content: /*
   * 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;
  
  // Android-changed: removed ValueBased paragraph.
  
  /**
   * 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(Object) orElse()}
   * (return a default value if value not present) and
   * {@link #ifPresent(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
       * @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";
      }
  }
  