465 lines
17 KiB
Java
465 lines
17 KiB
Java
/*
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* Copyright (c) 2012, 2020, Oracle and/or its affiliates. All rights reserved.
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* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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*
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* This code is free software; you can redistribute it and/or modify it
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* under the terms of the GNU General Public License version 2 only, as
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* published by the Free Software Foundation. Oracle designates this
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* particular file as subject to the "Classpath" exception as provided
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* by Oracle in the LICENSE file that accompanied this code.
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*
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* This code is distributed in the hope that it will be useful, but WITHOUT
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* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
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* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
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* version 2 for more details (a copy is included in the LICENSE file that
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* accompanied this code).
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*
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* You should have received a copy of the GNU General Public License version
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* 2 along with this work; if not, write to the Free Software Foundation,
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* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
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*
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* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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* or visit www.oracle.com if you need additional information or have any
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* questions.
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*/
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package java.util;
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import java.util.function.Consumer;
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import java.util.function.Function;
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import java.util.function.Predicate;
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import java.util.function.Supplier;
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import java.util.stream.Stream;
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/**
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* A container object which may or may not contain a non-{@code null} value.
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* If a value is present, {@code isPresent()} returns {@code true}. If no
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* value is present, the object is considered <i>empty</i> and
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* {@code isPresent()} returns {@code false}.
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*
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* <p>Additional methods that depend on the presence or absence of a contained
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* value are provided, such as {@link #orElse(Object) orElse()}
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* (returns a default value if no value is present) and
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* {@link #ifPresent(Consumer) ifPresent()} (performs an
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* action if a value is present).
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*
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* <p>This is a <a href="https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/lang/doc-files/ValueBased.html">value-based</a>
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* class; programmers should treat instances that are
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* {@linkplain #equals(Object) equal} as interchangeable and should not
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* use instances for synchronization, or unpredictable behavior may
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* occur. For example, in a future release, synchronization may fail.
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*
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* @apiNote
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* {@code Optional} is primarily intended for use as a method return type where
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* there is a clear need to represent "no result," and where using {@code null}
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* is likely to cause errors. A variable whose type is {@code Optional} should
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* never itself be {@code null}; it should always point to an {@code Optional}
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* instance.
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*
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* @param <T> the type of value
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* @since 1.8
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*/
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@jdk.internal.ValueBased
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public final class Optional<T> {
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/**
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* Common instance for {@code empty()}.
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*/
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private static final Optional<?> EMPTY = new Optional<>(null);
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/**
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* If non-null, the value; if null, indicates no value is present
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*/
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private final T value;
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/**
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* Returns an empty {@code Optional} instance. No value is present for this
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* {@code Optional}.
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*
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* @apiNote
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* Though it may be tempting to do so, avoid testing if an object is empty
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* by comparing with {@code ==} or {@code !=} against instances returned by
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* {@code Optional.empty()}. There is no guarantee that it is a singleton.
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* Instead, use {@link #isEmpty()} or {@link #isPresent()}.
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*
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* @param <T> The type of the non-existent value
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* @return an empty {@code Optional}
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*/
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public static<T> Optional<T> empty() {
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@SuppressWarnings("unchecked")
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Optional<T> t = (Optional<T>) EMPTY;
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return t;
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}
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/**
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* Constructs an instance with the described value.
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*
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* @param value the value to describe; it's the caller's responsibility to
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* ensure the value is non-{@code null} unless creating the singleton
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* instance returned by {@code empty()}.
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*/
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private Optional(T value) {
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this.value = value;
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}
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/**
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* Returns an {@code Optional} describing the given non-{@code null}
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* value.
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*
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* @param value the value to describe, which must be non-{@code null}
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* @param <T> the type of the value
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* @return an {@code Optional} with the value present
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* @throws NullPointerException if value is {@code null}
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*/
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public static <T> Optional<T> of(T value) {
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return new Optional<>(Objects.requireNonNull(value));
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}
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/**
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* Returns an {@code Optional} describing the given value, if
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* non-{@code null}, otherwise returns an empty {@code Optional}.
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*
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* @param value the possibly-{@code null} value to describe
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* @param <T> the type of the value
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* @return an {@code Optional} with a present value if the specified value
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* is non-{@code null}, otherwise an empty {@code Optional}
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*/
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@SuppressWarnings("unchecked")
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public static <T> Optional<T> ofNullable(T value) {
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return value == null ? (Optional<T>) EMPTY
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: new Optional<>(value);
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}
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/**
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* If a value is present, returns the value, otherwise throws
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* {@code NoSuchElementException}.
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*
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* @apiNote
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* The preferred alternative to this method is {@link #orElseThrow()}.
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*
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* @return the non-{@code null} value described by this {@code Optional}
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* @throws NoSuchElementException if no value is present
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*/
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public T get() {
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if (value == null) {
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throw new NoSuchElementException("No value present");
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}
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return value;
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}
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/**
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* If a value is present, returns {@code true}, otherwise {@code false}.
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*
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* @return {@code true} if a value is present, otherwise {@code false}
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*/
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public boolean isPresent() {
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return value != null;
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}
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/**
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* If a value is not present, returns {@code true}, otherwise
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* {@code false}.
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*
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* @return {@code true} if a value is not present, otherwise {@code false}
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* @since 11
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*/
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public boolean isEmpty() {
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return value == null;
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}
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/**
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* If a value is present, performs the given action with the value,
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* otherwise does nothing.
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*
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* @param action the action to be performed, if a value is present
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* @throws NullPointerException if value is present and the given action is
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* {@code null}
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*/
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public void ifPresent(Consumer<? super T> action) {
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if (value != null) {
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action.accept(value);
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}
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}
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/**
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* If a value is present, performs the given action with the value,
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* otherwise performs the given empty-based action.
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*
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* @param action the action to be performed, if a value is present
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* @param emptyAction the empty-based action to be performed, if no value is
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* present
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* @throws NullPointerException if a value is present and the given action
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* is {@code null}, or no value is present and the given empty-based
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* action is {@code null}.
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* @since 9
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*/
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public void ifPresentOrElse(Consumer<? super T> action, Runnable emptyAction) {
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if (value != null) {
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action.accept(value);
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} else {
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emptyAction.run();
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}
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}
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/**
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* If a value is present, and the value matches the given predicate,
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* returns an {@code Optional} describing the value, otherwise returns an
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* empty {@code Optional}.
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*
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* @param predicate the predicate to apply to a value, if present
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* @return an {@code Optional} describing the value of this
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* {@code Optional}, if a value is present and the value matches the
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* given predicate, otherwise an empty {@code Optional}
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* @throws NullPointerException if the predicate is {@code null}
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*/
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public Optional<T> filter(Predicate<? super T> predicate) {
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Objects.requireNonNull(predicate);
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if (!isPresent()) {
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return this;
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} else {
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return predicate.test(value) ? this : empty();
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}
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}
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/**
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* If a value is present, returns an {@code Optional} describing (as if by
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* {@link #ofNullable}) the result of applying the given mapping function to
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* the value, otherwise returns an empty {@code Optional}.
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*
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* <p>If the mapping function returns a {@code null} result then this method
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* returns an empty {@code Optional}.
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*
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* @apiNote
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* This method supports post-processing on {@code Optional} values, without
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* the need to explicitly check for a return status. For example, the
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* following code traverses a stream of URIs, selects one that has not
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* yet been processed, and creates a path from that URI, returning
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* an {@code Optional<Path>}:
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*
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* <pre>{@code
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* Optional<Path> p =
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* uris.stream().filter(uri -> !isProcessedYet(uri))
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* .findFirst()
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* .map(Paths::get);
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* }</pre>
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*
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* Here, {@code findFirst} returns an {@code Optional<URI>}, and then
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* {@code map} returns an {@code Optional<Path>} for the desired
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* URI if one exists.
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*
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* @param mapper the mapping function to apply to a value, if present
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* @param <U> The type of the value returned from the mapping function
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* @return an {@code Optional} describing the result of applying a mapping
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* function to the value of this {@code Optional}, if a value is
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* present, otherwise an empty {@code Optional}
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* @throws NullPointerException if the mapping function is {@code null}
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*/
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public <U> Optional<U> map(Function<? super T, ? extends U> mapper) {
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Objects.requireNonNull(mapper);
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if (!isPresent()) {
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return empty();
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} else {
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return Optional.ofNullable(mapper.apply(value));
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}
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}
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/**
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* If a value is present, returns the result of applying the given
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* {@code Optional}-bearing mapping function to the value, otherwise returns
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* an empty {@code Optional}.
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*
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* <p>This method is similar to {@link #map(Function)}, but the mapping
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* function is one whose result is already an {@code Optional}, and if
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* invoked, {@code flatMap} does not wrap it within an additional
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* {@code Optional}.
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*
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* @param <U> The type of value of the {@code Optional} returned by the
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* mapping function
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* @param mapper the mapping function to apply to a value, if present
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* @return the result of applying an {@code Optional}-bearing mapping
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* function to the value of this {@code Optional}, if a value is
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* present, otherwise an empty {@code Optional}
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* @throws NullPointerException if the mapping function is {@code null} or
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* returns a {@code null} result
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*/
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public <U> Optional<U> flatMap(Function<? super T, ? extends Optional<? extends U>> mapper) {
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Objects.requireNonNull(mapper);
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if (!isPresent()) {
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return empty();
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} else {
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@SuppressWarnings("unchecked")
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Optional<U> r = (Optional<U>) mapper.apply(value);
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return Objects.requireNonNull(r);
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}
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}
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/**
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* If a value is present, returns an {@code Optional} describing the value,
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* otherwise returns an {@code Optional} produced by the supplying function.
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*
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* @param supplier the supplying function that produces an {@code Optional}
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* to be returned
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* @return returns an {@code Optional} describing the value of this
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* {@code Optional}, if a value is present, otherwise an
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* {@code Optional} produced by the supplying function.
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* @throws NullPointerException if the supplying function is {@code null} or
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* produces a {@code null} result
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* @since 9
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*/
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public Optional<T> or(Supplier<? extends Optional<? extends T>> supplier) {
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Objects.requireNonNull(supplier);
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if (isPresent()) {
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return this;
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} else {
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@SuppressWarnings("unchecked")
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Optional<T> r = (Optional<T>) supplier.get();
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return Objects.requireNonNull(r);
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}
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}
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/**
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* If a value is present, returns a sequential {@link Stream} containing
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* only that value, otherwise returns an empty {@code Stream}.
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*
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* @apiNote
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* This method can be used to transform a {@code Stream} of optional
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* elements to a {@code Stream} of present value elements:
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* <pre>{@code
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* Stream<Optional<T>> os = ..
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* Stream<T> s = os.flatMap(Optional::stream)
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* }</pre>
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*
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* @return the optional value as a {@code Stream}
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* @since 9
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*/
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public Stream<T> stream() {
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if (!isPresent()) {
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return Stream.empty();
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} else {
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return Stream.of(value);
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}
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}
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/**
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* If a value is present, returns the value, otherwise returns
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* {@code other}.
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*
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* @param other the value to be returned, if no value is present.
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* May be {@code null}.
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* @return the value, if present, otherwise {@code other}
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*/
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public T orElse(T other) {
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return value != null ? value : other;
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}
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/**
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* If a value is present, returns the value, otherwise returns the result
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* produced by the supplying function.
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*
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* @param supplier the supplying function that produces a value to be returned
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* @return the value, if present, otherwise the result produced by the
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* supplying function
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* @throws NullPointerException if no value is present and the supplying
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* function is {@code null}
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*/
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public T orElseGet(Supplier<? extends T> supplier) {
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return value != null ? value : supplier.get();
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}
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/**
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* If a value is present, returns the value, otherwise throws
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* {@code NoSuchElementException}.
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*
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* @return the non-{@code null} value described by this {@code Optional}
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* @throws NoSuchElementException if no value is present
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* @since 10
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*/
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public T orElseThrow() {
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if (value == null) {
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throw new NoSuchElementException("No value present");
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}
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return value;
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}
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/**
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* If a value is present, returns the value, otherwise throws an exception
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* produced by the exception supplying function.
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*
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* @apiNote
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* A method reference to the exception constructor with an empty argument
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* list can be used as the supplier. For example,
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* {@code IllegalStateException::new}
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*
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* @param <X> Type of the exception to be thrown
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* @param exceptionSupplier the supplying function that produces an
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* exception to be thrown
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* @return the value, if present
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* @throws X if no value is present
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* @throws NullPointerException if no value is present and the exception
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* supplying function is {@code null}
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*/
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public <X extends Throwable> T orElseThrow(Supplier<? extends X> exceptionSupplier) throws X {
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if (value != null) {
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return value;
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} else {
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throw exceptionSupplier.get();
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}
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}
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/**
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* Indicates whether some other object is "equal to" this {@code Optional}.
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* The other object is considered equal if:
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* <ul>
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* <li>it is also an {@code Optional} and;
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* <li>both instances have no value present or;
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* <li>the present values are "equal to" each other via {@code equals()}.
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* </ul>
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*
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* @param obj an object to be tested for equality
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* @return {@code true} if the other object is "equal to" this object
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* otherwise {@code false}
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*/
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@Override
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public boolean equals(Object obj) {
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if (this == obj) {
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return true;
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}
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if (!(obj instanceof Optional)) {
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return false;
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}
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Optional<?> other = (Optional<?>) obj;
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return Objects.equals(value, other.value);
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}
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/**
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* Returns the hash code of the value, if present, otherwise {@code 0}
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* (zero) if no value is present.
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*
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* @return hash code value of the present value or {@code 0} if no value is
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* present
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*/
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@Override
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public int hashCode() {
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return Objects.hashCode(value);
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}
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/**
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* Returns a non-empty string representation of this {@code Optional}
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* suitable for debugging. The exact presentation format is unspecified and
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* may vary between implementations and versions.
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*
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* @implSpec
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* If a value is present the result must include its string representation
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* in the result. Empty and present {@code Optional}s must be unambiguously
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* differentiable.
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*
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* @return the string representation of this instance
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*/
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@Override
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public String toString() {
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return value != null
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? ("Optional[" + value + "]")
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: "Optional.empty";
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}
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}
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