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 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

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 *

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package java.util;

import java.util.function.DoubleConsumer;

import java.util.function.DoubleSupplier;

import java.util.function.Supplier;

/**

 * A container object which may or may not contain a {@code double} value.

 * If a value is present, {@code isPresent()} will return {@code true} and

 * {@code getAsDouble()} will return the value.

 *

 * <p>Additional methods that depend on the presence or absence of a contained

 * value are provided, such as {@link #orElse(double) orElse()}

 * (return a default value if value not present) and

 * {@link #ifPresent(java.util.function.DoubleConsumer) ifPresent()} (execute a block

 * of code if the value is present).

 *

 * <p>This is a <a href="../lang/doc-files/ValueBased.html">value-based</a>

 * class; use of identity-sensitive operations (including reference equality

 * ({@code ==}), identity hash code, or synchronization) on instances of

 * {@code OptionalDouble} may have unpredictable results and should be avoided.

 *

 * @since 1.8

 */

public final class OptionalDouble {

    /**

     * Common instance for {@code empty()}.

     */

    private static final OptionalDouble EMPTY = new OptionalDouble();

    /**

     * If true then the value is present, otherwise indicates no value is present

     */

    private final boolean isPresent;

    private final double value;

    /**

     * Construct an empty instance.

     *

     * @implNote generally only one empty instance, {@link OptionalDouble#EMPTY},

     * should exist per VM.

     */

    private OptionalDouble() {

        this.isPresent = false;

        this.value = Double.NaN;

    }

    /**

     * Returns an empty {@code OptionalDouble} instance.  No value is present for this

     * OptionalDouble.

     *

     * @apiNote Though it may be tempting to do so, avoid testing if an object

     * is empty by comparing with {@code ==} against instances returned by

     * {@code Option.empty()}. There is no guarantee that it is a singleton.

     * Instead, use {@link #isPresent()}.

     *

     *  @return an empty {@code OptionalDouble}.

     */

    public static OptionalDouble empty() {

        return EMPTY;

    }

    /**

     * Construct an instance with the value present.

     *

     * @param value the double value to be present.

     */

    private OptionalDouble(double value) {

        this.isPresent = true;

        this.value = value;

    }

    /**

     * Return an {@code OptionalDouble} with the specified value present.

     *

     * @param value the value to be present

     * @return an {@code OptionalDouble} with the value present

     */

    public static OptionalDouble of(double value) {

        return new OptionalDouble(value);

    }

    /**

     * If a value is present in this {@code OptionalDouble}, returns the value,

     * otherwise throws {@code NoSuchElementException}.

     *

     * @return the value held by this {@code OptionalDouble}

     * @throws NoSuchElementException if there is no value present

     *

     * @see OptionalDouble#isPresent()

     */

    public double getAsDouble() {

        if (!isPresent) {

            throw new NoSuchElementException("No value present");

        }

        return value;

    }

    /**

     * Return {@code true} if there is a value present, otherwise {@code false}.

     *

     * @return {@code true} if there is a value present, otherwise {@code false}

     */

    public boolean isPresent() {

        return isPresent;

    }

    /**

     * Have the specified consumer accept the value if a value is present,

     * otherwise do nothing.

     *

     * @param consumer block to be executed if a value is present

     * @throws NullPointerException if value is present and {@code consumer} is

     * null

     */

    public void ifPresent(DoubleConsumer consumer) {

            consumer.accept(value);

    }

    /**

     * Return the value if present, otherwise return {@code other}.

     *

     * @param other the value to be returned if there is no value present

     * @return the value, if present, otherwise {@code other}

     */

    public double orElse(double other) {

        return isPresent ? value : other;

    }

    /**

     * Return the value if present, otherwise invoke {@code other} and return

     * the result of that invocation.

     *

     * @param other a {@code DoubleSupplier} whose result is returned if no value

     * is present

     * @return the value if present otherwise the result of {@code other.getAsDouble()}

     * @throws NullPointerException if value is not present and {@code other} is

     * null

     */

    public double orElseGet(DoubleSupplier other) {

        return isPresent ? value : other.getAsDouble();

    }

    /**

     * Return the contained value, if present, otherwise throw an exception

     * to be created by the provided supplier.

     *

     * @apiNote A method reference to the exception constructor with an empty

     * argument list can be used as the supplier. For example,

     * {@code IllegalStateException::new}

     *

     * @param <X> Type of the exception to be thrown

     * @param exceptionSupplier The supplier which will return the exception to

     * be thrown

     * @return the present value

     * @throws X if there is no value present

     * @throws NullPointerException if no value is present and

     * {@code exceptionSupplier} is null

     */

    public<X extends Throwable> double orElseThrow(Supplier<? extends X> exceptionSupplier) throws X {

        if (isPresent) {

            return value;

        } else {

            throw exceptionSupplier.get();

        }

    }

    /**

     * Indicates whether some other object is "equal to" this OptionalDouble. The

     * other object is considered equal if:

     * <ul>

     * <li>it is also an {@code OptionalDouble} and;

     * <li>both instances have no value present or;

     * <li>the present values are "equal to" each other via {@code Double.compare() == 0}.

     * </ul>

     *

     * @param obj an object to be tested for equality

     * @return {code true} if the other object is "equal to" this object

     * otherwise {@code false}

     */

    @Override

    public boolean equals(Object obj) {

        if (this == obj) {

            return true;

        }

        if (!(obj instanceof OptionalDouble)) {

            return false;

        }

        OptionalDouble other = (OptionalDouble) obj;

        return (isPresent && other.isPresent)

               ? Double.compare(value, other.value) == 0

               : isPresent == other.isPresent;

    }

    /**

     * Returns the hash code value of the present value, if any, or 0 (zero) if

     * no value is present.

     *

     * @return hash code value of the present value or 0 if no value is present

     */

    @Override

    public int hashCode() {

        return isPresent ? Double.hashCode(value) : 0;

    }

    /**

     * {@inheritDoc}

     *

     * Returns a non-empty string representation of this object suitable for

     * debugging. The exact presentation format is unspecified and may vary

     * between implementations and versions.

     *

     * @implSpec If a value is present the result must include its string

     * representation in the result. Empty and present instances must be

     * unambiguously differentiable.

     *

     * @return the string representation of this instance

     */

    @Override

    public String toString() {

        return isPresent

                ? String.format("OptionalDouble[%s]", value)

                : "OptionalDouble.empty";

    }

}