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.LongConsumer; import java.util.function.LongSupplier; import java.util.function.Supplier; import java.util.stream.LongStream; /** * A container object which may or may not contain a {@code long} 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(long) orElse()} * (returns a default value if no value is present) and * {@link #ifPresent(LongConsumer) 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 OptionalLong} may have unpredictable results and should be avoided. * * @apiNote * {@code OptionalLong} is primarily intended for use as a method return type where * there is a clear need to represent "no result." A variable whose type is * {@code OptionalLong} should never itself be {@code null}; it should always point * to an {@code OptionalLong} instance. * * @since 1.8 */ public final class OptionalLong { /** * Common instance for {@code empty()}. */ private static final OptionalLong EMPTY = new OptionalLong(); /** * If true then the value is present, otherwise indicates no value is present */ private final boolean isPresent; private final long value; /** * Construct an empty instance. * * @implNote generally only one empty instance, {@link OptionalLong#EMPTY}, * should exist per VM. */ private OptionalLong() { this.isPresent = false; this.value = 0; } /** * Returns an empty {@code OptionalLong} instance. No value is present for * this {@code OptionalLong}. * * @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 OptionalLong.empty()}. There is no guarantee that it is a singleton. * Instead, use {@link #isPresent()}. * * @return an empty {@code OptionalLong}. */ public static OptionalLong empty() { return EMPTY; } /** * Construct an instance with the described value. * * @param value the long value to describe */ private OptionalLong(long value) { this.isPresent = true; this.value = value; } /** * Returns an {@code OptionalLong} describing the given value. * * @param value the value to describe * @return an {@code OptionalLong} with the value present */ public static OptionalLong of(long value) { return new OptionalLong(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 value described by this {@code OptionalLong} * @throws NoSuchElementException if no value is present */ public long getAsLong() { if (!isPresent) { 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 isPresent; } /** * 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 !isPresent; } /** * 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(LongConsumer action) { if (isPresent) { 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(LongConsumer action, Runnable emptyAction) { if (isPresent) { action.accept(value); } else { emptyAction.run(); } } /** * If a value is present, returns a sequential {@link LongStream} containing * only that value, otherwise returns an empty {@code LongStream}. * * @apiNote * This method can be used to transform a {@code Stream} of optional longs * to an {@code LongStream} of present longs: * <pre>{@code * Stream<OptionalLong> os = .. * LongStream s = os.flatMapToLong(OptionalLong::stream) * }</pre> * * @return the optional value as an {@code LongStream} * @since 9 */ public LongStream stream() { if (isPresent) { return LongStream.of(value); } else { return LongStream.empty(); } } /** * If a value is present, returns the value, otherwise returns * {@code other}. * * @param other the value to be returned, if no value is present * @return the value, if present, otherwise {@code other} */ public long orElse(long other) { return isPresent ? 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 long orElseGet(LongSupplier supplier) { return isPresent ? value : supplier.getAsLong(); } /** * If a value is present, returns the value, otherwise throws * {@code NoSuchElementException}. * * @return the value described by this {@code OptionalLong} * @throws NoSuchElementException if no value is present * @since 10 */ public long orElseThrow() { if (!isPresent) { 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> long orElseThrow(Supplier<? extends X> exceptionSupplier) throws X { if (isPresent) { return value; } else { throw exceptionSupplier.get(); } } /** * Indicates whether some other object is "equal to" this * {@code OptionalLong}. The other object is considered equal if: * <ul> * <li>it is also an {@code OptionalLong} and; * <li>both instances have no value present or; * <li>the present values are "equal to" each other via {@code ==}. * </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 OptionalLong)) { return false; } OptionalLong other = (OptionalLong) obj; return (isPresent && other.isPresent) ? value == other.value : isPresent == other.isPresent; } /** * 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 isPresent ? Long.hashCode(value) : 0; } /** * Returns a non-empty string representation of this {@code OptionalLong} * 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 OptionalLong}s must be * unambiguously differentiable. * * @return the string representation of this instance */ @Override public String toString() { return isPresent ? String.format("OptionalLong[%s]", value) : "OptionalLong.empty"; } }