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	/*
	* Copyright (c) 2012, 2020, Oracle and/or its affiliates. All rights reserved.
	* 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
	* or visit www.oracle.com if you need additional information or have any
	* questions.
	*/
	package java.util;

	import java.util.function.Consumer;
	import java.util.function.Function;
	import java.util.function.Predicate;
	import java.util.function.Supplier;
	import java.util.stream.Stream;

	/**
	* A container object which may or may not contain a non-{@code null} 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(Object) orElse()}
	* (returns a default value if no value is present) and
	* {@link #ifPresent(Consumer) ifPresent()} (performs an
	* action if a value is present).
	*
	* <p>This is a <a href="{@docRoot}/java.base/java/lang/doc-files/ValueBased.html">value-based</a>
	* class; programmers should treat instances that are
	* {@linkplain #equals(Object) equal} as interchangeable and should not
	* use instances for synchronization, or unpredictable behavior may
	* occur. For example, in a future release, synchronization may fail.
	*
	* @apiNote
	* {@code Optional} is primarily intended for use as a method return type where
	* there is a clear need to represent "no result," and where using {@code null}
	* is likely to cause errors. A variable whose type is {@code Optional} should
	* never itself be {@code null}; it should always point to an {@code Optional}
	* instance.
	*
	* @param <T> the type of value
	* @since 1.8
	*/
	@jdk.internal.ValueBased
	public final class Optional<T> {
	/**
	* Common instance for {@code empty()}.
	*/
	private static final Optional<?> EMPTY = new Optional<>(null);

	/**
	* If non-null, the value; if null, indicates no value is present
	*/
	private final T value;

	/**
	* Returns an empty {@code Optional} instance. No value is present for this
	* {@code Optional}.
	*
	* @apiNote
	* Though it may be tempting to do so, avoid testing if an object is empty
	* by comparing with {@code ==} or {@code !=} against instances returned by
	* {@code Optional.empty()}. There is no guarantee that it is a singleton.
	* Instead, use {@link #isEmpty()} or {@link #isPresent()}.
	*
	* @param <T> The type of the non-existent value
	* @return an empty {@code Optional}
	*/
	public static<T> Optional<T> empty() {
	@SuppressWarnings("unchecked")
	Optional<T> t = (Optional<T>) EMPTY;
	return t;
	}

	/**
	* Constructs an instance with the described value.
	*
	* @param value the value to describe; it's the caller's responsibility to
	* ensure the value is non-{@code null} unless creating the singleton
	* instance returned by {@code empty()}.
	*/
	private Optional(T value) {
	this.value = value;
	}

	/**
	* Returns an {@code Optional} describing the given non-{@code null}
	* value.
	*
	* @param value the value to describe, which must be non-{@code null}
	* @param <T> the type of the value
	* @return an {@code Optional} with the value present
	* @throws NullPointerException if value is {@code null}
	*/
	public static <T> Optional<T> of(T value) {
	return new Optional<>(Objects.requireNonNull(value));
	}

	/**
	* Returns an {@code Optional} describing the given value, if
	* non-{@code null}, otherwise returns an empty {@code Optional}.
	*
	* @param value the possibly-{@code null} value to describe
	* @param <T> the type of the value
	* @return an {@code Optional} with a present value if the specified value
	* is non-{@code null}, otherwise an empty {@code Optional}
	*/
	@SuppressWarnings("unchecked")
	public static <T> Optional<T> ofNullable(T value) {
	return value == null ? (Optional<T>) EMPTY
	: new Optional<>(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 non-{@code null} value described by this {@code Optional}
	* @throws NoSuchElementException if no value is present
	*/
	public T get() {
	if (value == null) {
	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 value != null;
	}

	/**
	* 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 value == null;
	}

	/**
	* 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(Consumer<? super T> action) {
	if (value != null) {
	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(Consumer<? super T> action, Runnable emptyAction) {
	if (value != null) {
	action.accept(value);
	} else {
	emptyAction.run();
	}
	}

	/**
	* If a value is present, and the value matches the given predicate,
	* returns an {@code Optional} describing the value, otherwise returns an
	* empty {@code Optional}.
	*
	* @param predicate the predicate to apply to a value, if present
	* @return an {@code Optional} describing the value of this
	* {@code Optional}, if a value is present and the value matches the
	* given predicate, otherwise an empty {@code Optional}
	* @throws NullPointerException if the predicate is {@code null}
	*/
	public Optional<T> filter(Predicate<? super T> predicate) {
	Objects.requireNonNull(predicate);
	if (!isPresent()) {
	return this;
	} else {
	return predicate.test(value) ? this : empty();
	}
	}

	/**
	* If a value is present, returns an {@code Optional} describing (as if by
	* {@link #ofNullable}) the result of applying the given mapping function to
	* the value, otherwise returns an empty {@code Optional}.
	*
	* <p>If the mapping function returns a {@code null} result then this method
	* returns an empty {@code Optional}.
	*
	* @apiNote
	* This method supports post-processing on {@code Optional} values, without
	* the need to explicitly check for a return status. For example, the
	* following code traverses a stream of URIs, selects one that has not
	* yet been processed, and creates a path from that URI, returning
	* an {@code Optional<Path>}:
	*
	* <pre>{@code
	* Optional<Path> p =
	* uris.stream().filter(uri -> !isProcessedYet(uri))
	* .findFirst()
	* .map(Paths::get);
	* }</pre>
	*
	* Here, {@code findFirst} returns an {@code Optional<URI>}, and then
	* {@code map} returns an {@code Optional<Path>} for the desired
	* URI if one exists.
	*
	* @param mapper the mapping function to apply to a value, if present
	* @param <U> The type of the value returned from the mapping function
	* @return an {@code Optional} describing the result of applying a mapping
	* function to the value of this {@code Optional}, if a value is
	* present, otherwise an empty {@code Optional}
	* @throws NullPointerException if the mapping function is {@code null}
	*/
	public <U> Optional<U> map(Function<? super T, ? extends U> mapper) {
	Objects.requireNonNull(mapper);
	if (!isPresent()) {
	return empty();
	} else {
	return Optional.ofNullable(mapper.apply(value));
	}
	}

	/**
	* If a value is present, returns the result of applying the given
	* {@code Optional}-bearing mapping function to the value, otherwise returns
	* an empty {@code Optional}.
	*
	* <p>This method is similar to {@link #map(Function)}, but the mapping
	* function is one whose result is already an {@code Optional}, and if
	* invoked, {@code flatMap} does not wrap it within an additional
	* {@code Optional}.
	*
	* @param <U> The type of value of the {@code Optional} returned by the
	* mapping function
	* @param mapper the mapping function to apply to a value, if present
	* @return the result of applying an {@code Optional}-bearing mapping
	* function to the value of this {@code Optional}, if a value is
	* present, otherwise an empty {@code Optional}
	* @throws NullPointerException if the mapping function is {@code null} or
	* returns a {@code null} result
	*/
	public <U> Optional<U> flatMap(Function<? super T, ? extends Optional<? extends U>> mapper) {
	Objects.requireNonNull(mapper);
	if (!isPresent()) {
	return empty();
	} else {
	@SuppressWarnings("unchecked")
	Optional<U> r = (Optional<U>) mapper.apply(value);
	return Objects.requireNonNull(r);
	}
	}

	/**
	* If a value is present, returns an {@code Optional} describing the value,
	* otherwise returns an {@code Optional} produced by the supplying function.
	*
	* @param supplier the supplying function that produces an {@code Optional}
	* to be returned
	* @return returns an {@code Optional} describing the value of this
	* {@code Optional}, if a value is present, otherwise an
	* {@code Optional} produced by the supplying function.
	* @throws NullPointerException if the supplying function is {@code null} or
	* produces a {@code null} result
	* @since 9
	*/
	public Optional<T> or(Supplier<? extends Optional<? extends T>> supplier) {
	Objects.requireNonNull(supplier);
	if (isPresent()) {
	return this;
	} else {
	@SuppressWarnings("unchecked")
	Optional<T> r = (Optional<T>) supplier.get();
	return Objects.requireNonNull(r);
	}
	}

	/**
	* If a value is present, returns a sequential {@link Stream} containing
	* only that value, otherwise returns an empty {@code Stream}.
	*
	* @apiNote
	* This method can be used to transform a {@code Stream} of optional
	* elements to a {@code Stream} of present value elements:
	* <pre>{@code
	* Stream<Optional<T>> os = ..
	* Stream<T> s = os.flatMap(Optional::stream)
	* }</pre>
	*
	* @return the optional value as a {@code Stream}
	* @since 9
	*/
	public Stream<T> stream() {
	if (!isPresent()) {
	return Stream.empty();
	} else {
	return Stream.of(value);
	}
	}

	/**
	* If a value is present, returns the value, otherwise returns
	* {@code other}.
	*
	* @param other the value to be returned, if no value is present.
	* May be {@code null}.
	* @return the value, if present, otherwise {@code other}
	*/
	public T orElse(T other) {
	return value != null ? 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 T orElseGet(Supplier<? extends T> supplier) {
	return value != null ? value : supplier.get();
	}

	/**
	* If a value is present, returns the value, otherwise throws
	* {@code NoSuchElementException}.
	*
	* @return the non-{@code null} value described by this {@code Optional}
	* @throws NoSuchElementException if no value is present
	* @since 10
	*/
	public T orElseThrow() {
	if (value == null) {
	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> T orElseThrow(Supplier<? extends X> exceptionSupplier) throws X {
	if (value != null) {
	return value;
	} else {
	throw exceptionSupplier.get();
	}
	}

	/**
	* Indicates whether some other object is "equal to" this {@code Optional}.
	* The other object is considered equal if:
	* <ul>
	* <li>it is also an {@code Optional} and;
	* <li>both instances have no value present or;
	* <li>the present values are "equal to" each other via {@code equals()}.
	* </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;
	}

	return obj instanceof Optional<?> other
	&& Objects.equals(value, other.value);
	}

	/**
	* 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 Objects.hashCode(value);
	}

	/**
	* Returns a non-empty string representation of this {@code Optional}
	* 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 Optional}s must be unambiguously
	* differentiable.
	*
	* @return the string representation of this instance
	*/
	@Override
	public String toString() {
	return value != null
	? String.format("Optional[%s]", value)
	: "Optional.empty";
	}
	}

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