/*

 * 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

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 */

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).

 *

 * @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

     */

    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

     */

    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;

        }

        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";

    }

}