| /* |
| * 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; |
| } |