/*
* 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;
}