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.Supplier;
29
30 /**
31 * A container object which may or may not contain a non-null value.
32 * If a value is present, {@code isPresent()} will return {@code true} and
33 * {@code get()} will return the value.
34 *
35 * <p>Additional methods that depend on the presence or absence of a contained
36 * value are provided, such as {@link #orElse(java.lang.Object) orElse()}
37 * (return a default value if value not present) and
38 * {@link #ifPresent(java.util.function.Consumer) ifPresent()} (execute a block
39 * of code if the value is present).
40 *
41 * @since 1.8
42 */
43 public final class Optional<T> {
44 /**
45 * Common instance for {@code empty()}.
46 */
47 private static final Optional<?> EMPTY = new Optional<>();
48
49 /**
50 * If non-null, the value; if null, indicates no value is present
51 */
52 private final T value;
53
54 /**
55 * Construct an empty instance.
56 *
57 * @implNote Generally only one empty instance, {@link Optional#EMPTY},
58 * should exist per VM.
59 */
60 private Optional() {
61 this.value = null;
62 }
63
64 /**
65 * Returns an empty {@code Optional} instance. No value is present for this
66 * Optional.
67 *
68 * @apiNote Though it may be tempting to do so, avoid testing if an object
69 * is empty by comparing with {@code ==} against instances returned by
70 * {@code Option.empty()}. There is no guarantee that it is a singleton.
71 * Instead, use {@link #isPresent()}.
72 *
73 * @param <T> Type of the non-existent value
74 * @return an empty {@code Optional}
75 */
76 public static<T> Optional<T> empty() {
77 @SuppressWarnings("unchecked")
78 Optional<T> t = (Optional<T>) EMPTY;
79 return t;
80 }
81
82 /**
83 * Construct an instance with the value present.
84 *
85 * @param value the non-null value to be present
86 */
87 private Optional(T value) {
88 this.value = Objects.requireNonNull(value);
89 }
90
91 /**
92 * Return an {@code Optional} with the specified present value.
93 *
94 * @param value the value to be present, which must be non-null
95 * @return an {@code Optional} with the value present
96 */
97 public static <T> Optional<T> of(T value) {
98 return new Optional<>(value);
99 }
100
101 /**
102 * If a value is present in this {@code Optional}, returns the value,
103 * otherwise throws {@code NoSuchElementException}.
104 *
105 * @return the non-null value held by this {@code Optional}
106 * @throws NoSuchElementException if there is no value present
107 *
108 * @see Optional#isPresent()
109 */
110 public T get() {
111 if (value == null) {
112 throw new NoSuchElementException("No value present");
113 }
114 return value;
115 }
116
117 /**
118 * Return {@code true} if there is a value present, otherwise {@code false}.
119 *
120 * @return {@code true} if there is a value present, otherwise {@code false}
121 */
122 public boolean isPresent() {
123 return value != null;
124 }
125
126 /**
127 * Have the specified consumer accept the value if a value is present,
128 * otherwise do nothing.
129 *
130 * @param consumer block to be executed if a value is present
131 * @throws NullPointerException if value is present and {@code consumer} is
132 * null
133 */
134 public void ifPresent(Consumer<? super T> consumer) {
135 if (value != null)
136 consumer.accept(value);
137 }
138
139 /**
140 * Return the value if present, otherwise return {@code other}.
141 *
142 * @param other the value to be returned if there is no value present, may
143 * be null
144 * @return the value, if present, otherwise {@code other}
145 */
146 public T orElse(T other) {
147 return value != null ? value : other;
148 }
149
150 /**
151 * Return the value if present, otherwise invoke {@code other} and return
152 * the result of that invocation.
153 *
154 * @param other a {@code Supplier} whose result is returned if no value
155 * is present
156 * @return the value if present otherwise the result of {@code other.get()}
157 * @throws NullPointerException if value is not present and {@code other} is
158 * null
159 */
160 public T orElseGet(Supplier<? extends T> other) {
161 return value != null ? value : other.get();
162 }
163
164 /**
165 * Return the contained value, if present, otherwise throw an exception
166 * to be created by the provided supplier.
167 *
168 * @apiNote A method reference to the exception constructor with an empty
169 * argument list can be used as the supplier. For example,
170 * {@code IllegalStateException::new}
171 *
172 * @param <X> Type of the exception to be thrown
173 * @param exceptionSupplier The supplier which will return the exception to
174 * be thrown
175 * @return the present value
176 * @throws X if there is no value present
177 * @throws NullPointerException if no value is present and
178 * {@code exceptionSupplier} is null
179 */
180 public <X extends Throwable> T orElseThrow(Supplier<? extends X> exceptionSupplier) throws X {
181 if (value != null) {
182 return value;
183 } else {
184 throw exceptionSupplier.get();
185 }
186 }
187
188 /**
189 * Indicates whether some other object is "equal to" this Optional. The
190 * other object is considered equal if:
191 * <ul>
192 * <li>it is also an {@code Optional} and;
193 * <li>both instances have no value present or;
194 * <li>the present values are "equal to" each other via {@code equals()}.
195 * </ul>
196 *
197 * @param obj an object to be tested for equality
198 * @return {code true} if the other object is "equal to" this object
199 * otherwise {@code false}
200 */
201 @Override
202 public boolean equals(Object obj) {
203 if (this == obj) {
204 return true;
205 }
206
207 if (!(obj instanceof Optional)) {
208 return false;
209 }
210
211 Optional other = (Optional) obj;
212 return Objects.equals(value, other.value);
213 }
214
215 /**
216 * Returns the hash code value of the present value, if any, or 0 (zero) if
217 * no value is present.
218 *
219 * @return hash code value of the present value or 0 if no value is present
220 */
221 @Override
222 public int hashCode() {
223 return Objects.hashCode(value);
224 }
225
226 /**
227 * Returns a non-empty string representation of this Optional suitable for
228 * debugging. The exact presentation format is unspecified and may vary
229 * between implementations and versions.
230 *
231 * @implSpec If a value is present the result must include its string
232 * representation in the result. Empty and present Optionals must be
233 * unambiguously differentiable.
234 *
235 * @return the string representation of this instance
236 */
237 @Override
238 public String toString() {
239 return value != null
240 ? String.format("Optional[%s]", value)
241 : "Optional.empty";
242 }
243 }