Java tutorial
/* * Copyright (c) 2012, 2018, Oracle and/or its affiliates. All rights reserved. * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. * * This code is free software; you can redistribute it and/or modify it * under the terms of the GNU General Public License version 2 only, as * published by the Free Software Foundation. Oracle designates this * particular file as subject to the "Classpath" exception as provided * by Oracle in the LICENSE file that accompanied this code. * * This code is distributed in the hope that it will be useful, but WITHOUT * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License * version 2 for more details (a copy is included in the LICENSE file that * accompanied this code). * * You should have received a copy of the GNU General Public License version * 2 along with this work; if not, write to the Free Software Foundation, * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. * * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA * or visit www.oracle.com if you need additional information or have any * questions. */ package java.util; import java.util.function.Consumer; import java.util.function.Function; import java.util.function.Predicate; import java.util.function.Supplier; import java.util.stream.Stream; /** * A container object which may or may not contain a non-{@code null} value. * If a value is present, {@code isPresent()} returns {@code true}. If no * value is present, the object is considered <i>empty</i> and * {@code isPresent()} returns {@code false}. * * <p>Additional methods that depend on the presence or absence of a contained * value are provided, such as {@link #orElse(Object) orElse()} * (returns a default value if no value is present) and * {@link #ifPresent(Consumer) ifPresent()} (performs an * action if a value is present). * * <p>This is a <a href="../lang/doc-files/ValueBased.html">value-based</a> * class; use of identity-sensitive operations (including reference equality * ({@code ==}), identity hash code, or synchronization) on instances of * {@code Optional} may have unpredictable results and should be avoided. * * @apiNote * {@code Optional} is primarily intended for use as a method return type where * there is a clear need to represent "no result," and where using {@code null} * is likely to cause errors. A variable whose type is {@code Optional} should * never itself be {@code null}; it should always point to an {@code Optional} * instance. * * @param <T> the type of value * @since 1.8 */ public final class Optional<T> { /** * Common instance for {@code empty()}. */ private static final Optional<?> EMPTY = new Optional<>(null); /** * If non-null, the value; if null, indicates no value is present */ private final T value; /** * Returns an empty {@code Optional} instance. No value is present for this * {@code Optional}. * * @apiNote * Though it may be tempting to do so, avoid testing if an object is empty * by comparing with {@code ==} against instances returned by * {@code Optional.empty()}. There is no guarantee that it is a singleton. * Instead, use {@link #isPresent()}. * * @param <T> The type of the non-existent value * @return an empty {@code Optional} */ public static <T> Optional<T> empty() { @SuppressWarnings("unchecked") Optional<T> t = (Optional<T>) EMPTY; return t; } /** * Constructs an instance with the described value. * * @param value the value to describe; it's the caller's responsibility to * ensure the value is non-{@code null} unless creating the singleton * instance returned by {@code empty()}. */ private Optional(T value) { this.value = value; } /** * Returns an {@code Optional} describing the given non-{@code null} * value. * * @param value the value to describe, which must be non-{@code null} * @param <T> the type of the value * @return an {@code Optional} with the value present * @throws NullPointerException if value is {@code null} */ public static <T> Optional<T> of(T value) { return new Optional<>(Objects.requireNonNull(value)); } /** * Returns an {@code Optional} describing the given value, if * non-{@code null}, otherwise returns an empty {@code Optional}. * * @param value the possibly-{@code null} value to describe * @param <T> the type of the value * @return an {@code Optional} with a present value if the specified value * is non-{@code null}, otherwise an empty {@code Optional} */ @SuppressWarnings("unchecked") public static <T> Optional<T> ofNullable(T value) { return value == null ? (Optional<T>) EMPTY : new Optional<>(value); } /** * If a value is present, returns the value, otherwise throws * {@code NoSuchElementException}. * * @apiNote * The preferred alternative to this method is {@link #orElseThrow()}. * * @return the non-{@code null} value described by this {@code Optional} * @throws NoSuchElementException if no value is present */ public T get() { if (value == null) { throw new NoSuchElementException("No value present"); } return value; } /** * If a value is present, returns {@code true}, otherwise {@code false}. * * @return {@code true} if a value is present, otherwise {@code false} */ public boolean isPresent() { return value != null; } /** * If a value is not present, returns {@code true}, otherwise * {@code false}. * * @return {@code true} if a value is not present, otherwise {@code false} * @since 11 */ public boolean isEmpty() { return value == null; } /** * If a value is present, performs the given action with the value, * otherwise does nothing. * * @param action the action to be performed, if a value is present * @throws NullPointerException if value is present and the given action is * {@code null} */ public void ifPresent(Consumer<? super T> action) { if (value != null) { action.accept(value); } } /** * If a value is present, performs the given action with the value, * otherwise performs the given empty-based action. * * @param action the action to be performed, if a value is present * @param emptyAction the empty-based action to be performed, if no value is * present * @throws NullPointerException if a value is present and the given action * is {@code null}, or no value is present and the given empty-based * action is {@code null}. * @since 9 */ public void ifPresentOrElse(Consumer<? super T> action, Runnable emptyAction) { if (value != null) { action.accept(value); } else { emptyAction.run(); } } /** * If a value is present, and the value matches the given predicate, * returns an {@code Optional} describing the value, otherwise returns an * empty {@code Optional}. * * @param predicate the predicate to apply to a value, if present * @return an {@code Optional} describing the value of this * {@code Optional}, if a value is present and the value matches the * given predicate, otherwise an empty {@code Optional} * @throws NullPointerException if the predicate is {@code null} */ public Optional<T> filter(Predicate<? super T> predicate) { Objects.requireNonNull(predicate); if (!isPresent()) { return this; } else { return predicate.test(value) ? this : empty(); } } /** * If a value is present, returns an {@code Optional} describing (as if by * {@link #ofNullable}) the result of applying the given mapping function to * the value, otherwise returns an empty {@code Optional}. * * <p>If the mapping function returns a {@code null} result then this method * returns an empty {@code Optional}. * * @apiNote * This method supports post-processing on {@code Optional} values, without * the need to explicitly check for a return status. For example, the * following code traverses a stream of URIs, selects one that has not * yet been processed, and creates a path from that URI, returning * an {@code Optional<Path>}: * * <pre>{@code * Optional<Path> p = * uris.stream().filter(uri -> !isProcessedYet(uri)) * .findFirst() * .map(Paths::get); * }</pre> * * Here, {@code findFirst} returns an {@code Optional<URI>}, and then * {@code map} returns an {@code Optional<Path>} for the desired * URI if one exists. * * @param mapper the mapping function to apply to a value, if present * @param <U> The type of the value returned from the mapping function * @return an {@code Optional} describing the result of applying a mapping * function to the value of this {@code Optional}, if a value is * present, otherwise an empty {@code Optional} * @throws NullPointerException if the mapping function is {@code null} */ public <U> Optional<U> map(Function<? super T, ? extends U> mapper) { Objects.requireNonNull(mapper); if (!isPresent()) { return empty(); } else { return Optional.ofNullable(mapper.apply(value)); } } /** * If a value is present, returns the result of applying the given * {@code Optional}-bearing mapping function to the value, otherwise returns * an empty {@code Optional}. * * <p>This method is similar to {@link #map(Function)}, but the mapping * function is one whose result is already an {@code Optional}, and if * invoked, {@code flatMap} does not wrap it within an additional * {@code Optional}. * * @param <U> The type of value of the {@code Optional} returned by the * mapping function * @param mapper the mapping function to apply to a value, if present * @return the result of applying an {@code Optional}-bearing mapping * function to the value of this {@code Optional}, if a value is * present, otherwise an empty {@code Optional} * @throws NullPointerException if the mapping function is {@code null} or * returns a {@code null} result */ public <U> Optional<U> flatMap(Function<? super T, ? extends Optional<? extends U>> mapper) { Objects.requireNonNull(mapper); if (!isPresent()) { return empty(); } else { @SuppressWarnings("unchecked") Optional<U> r = (Optional<U>) mapper.apply(value); return Objects.requireNonNull(r); } } /** * If a value is present, returns an {@code Optional} describing the value, * otherwise returns an {@code Optional} produced by the supplying function. * * @param supplier the supplying function that produces an {@code Optional} * to be returned * @return returns an {@code Optional} describing the value of this * {@code Optional}, if a value is present, otherwise an * {@code Optional} produced by the supplying function. * @throws NullPointerException if the supplying function is {@code null} or * produces a {@code null} result * @since 9 */ public Optional<T> or(Supplier<? extends Optional<? extends T>> supplier) { Objects.requireNonNull(supplier); if (isPresent()) { return this; } else { @SuppressWarnings("unchecked") Optional<T> r = (Optional<T>) supplier.get(); return Objects.requireNonNull(r); } } /** * If a value is present, returns a sequential {@link Stream} containing * only that value, otherwise returns an empty {@code Stream}. * * @apiNote * This method can be used to transform a {@code Stream} of optional * elements to a {@code Stream} of present value elements: * <pre>{@code * Stream<Optional<T>> os = .. * Stream<T> s = os.flatMap(Optional::stream) * }</pre> * * @return the optional value as a {@code Stream} * @since 9 */ public Stream<T> stream() { if (!isPresent()) { return Stream.empty(); } else { return Stream.of(value); } } /** * If a value is present, returns the value, otherwise returns * {@code other}. * * @param other the value to be returned, if no value is present. * May be {@code null}. * @return the value, if present, otherwise {@code other} */ public T orElse(T other) { return value != null ? value : other; } /** * If a value is present, returns the value, otherwise returns the result * produced by the supplying function. * * @param supplier the supplying function that produces a value to be returned * @return the value, if present, otherwise the result produced by the * supplying function * @throws NullPointerException if no value is present and the supplying * function is {@code null} */ public T orElseGet(Supplier<? extends T> supplier) { return value != null ? value : supplier.get(); } /** * If a value is present, returns the value, otherwise throws * {@code NoSuchElementException}. * * @return the non-{@code null} value described by this {@code Optional} * @throws NoSuchElementException if no value is present * @since 10 */ public T orElseThrow() { if (value == null) { throw new NoSuchElementException("No value present"); } return value; } /** * If a value is present, returns the value, otherwise throws an exception * produced by the exception supplying function. * * @apiNote * A method reference to the exception constructor with an empty argument * list can be used as the supplier. For example, * {@code IllegalStateException::new} * * @param <X> Type of the exception to be thrown * @param exceptionSupplier the supplying function that produces an * exception to be thrown * @return the value, if present * @throws X if no value is present * @throws NullPointerException if no value is present and the exception * supplying function is {@code null} */ public <X extends Throwable> T orElseThrow(Supplier<? extends X> exceptionSupplier) throws X { if (value != null) { return value; } else { throw exceptionSupplier.get(); } } /** * Indicates whether some other object is "equal to" this {@code Optional}. * The other object is considered equal if: * <ul> * <li>it is also an {@code Optional} and; * <li>both instances have no value present or; * <li>the present values are "equal to" each other via {@code equals()}. * </ul> * * @param obj an object to be tested for equality * @return {@code true} if the other object is "equal to" this object * otherwise {@code false} */ @Override public boolean equals(Object obj) { if (this == obj) { return true; } if (!(obj instanceof Optional)) { return false; } Optional<?> other = (Optional<?>) obj; return Objects.equals(value, other.value); } /** * Returns the hash code of the value, if present, otherwise {@code 0} * (zero) if no value is present. * * @return hash code value of the present value or {@code 0} if no value is * present */ @Override public int hashCode() { return Objects.hashCode(value); } /** * Returns a non-empty string representation of this {@code Optional} * suitable for debugging. The exact presentation format is unspecified and * may vary between implementations and versions. * * @implSpec * If a value is present the result must include its string representation * in the result. Empty and present {@code Optional}s must be unambiguously * differentiable. * * @return the string representation of this instance */ @Override public String toString() { return value != null ? String.format("Optional[%s]", value) : "Optional.empty"; } }