/*

 * Copyright (c) 2012, 2017, 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="../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.

 *

 * @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

 */

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

     * {@code 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 Optional.empty()}.  There is no guarantee that it is a singleton.

     * Instead, use {@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 non-{@code null} value to describe

     * @throws NullPointerException if value is {@code null}

     */

    private Optional(T value) {

        this.value = Objects.requireNonNull(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<>(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}

     */

    public static <T> Optional<T> ofNullable(T value) {

        return value == null ? empty() : of(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 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;

        }

        if (!(obj instanceof Optional)) {

            return false;

        }

        Optional<?> other = (Optional<?>) obj;

        return 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";

    }

}