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 * Copyright (c) 2012, 2018, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
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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;
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 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;
 }

 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";
 }
}