content: /**
   * Copyright (C) 2011 The Guava Authors
   *
   * Licensed under the Apache License, Version 2.0 (the "License");
   * you may not use this file except in compliance with the License.
   * You may obtain a copy of the License at
   *
   * http://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS,
   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   * See the License for the specific language governing permissions and
   * limitations under the License.
   */
  package com.fernandocejas.arrow.optional;
  
  import com.fernandocejas.arrow.collections.AbstractIterator;
  import com.fernandocejas.arrow.functions.Function;
  import java.io.Serializable;
  import java.util.Iterator;
  import java.util.Set;
  import org.jetbrains.annotations.Nullable;
  
  import static com.fernandocejas.arrow.checks.Preconditions.checkNotNull;
  
  /**
   * An immutable object that may contain a non-null reference to another object. Each
   * instance of this type either contains a non-null reference, or contains nothing (in
   * which case we say that the reference is "absent"); it is never said to "contain {@code
   * null}".
   *
   * <p>A non-null {@code Optional<T>} reference can be used as a replacement for a nullable
   * {@code T} reference. It allows you to represent "a {@code T} that must be present" and
   * a "a {@code T} that might be absent" as two distinct types in your program, which can
   * aid clarity.
   *
   * <p>Some uses of this class include
   *
   * <ul>
   * <li>As a method return type, as an alternative to returning {@code null} to indicate
   * that no value was available
   * <li>To distinguish between "unknown" (for example, not present in a map) and "known to
   * have no value" (present in the map, with value {@code Optional.absent()})
   * <li>To wrap nullable references for storage in a collection that does not support
   * {@code null} (though there are
   * <a href="http://code.google.com/p/guava-libraries/wiki/LivingWithNullHostileCollections">
   * several other approaches to this</a> that should be considered first)
   * </ul>
   *
   * <p>A common alternative to using this class is to find or create a suitable
   * <a href="http://en.wikipedia.org/wiki/Null_Object_pattern">null object</a> for the
   * type in question.
   *
   * <p>This class is not intended as a direct analogue of any existing "option" or "maybe"
   * construct from other programming environments, though it may bear some similarities.
   *
   * <p>See the Guava User Guide article on <a
   * href="http://code.google.com/p/guava-libraries/wiki/UsingAndAvoidingNullExplained#Optional">
   * using {@code Optional}</a>.
   *
   * @param <T> the type of instance that can be contained. {@code Optional} is naturally
   * covariant on this type, so it is safe to cast an {@code Optional<T>} to {@code
   * Optional<S>} for any supertype {@code S} of {@code T}.
   * @author Kurt Alfred Kluever
   * @author Kevin Bourrillion
   * @since 10.0
   *
   * <p><b>This class contains code derived from <a href="https://github.com/google/guava">Google
   * Guava</a></b>
   */
  public abstract class Optional<T> implements Serializable {
    private static final long serialVersionUID = 0;
  
    /**
     * Returns an {@code Optional} instance with no contained reference.
     */
    public static <T> Optional<T> absent() {
      return Absent.withType();
    }
  
    /**
     * Returns an {@code Optional} instance containing the given non-null reference.
     */
    public static <T> Optional<T> of(T reference) {
      return new Present<>(checkNotNull(reference));
    }
  
    /**
     * If {@code nullableReference} is non-null, returns an {@code Optional} instance containing that
     * reference; otherwise returns {@link Optional#absent}.
     */
    public static <T> Optional<T> fromNullable(@Nullable T nullableReference) {
      return (nullableReference == null)
          ? Optional.<T>absent()
          : new Present<>(nullableReference);
    }
  
    Optional() {
    }
  
    /**
     * Returns {@code true} if this holder contains a (non-null) instance.
     */
    public abstract boolean isPresent();
  
    /**
     * Returns the contained instance, which must be present. If the instance might be
     * absent, use {@link #or(Object)} or {@link #orNull} instead.
     *
     * @throws IllegalStateException if the instance is absent ({@link #isPresent} returns
     * {@code false})
     */
    public abstract T get();
  
    /**
     * Returns the contained instance if it is present; {@code defaultValue} otherwise. If
     * no default value should be required because the instance is known to be present, use
     * {@link #get()} instead. For a default value of {@code null}, use {@link #orNull}.
     *
     * <p>Note about generics: The signature {@code public T or(T defaultValue)} is overly
     * restrictive. However, the ideal signature, {@code public <S super T> S or(S)}, is not legal
     * Java. As a result, some sensible operations involving subtypes are compile errors:
     * <pre>   {@code
     *
     *   Optional<Integer> optionalInt = getSomeOptionalInt();
     *   Number value = optionalInt.or(0.5); // error
     *
     *   FluentIterable<? extends Number> numbers = getSomeNumbers();
     *   Optional<? extends Number> first = numbers.first();
     *   Number value = first.or(0.5); // error}</pre>
     *
     * <p>As a workaround, it is always safe to cast an {@code Optional<? extends T>} to {@code
     * Optional<T>}. Casting either of the above example {@code Optional} instances to {@code
     * Optional<Number>} (where {@code Number} is the desired output type) solves the problem:
     * <pre>   {@code
     *
     *   Optional<Number> optionalInt = (Optional) getSomeOptionalInt();
     *   Number value = optionalInt.or(0.5); // fine
     *
     *   FluentIterable<? extends Number> numbers = getSomeNumbers();
     *   Optional<Number> first = (Optional) numbers.first();
     *   Number value = first.or(0.5); // fine}</pre>
     */
    public abstract T or(T defaultValue);
  
    /**
     * Returns this {@code Optional} if it has a value present; {@code secondChoice}
     * otherwise.
     */
    public abstract Optional<T> or(Optional<? extends T> secondChoice);
  
    /**
     * Returns the contained instance if it is present; {@code null} otherwise. If the
     * instance is known to be present, use {@link #get()} instead.
     */
    @Nullable
    public abstract T orNull();
  
    /**
     * Returns an immutable singleton {@link Set} whose only element is the contained instance
     * if it is present; an empty immutable {@link Set} otherwise.
     *
     * @since 11.0
     */
    public abstract Set<T> asSet();
  
    /**
     * If the instance is present, it is transformed with the given {@link Function}; otherwise,
     * {@link Optional#absent} is returned. If the function returns {@code null}, a
     * {@link NullPointerException} is thrown.
     *
     * @throws NullPointerException if the function returns {@code null}
     * @since 12.0
     */
    public abstract <V> Optional<V> transform(Function<? super T, V> function);
  
    /**
     * Returns {@code true} if {@code object} is an {@code Optional} instance, and either
     * the contained references are {@linkplain Object#equals equal} to each other or both
     * are absent. Note that {@code Optional} instances of differing parameterized types can
     * be equal.
     */
    @Override
    public abstract boolean equals(@Nullable Object object);
  
    /**
     * Returns a hash code for this instance.
     */
    @Override
    public abstract int hashCode();
  
    /**
     * Returns a string representation for this instance. The form of this string
     * representation is unspecified.
     */
    @Override
    public abstract String toString();
  
    /**
     * Returns the value of each present instance from the supplied {@code optionals}, in order,
     * skipping over occurrences of {@link Optional#absent}. Iterators are unmodifiable and are
     * evaluated lazily.
     *
     * @since 11.0 (generics widened in 13.0)
     */
    public static <T> Iterable<T> presentInstances(
        final Iterable<? extends Optional<? extends T>> optionals) {
      checkNotNull(optionals);
      return new Iterable<T>() {
        @Override
        public Iterator<T> iterator() {
          return new AbstractIterator<T>() {
            private final Iterator<? extends Optional<? extends T>> iterator =
                checkNotNull(optionals.iterator());
  
            @Override
            protected T computeNext() {
              while (iterator.hasNext()) {
                Optional<? extends T> optional = iterator.next();
                if (optional.isPresent()) {
                  return optional.get();
                }
              }
              return endOfData();
            }
          };
        }
      };
    }
  }
  