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	/*
	* Copyright (c) 2012, 2013, 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;

	/**
	* A container object which may or may not contain a non-null value.
	* If a value is present, {@code isPresent()} will return {@code true} and
	* {@code get()} will return the value.
	*
	* <p>Additional methods that depend on the presence or absence of a contained
	* value are provided, such as {@link #orElse(java.lang.Object) orElse()}
	* (return a default value if value not present) and
	* {@link #ifPresent(java.util.function.Consumer) 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 Optional} may have unpredictable results and should be avoided.
	*
	* @since 1.8
	*/
	public final class Optional<T> {
	/**
	* Common instance for {@code empty()}.
	*/
	private static final Optional<?> EMPTY = new Optional<>();

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

	/**
	* Constructs an empty instance.
	*
	* @implNote Generally only one empty instance, {@link Optional#EMPTY},
	* should exist per VM.
	*/
	private Optional() {
	this.value = null;
	}

	/**
	* Returns an empty {@code Optional} instance. No value is present for this
	* Optional.
	*
	* @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()}.
	*
	* @param <T> 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 value present.
	*
	* @param value the non-null value to be present
	* @throws NullPointerException if value is null
	*/
	private Optional(T value) {
	this.value = Objects.requireNonNull(value);
	}

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

	/**
	* Returns an {@code Optional} describing the specified value, if non-null,
	* otherwise returns an empty {@code Optional}.
	*
	* @param <T> the class of the value
	* @param value the possibly-null value to describe
	* @return an {@code Optional} with a present value if the specified value
	* is non-null, otherwise an empty {@code Optional}
	*/
	public static <T> Optional<T> ofNullable(T value) {
	return value == null ? empty() : of(value);
	}

	/**
	* If a value is present in this {@code Optional}, returns the value,
	* otherwise throws {@code NoSuchElementException}.
	*
	* @return the non-null value held by this {@code Optional}
	* @throws NoSuchElementException if there is no value present
	*
	* @see Optional#isPresent()
	*/
	public T get() {
	if (value == null) {
	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 value != null;
	}

	/**
	* If a value is present, invoke the specified consumer with the value,
	* 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(Consumer<? super T> consumer) {
	if (value != null)
	consumer.accept(value);
	}

	/**
	* If a value is present, and the value matches the given predicate,
	* return an {@code Optional} describing the value, otherwise return an
	* empty {@code Optional}.
	*
	* @param predicate a predicate to apply to the 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 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, apply the provided mapping function to it,
	* and if the result is non-null, return an {@code Optional} describing the
	* result. Otherwise return an empty {@code Optional}.
	*
	* @apiNote This method supports post-processing on optional values, without
	* the need to explicitly check for a return status. For example, the
	* following code traverses a stream of file names, selects one that has
	* not yet been processed, and then opens that file, returning an
	* {@code Optional<FileInputStream>}:
	*
	* <pre>{@code
	* Optional<FileInputStream> fis =
	* names.stream().filter(name -> !isProcessedYet(name))
	* .findFirst()
	* .map(name -> new FileInputStream(name));
	* }</pre>
	*
	* Here, {@code findFirst} returns an {@code Optional<String>}, and then
	* {@code map} returns an {@code Optional<FileInputStream>} for the desired
	* file if one exists.
	*
	* @param <U> The type of the result of the mapping function
	* @param mapper a mapping function to apply to the value, if present
	* @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 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, apply the provided {@code Optional}-bearing
	* mapping function to it, return that result, otherwise return an empty
	* {@code Optional}. This method is similar to {@link #map(Function)},
	* but the provided mapper is one whose result is already an {@code Optional},
	* and if invoked, {@code flatMap} does not wrap it with an additional
	* {@code Optional}.
	*
	* @param <U> The type parameter to the {@code Optional} returned by
	* @param mapper a mapping function to apply to the value, if present
	* the mapping function
	* @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 null or returns
	* a null result
	*/
	public<U> Optional<U> flatMap(Function<? super T, Optional<U>> mapper) {
	Objects.requireNonNull(mapper);
	if (!isPresent())
	return empty();
	else {
	return Objects.requireNonNull(mapper.apply(value));
	}
	}

	/**
	* Return the value if present, otherwise return {@code other}.
	*
	* @param other the value to be returned if there is no value present, may
	* be null
	* @return the value, if present, otherwise {@code other}
	*/
	public T orElse(T other) {
	return value != null ? value : other;
	}

	/**
	* Return the value if present, otherwise invoke {@code other} and return
	* the result of that invocation.
	*
	* @param other a {@code Supplier} whose result is returned if no value
	* is present
	* @return the value if present otherwise the result of {@code other.get()}
	* @throws NullPointerException if value is not present and {@code other} is
	* null
	*/
	public T orElseGet(Supplier<? extends T> other) {
	return value != null ? value : other.get();
	}

	/**
	* 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> 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 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;
	}

	if (!(obj instanceof Optional)) {
	return false;
	}

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

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

	/**
	* Returns a non-empty string representation of this 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 Optionals 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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