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 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
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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}. If no
 * value is present, the object is considered <i>empty</i> and
 * {@code isPresent()} returns {@code false}.
 *
 * <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 preferred alternative to this method is {@link #orElseThrow()}.
 *
 * @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 not present, returns {@code true}, otherwise
 * {@code false}.
 *
 * @return {@code true} if a value is not present, otherwise {@code false}
 * @since 11
 */
 public boolean isEmpty() {
 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
 * {@code NoSuchElementException}.
 *
 * @return the value described by this {@code OptionalDouble}
 * @throws NoSuchElementException if no value is present
 * @since 10
 */
 public double orElseThrow() {
 if (!isPresent) {
 throw new NoSuchElementException("No value present");
 }
 return value;
 }

 /**
 * 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";
 }
}