/*
 * 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 java.io.Serializable;
import java.util.Iterator;
import java.util.Set;

import javax.annotation.Nullable;

import com.google.common.annotations.Beta;
import com.google.common.annotations.GwtCompatible;

/**
 * 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;
}
