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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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 * 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
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 * 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,
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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;
import java.util.stream.DoubleStream;

/**
 * A container object which may or may not contain a {@code double} value.  If a
 * value is present, {@code isPresent()} returns {@code true} and
 * {@code getAsDouble()} returns the value.
 *
 * <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
     * The methods {@link #orElse(double) orElse} and
     * {@link #orElseGet(DoubleSupplier) orElseGet}
     * are generally preferable to this method, as they return a substitute
     * value if the value is absent, instead of throwing an exception.
     *
     * @return the value described by this {@code OptionalDouble}
     * @throws NoSuchElementException if no value is present
     * @see OptionalDouble#isPresent()
     */
    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";
    }
}