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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

 *

 * This code is free software; you can redistribute it and/or modify it

 * under the terms of the GNU General Public License version 2 only, as

 * published by the Free Software Foundation.  Oracle designates this

 * particular file as subject to the "Classpath" exception as provided

 * by Oracle in the LICENSE file that accompanied this code.

 *

 * This code is distributed in the hope that it will be useful, but WITHOUT

 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

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

 * version 2 for more details (a copy is included in the LICENSE file that

 * accompanied this code).

 *

 * You should have received a copy of the GNU General Public License version

 * 2 along with this work; if not, write to the Free Software Foundation,

 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

 *

 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

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 */

package java.util;

import java.util.function.DoubleConsumer;

import java.util.function.DoubleSupplier;

import java.util.function.Supplier;

import java.util.stream.DoubleStream;

/**

 *

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

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

 * (returns a default value if no value is present) and

 * {@link #ifPresent(DoubleConsumer) ifPresent()} (performs

 * an action if a 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.

 *

 * @apiNote

 * {@code OptionalDouble} is primarily intended for use as a method return type where

 * there is a clear need to represent "no result." A variable whose type is

 * {@code OptionalDouble} should never itself be {@code null}; it should always point

 * to an {@code OptionalDouble} instance.

 *

 * @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 {@code 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 OptionalDouble.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 described value.

     *

     * @param value the double value to describe.

     */

    private OptionalDouble(double value) {

        this.isPresent = true;

        this.value = value;

    }

    /**

     * Returns an {@code OptionalDouble} describing the given value.

     *

     * @param value the value to describe

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

     */

    public static OptionalDouble of(double value) {

        return new OptionalDouble(value);

    }

    /**

     * If a value is present, returns the value, otherwise throws

     * {@code NoSuchElementException}.

     *

     * @apiNote

     *

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

     * @throws NoSuchElementException if no value is present

     */

    public double getAsDouble() {

        if (!isPresent) {

            throw new NoSuchElementException("No value present");

        }

        return value;

    }

    /**

     * If a value is present, returns {@code true}, otherwise {@code false}.

     *

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

     */

    public boolean isPresent() {

        return isPresent;

    }

    /**

     * If a value is present, performs the given action with the value,

     * otherwise does nothing.

     *

     * @param action the action to be performed, if a value is present

     * @throws NullPointerException if value is present and the given action is

     *         {@code null}

     */

    public void ifPresent(DoubleConsumer action) {

        if (isPresent) {

            action.accept(value);

        }

    }

    /**

     * If a value is present, performs the given action with the value,

     * otherwise performs the given empty-based action.

     *

     * @param action the action to be performed, if a value is present

     * @param emptyAction the empty-based action to be performed, if no value is

     * present

     * @throws NullPointerException if a value is present and the given action

     *         is {@code null}, or no value is present and the given empty-based

     *         action is {@code null}.

     * @since 9

     */

    public void ifPresentOrElse(DoubleConsumer action, Runnable emptyAction) {

        if (isPresent) {

            action.accept(value);

        } else {

            emptyAction.run();

        }

    }

    /**

     * If a value is present, returns a sequential {@link DoubleStream}

     * containing only that value, otherwise returns an empty

     * {@code DoubleStream}.

     *

     * @apiNote

     * This method can be used to transform a {@code Stream} of optional doubles

     * to a {@code DoubleStream} of present doubles:

     * <pre>{@code

     *     Stream<OptionalDouble> os = ..

     *     DoubleStream s = os.flatMapToDouble(OptionalDouble::stream)

     * }</pre>

     *

     * @return the optional value as a {@code DoubleStream}

     * @since 9

     */

    public DoubleStream stream() {

        if (isPresent) {

            return DoubleStream.of(value);

        } else {

            return DoubleStream.empty();

        }

    }

    /**

     * If a value is present, returns the value, otherwise returns

     * {@code other}.

     *

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

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

     */

    public double orElse(double other) {

        return isPresent ? value : other;

    }

    /**

     * If a value is present, returns the value, otherwise returns the result

     * produced by the supplying function.

     *

     * @param supplier the supplying function that produces a value to be returned

     * @return the value, if present, otherwise the result produced by the

     *         supplying function

     * @throws NullPointerException if no value is present and the supplying

     *         function is {@code null}

     */

    public double orElseGet(DoubleSupplier supplier) {

        return isPresent ? value : supplier.getAsDouble();

    }

    /**

     * If a value is present, returns the value, otherwise throws an exception

     * produced by the exception supplying function.

     *

     * @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 supplying function that produces an

     *        exception to be thrown

     * @return the value, if present

     * @throws X if no value is present

     * @throws NullPointerException if no value is present and the exception

     *         supplying function is {@code 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

     * {@code 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 of the value, if present, otherwise {@code 0}

     * (zero) if no value is present.

     *

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

     *         present

     */

    @Override

    public int hashCode() {

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

    }

    /**

     * Returns a non-empty string representation of this {@code OptionalDouble}

     * 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 {@code OptionalDouble}s must be

     * unambiguously differentiable.

     *

     * @return the string representation of this instance

     */

    @Override

    public String toString() {

        return isPresent

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

                : "OptionalDouble.empty";

    }

}