison to {@code java.util.Optional}:</b> this class leaves the specific string 316 * representation unspecified, unlike the Java 8 equivalent. 317 */ 318 @Override 319 public abstract String toString(); 320 321 /** 322 * Returns the value of each present instance from the supplied {@code optionals}, in order, 323 * skipping over occurrences of {@link Optional#absent}. Iterators are unmodifiable and are 324 * evaluated lazily. 325 * 326 * <p><b>Comparison to {@code java.util.Optional}:</b> this method has no equivalent in Java 8's 327 * {@code Optional} class; use {@code 328 * optionals.stream().filter(Optional::isPresent).map(Optional::get)} instead. 329 * 330 * <p><b>Java 9 users:</b> use {@code optionals.stream().flatMap(Optional::stream)} instead. 331 * 332 * @since 11.0 (generics widened in 13.0) 333 */ 334 @Beta 335 public static <T> Iterable<T> presentInstances( 336 final Iterable<? extends Optional<? extends T>> optionals) { 337 checkNotNull(optionals); 338 return new Iterable<T>() { 339 @Override 340 public Iterator<T> iterator() { 341 return new AbstractIterator<T>() { 342 private final Iterator<? extends Optional<? extends T>> iterator = 343 checkNotNull(optionals.iterator()); 344 345 @Override 346 @CheckForNull 347 protected T computeNext() { 348 while (iterator.hasNext()) { 349 Optional<? extends T> optional = iterator.next(); 350 if (optional.isPresent()) { 351 return optional.get(); 352 } 353 } 354 return endOfData(); 355 } 356 }; 357 } 358 }; 359 } 360 361 private static final long serialVersionUID = 0; 362}