cn.keayuan.util.Optional Maven / Gradle / Ivy
package cn.keayuan.util;
import java.util.NoSuchElementException;
import java.util.Objects;
import cn.keayuan.util.function.Consumer;
import cn.keayuan.util.function.Function;
import cn.keayuan.util.function.Predicate;
import cn.keayuan.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
* Optional.
*
* @param Type of the non-existent value
* @return an empty {@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 Option.empty()}. There is no guarantee that it is a singleton.
* Instead, use {@link #isPresent()}.
*/
public static Optional empty() {
@SuppressWarnings("unchecked") Optional t = (Optional) EMPTY;
return t;
}
/**
* Constructs an instance with the value present.
*
* @param value the non-null value to be present
* @throws NullPointerException if value is null
*/
private Optional(T value) {
this.value = Objects.requireNonNull(value);
}
/**
* Returns an {@code Optional} with the specified present non-null value.
*
* @param the class of the value
* @param value the value to be present, which must be non-null
* @return an {@code Optional} with the value present
* @throws NullPointerException if value is null
*/
public static Optional of(T value) {
return new Optional<>(value);
}
/**
* Returns an {@code Optional} describing the specified value, if non-null,
* otherwise returns an empty {@code Optional}.
*
* @param the class of the value
* @param value the possibly-null value to describe
* @return an {@code Optional} with a present value if the specified value
* is non-null, otherwise an empty {@code Optional}
*/
public static Optional ofNullable(T value) {
return value == null ? empty() : of(value);
}
/**
* If a value is present in this {@code Optional}, returns the value,
* otherwise throws {@code NoSuchElementException}.
*
* @return the non-null value held by this {@code Optional}
* @throws NoSuchElementException if there is no value present
* @see Optional#isPresent()
*/
public T get() {
if (value == null) {
throw new NoSuchElementException("No value present");
}
return value;
}
/**
* Return {@code true} if there is a value present, otherwise {@code false}.
*
* @return {@code true} if there is a value present, otherwise {@code false}
*/
public boolean isPresent() {
return value != null;
}
/**
* If a value is present, invoke the specified consumer with the value,
* otherwise do nothing.
*
* @param consumer block to be executed if a value is present
* @throws NullPointerException if value is present and {@code consumer} is
* null
*/
public void ifPresent(Consumer consumer) {
if (value != null) consumer.accept(value);
}
/**
* If a value is present, and the value matches the given predicate,
* return an {@code Optional} describing the value, otherwise return an
* empty {@code Optional}.
*
* @param predicate a predicate to apply to the 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 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, apply the provided mapping function to it,
* and if the result is non-null, return an {@code Optional} describing the
* result. Otherwise return an empty {@code Optional}.
*
* @param The type of the result of the mapping function
* @param mapper a mapping function to apply to the value, if present
* @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 null
* @apiNote This method supports post-processing on optional values, without
* the need to explicitly check for a return status. For example, the
* following code traverses a stream of file names, selects one that has
* not yet been processed, and then opens that file, returning an
* {@code Optional}:
*
* {@code
* Optional fis =
* names.stream().filter(name -> !isProcessedYet(name))
* .findFirst()
* .map(name -> new FileInputStream(name));
* }
*
* Here, {@code findFirst} returns an {@code Optional}, and then
* {@code map} returns an {@code Optional} for the desired
* file if one exists.
*/
public Optional map(Function mapper) {
Objects.requireNonNull(mapper);
if (!isPresent()) return empty();
else {
return Optional.ofNullable(mapper.apply(value));
}
}
/**
* If a value is present, apply the provided {@code Optional}-bearing
* mapping function to it, return that result, otherwise return an empty
* {@code Optional}. This method is similar to {@link #map(Function)},
* but the provided mapper is one whose result is already an {@code Optional},
* and if invoked, {@code flatMap} does not wrap it with an additional
* {@code Optional}.
*
* @param The type parameter to the {@code Optional} returned by
* @param mapper a mapping function to apply to the value, if present
* the mapping function
* @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 null or returns
* a null result
*/
public Optional flatMap(Function> mapper) {
Objects.requireNonNull(mapper);
if (!isPresent()) return empty();
else {
return Objects.requireNonNull(mapper.apply(value));
}
}
/**
* Return the value if present, otherwise return {@code other}.
*
* @param other the value to be returned if there is no value present, may
* be null
* @return the value, if present, otherwise {@code other}
*/
public T orElse(T other) {
return value != null ? value : other;
}
/**
* Return the value if present, otherwise invoke {@code other} and return
* the result of that invocation.
*
* @param other a {@code Supplier} whose result is returned if no value
* is present
* @return the value if present otherwise the result of {@code other.get()}
* @throws NullPointerException if value is not present and {@code other} is
* null
*/
public T orElseGet(Supplier other) {
return value != null ? value : other.get();
}
/**
* Return the contained value, if present, otherwise throw an exception
* to be created by the provided supplier.
*
* @param Type of the exception to be thrown
* @param exceptionSupplier The supplier which will return the exception to
* be thrown
* @return the present value
* @throws X if there is no value present
* @throws NullPointerException if no value is present and
* {@code exceptionSupplier} is null
* @apiNote A method reference to the exception constructor with an empty
* argument list can be used as the supplier. For example,
* {@code IllegalStateException::new}
*/
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 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 ObjectUtils.equals(value, other.value);
}
/**
* Returns the hash code value of the present value, if any, or 0 (zero) if
* no value is present.
*
* @return hash code value of the present value or 0 if no value is present
*/
@Override
public int hashCode() {
return ObjectUtils.hashCode(value);
}
/**
* Returns a non-empty string representation of this Optional suitable for
* debugging. The exact presentation format is unspecified and may vary
* between implementations and versions.
*
* @return the string representation of this instance
* @implSpec If a value is present the result must include its string
* representation in the result. Empty and present Optionals must be
* unambiguously differentiable.
*/
@Override
public String toString() {
return value != null ? String.format("Optional[%s]", value) : "Optional.empty";
}
}