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