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 * @throws NullPointerException if value is null
89 */
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 */
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 */
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 */
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 */
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 */
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 */
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 */
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 */
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 */
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 */
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 */
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
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
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
339 public String toString() {
340 return value != null
341 ? String.format("Optional[%s]", value)
342 : "Optional.empty";
343 }
344 }