/*
* 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}".
*
*
* A non-null {@code Optional} 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.
*
*
* Some uses of this class include
*
*
* - As a method return type, as an alternative to returning {@code null} to
* indicate that no value was available
*
- 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()})
*
- To wrap nullable references for storage in a collection that does not
* support {@code null} (though there are
* several other approaches to this that should be considered first)
*
*
*
* A common alternative to using this class is to find or create a suitable
* null object
* for the type in question.
*
*
* 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.
*
*
* See the Guava User Guide article on
* using {@code Optional}.
*
* @param 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} to {@code
* Optional} 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 implements Serializable {
/**
* Returns an {@code Optional} instance with no contained reference.
*/
public static Optional absent() {
return Absent.withType();
}
/**
* Returns an {@code Optional} instance containing the given non-null reference.
*/
public static Optional 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 Optional fromNullable(@Nullable T nullableReference) {
return (nullableReference == null) ? Optional.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}.
*
*
* Note about generics: The signature {@code public T or(T defaultValue)} is
* overly restrictive. However, the ideal signature,
* {@code public S or(S)}, is not legal Java. As a result, some
* sensible operations involving subtypes are compile errors:
*
*
* {@code
*
* Optional 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}
*
*
*
* As a workaround, it is always safe to cast an {@code Optional extends T>}
* to {@code
* Optional}. Casting either of the above example {@code Optional} instances
* to {@code
* Optional} (where {@code Number} is the desired output type) solves
* the problem:
*
*
* {@code
*
* Optional optionalInt = (Optional) getSomeOptionalInt();
* Number value = optionalInt.or(0.5); // fine
*
* FluentIterable extends Number> numbers = getSomeNumbers();
* Optional first = (Optional) numbers.first();
* Number value = first.or(0.5); // fine}
*
*/
public abstract T or(T defaultValue);
/**
* Returns this {@code Optional} if it has a value present; {@code secondChoice}
* otherwise.
*/
public abstract Optional 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 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 Optional 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 Iterable presentInstances(final Iterable extends Optional extends T>> optionals) {
checkNotNull(optionals);
return new Iterable() {
@Override
public Iterator iterator() {
return new AbstractIterator() {
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;
}