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25package java.util;
26
27import java.util.function.IntConsumer;
28import java.util.function.IntSupplier;
29import java.util.function.Supplier;
30import java.util.stream.IntStream;
31
32/**
33 * A container object which may or may not contain an {@code int} value.  If a
34 * value is present, {@code isPresent()} returns {@code true} and
35 * {@code getAsInt()} returns 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(int) orElse()}
39 * (returns a default value if no value is present) and
40 * {@link #ifPresent(IntConsumer) ifPresent()} (performs an
41 * action if a value is present).
42 *
43 * <p>This is a <a href="../lang/doc-files/ValueBased.html">value-based</a>
44 * class; use of identity-sensitive operations (including reference equality
45 * ({@code ==}), identity hash code, or synchronization) on instances of
46 * {@code OptionalInt} may have unpredictable results and should be avoided.
47 *
48 * @apiNote
49 * {@code OptionalInt} is primarily intended for use as a method return type where
50 * there is a clear need to represent "no result." A variable whose type is
51 * {@code OptionalInt} should never itself be {@code null}; it should always point
52 * to an {@code OptionalInt} instance.
53 *
54 * @since 1.8
55 */
56public final class OptionalInt {
57    /**
58     * Common instance for {@code empty()}.
59     */
60    private static final OptionalInt EMPTY = new OptionalInt();
61
62    /**
63     * If true then the value is present, otherwise indicates no value is present
64     */
65    private final boolean isPresent;
66    private final int value;
67
68    /**
69     * Construct an empty instance.
70     *
71     * @implNote Generally only one empty instance, {@link OptionalInt#EMPTY},
72     * should exist per VM.
73     */
74    private OptionalInt() {
75        this.isPresent = false;
76        this.value = 0;
77    }
78
79    /**
80     * Returns an empty {@code OptionalInt} instance.  No value is present for
81     * this {@code OptionalInt}.
82     *
83     * @apiNote
84     * Though it may be tempting to do so, avoid testing if an object is empty
85     * by comparing with {@code ==} against instances returned by
86     * {@code OptionalInt.empty()}.  There is no guarantee that it is a singleton.
87     * Instead, use {@link #isPresent()}.
88     *
89     * @return an empty {@code OptionalInt}
90     */
91    public static OptionalInt empty() {
92        return EMPTY;
93    }
94
95    /**
96     * Construct an instance with the described value.
97     *
98     * @param value the int value to describe
99     */
100    private OptionalInt(int value) {
101        this.isPresent = true;
102        this.value = value;
103    }
104
105    /**
106     * Returns an {@code OptionalInt} describing the given value.
107     *
108     * @param value the value to describe
109     * @return an {@code OptionalInt} with the value present
110     */
111    public static OptionalInt of(int value) {
112        return new OptionalInt(value);
113    }
114
115    /**
116     * If a value is present, returns the value, otherwise throws
117     * {@code NoSuchElementException}.
118     *
119     * @apiNote
120     * The methods {@link #orElse(int) orElse} and
121     * {@link #orElseGet(IntSupplier) orElseGet}
122     * are generally preferable to this method, as they return a substitute
123     * value if the value is absent, instead of throwing an exception.
124     *
125     * @return the value described by this {@code OptionalInt}
126     * @throws NoSuchElementException if no value is present
127     * @see OptionalInt#isPresent()
128     */
129    public int getAsInt() {
130        if (!isPresent) {
131            throw new NoSuchElementException("No value present");
132        }
133        return value;
134    }
135
136    /**
137     * If a value is present, returns {@code true}, otherwise {@code false}.
138     *
139     * @return {@code true} if a value is present, otherwise {@code false}
140     */
141    public boolean isPresent() {
142        return isPresent;
143    }
144
145    /**
146     * If a value is present, performs the given action with the value,
147     * otherwise does nothing.
148     *
149     * @param action the action to be performed, if a value is present
150     * @throws NullPointerException if value is present and the given action is
151     *         {@code null}
152     */
153    public void ifPresent(IntConsumer action) {
154        if (isPresent) {
155            action.accept(value);
156        }
157    }
158
159    /**
160     * If a value is present, performs the given action with the value,
161     * otherwise performs the given empty-based action.
162     *
163     * @param action the action to be performed, if a value is present
164     * @param emptyAction the empty-based action to be performed, if no value is
165     *        present
166     * @throws NullPointerException if a value is present and the given action
167     *         is {@code null}, or no value is present and the given empty-based
168     *         action is {@code null}.
169     * @since 9
170     */
171    public void ifPresentOrElse(IntConsumer action, Runnable emptyAction) {
172        if (isPresent) {
173            action.accept(value);
174        } else {
175            emptyAction.run();
176        }
177    }
178
179    /**
180     * If a value is present, returns a sequential {@link IntStream} containing
181     * only that value, otherwise returns an empty {@code IntStream}.
182     *
183     * @apiNote
184     * This method can be used to transform a {@code Stream} of optional
185     * integers to an {@code IntStream} of present integers:
186     * <pre>{@code
187     *     Stream<OptionalInt> os = ..
188     *     IntStream s = os.flatMapToInt(OptionalInt::stream)
189     * }</pre>
190     *
191     * @return the optional value as an {@code IntStream}
192     * @since 9
193     */
194    public IntStream stream() {
195        if (isPresent) {
196            return IntStream.of(value);
197        } else {
198            return IntStream.empty();
199        }
200    }
201
202    /**
203     * If a value is present, returns the value, otherwise returns
204     * {@code other}.
205     *
206     * @param other the value to be returned, if no value is present
207     * @return the value, if present, otherwise {@code other}
208     */
209    public int orElse(int other) {
210        return isPresent ? value : other;
211    }
212
213    /**
214     * If a value is present, returns the value, otherwise returns the result
215     * produced by the supplying function.
216     *
217     * @param supplier the supplying function that produces a value to be returned
218     * @return the value, if present, otherwise the result produced by the
219     *         supplying function
220     * @throws NullPointerException if no value is present and the supplying
221     *         function is {@code null}
222     */
223    public int orElseGet(IntSupplier supplier) {
224        return isPresent ? value : supplier.getAsInt();
225    }
226
227    /**
228     * If a value is present, returns the value, otherwise throws an exception
229     * produced by the exception supplying function.
230     *
231     * @apiNote
232     * A method reference to the exception constructor with an empty argument
233     * list can be used as the supplier. For example,
234     * {@code IllegalStateException::new}
235     *
236     * @param <X> Type of the exception to be thrown
237     * @param exceptionSupplier the supplying function that produces an
238     *        exception to be thrown
239     * @return the value, if present
240     * @throws X if no value is present
241     * @throws NullPointerException if no value is present and the exception
242     *         supplying function is {@code null}
243     */
244    public<X extends Throwable> int orElseThrow(Supplier<? extends X> exceptionSupplier) throws X {
245        if (isPresent) {
246            return value;
247        } else {
248            throw exceptionSupplier.get();
249        }
250    }
251
252    /**
253     * Indicates whether some other object is "equal to" this
254     * {@code OptionalInt}.  The other object is considered equal if:
255     * <ul>
256     * <li>it is also an {@code OptionalInt} and;
257     * <li>both instances have no value present or;
258     * <li>the present values are "equal to" each other via {@code ==}.
259     * </ul>
260     *
261     * @param obj an object to be tested for equality
262     * @return {@code true} if the other object is "equal to" this object
263     *         otherwise {@code false}
264     */
265    @Override
266    public boolean equals(Object obj) {
267        if (this == obj) {
268            return true;
269        }
270
271        if (!(obj instanceof OptionalInt)) {
272            return false;
273        }
274
275        OptionalInt other = (OptionalInt) obj;
276        return (isPresent && other.isPresent)
277                ? value == other.value
278                : isPresent == other.isPresent;
279    }
280
281    /**
282     * Returns the hash code of the value, if present, otherwise {@code 0}
283     * (zero) if no value is present.
284     *
285     * @return hash code value of the present value or {@code 0} if no value is
286     *         present
287     */
288    @Override
289    public int hashCode() {
290        return isPresent ? Integer.hashCode(value) : 0;
291    }
292
293    /**
294     * Returns a non-empty string representation of this {@code OptionalInt}
295     * suitable for debugging.  The exact presentation format is unspecified and
296     * may vary between implementations and versions.
297     *
298     * @implSpec
299     * If a value is present the result must include its string representation
300     * in the result.  Empty and present {@code OptionalInt}s must be
301     * unambiguously differentiable.
302     *
303     * @return the string representation of this instance
304     */
305    @Override
306    public String toString() {
307        return isPresent
308                ? String.format("OptionalInt[%s]", value)
309                : "OptionalInt.empty";
310    }
311}
312