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
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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
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24 */
25package java.util;
26
27import java.util.function.Consumer;
28import java.util.function.Function;
29import java.util.function.Predicate;
30import 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 */
45public 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}
345