java.util.Optional Maven / Gradle / Ivy
/*
* Copyright (C) 2020 API Design
*
* This program is free software; you can redistribute it and/or
* modify it under the terms of the GNU General Public License
* as published by the Free Software Foundation; either version 2
* of the License, or (at your option) any later version.
*
* This program 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 for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program; if not, write to the Free Software
* Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
*/
package java.util;
import java.util.function.Consumer;
import java.util.function.Function;
import java.util.function.Predicate;
import java.util.function.Supplier;
public final class Optional {
/**
* Common instance for {@code empty()}.
*/
private static final Optional EMPTY = new Optional<>();
/**
* If non-null, the value; if null, indicates no value is present
*/
private final T value;
/**
* Constructs an empty instance.
*
* @implNote Generally only one empty instance, {@link Optional#EMPTY},
* should exist per VM.
*/
private Optional() {
this.value = null;
}
/**
* 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 The type of the non-existent value
* @return an empty {@code Optional}
*/
public static Optional empty() {
@SuppressWarnings("unchecked")
Optional t = (Optional) EMPTY;
return t;
}
/**
* Constructs an instance with the described value.
*
* @param value the non-{@code null} value to describe
* @throws NullPointerException if value is {@code null}
*/
private Optional(T value) {
this.value = Objects.requireNonNull(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 the type of the value
* @return an {@code Optional} with the value present
* @throws NullPointerException if value is {@code null}
*/
public static Optional of(T value) {
return new Optional<>(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 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}
*/
public static Optional ofNullable(T value) {
return value == null ? empty() : of(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 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 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 filter(Predicate 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}.
*
* 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}:
*
* {@code
* Optional p =
* uris.stream().filter(uri -> !isProcessedYet(uri))
* .findFirst()
* .map(Paths::get);
* }
*
* Here, {@code findFirst} returns an {@code Optional}, and then
* {@code map} returns an {@code Optional} for the desired
* URI if one exists.
*
* @param mapper the mapping function to apply to a value, if present
* @param 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 Optional map(Function 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}.
*
* 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 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 Optional flatMap(Function> mapper) {
Objects.requireNonNull(mapper);
if (!isPresent()) {
return empty();
} else {
@SuppressWarnings("unchecked")
Optional r = (Optional) 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 or(Supplier> supplier) {
Objects.requireNonNull(supplier);
if (isPresent()) {
return this;
} else {
@SuppressWarnings("unchecked")
Optional r = (Optional) 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:
* {@code
* Stream> os = ..
* Stream s = os.flatMap(Optional::stream)
* }
*
* @return the optional value as a {@code Stream}
* @since 9
public Stream 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 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 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 T orElseThrow(Supplier 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:
*
* - it is also an {@code Optional} and;
*
- both instances have no value present or;
*
- the present values are "equal to" each other via {@code equals()}.
*
*
* @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";
}
}