/*
    * 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.google.common.base;
  
  import static com.google.common.base.Preconditions.checkNotNull;
  
  import com.google.common.annotations.Beta;
  import com.google.common.annotations.GwtCompatible;
  
  import java.io.Serializable;
  import java.util.Iterator;
  import java.util.Set;
  
  import javax.annotation.Nullable;
  
  /**
   * 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
   */
  @GwtCompatible(serializable = true)
  public abstract class Optional<T> implements Serializable {
    /**
     * 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<T>(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<T>(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 supplier.get()} otherwise. If the
    * supplier returns {@code null}, a {@link NullPointerException} is thrown.
    *
    * @throws NullPointerException if the supplier returns {@code null}
    */
   @Beta
   public abstract T or(Supplier<? extends T> supplier);
 
   /**
    * 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)
    */
   @Beta
   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();
           }
         };
       }
     };
   }
 
   private static final long serialVersionUID = 0;
 }