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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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package sun.security.util;

import java.util.Objects;
import java.util.NoSuchElementException;

A container object which may or may not contain a non-null value. If a value is present, isPresent() will return true and get() will return the value. 
Additional methods that depend on the presence or absence of a contained value are provided, such as orElse() (return a default value if value not present) and ifPresent() (execute a block of code if the value is present). 
This is a value-based class; use of identity-sensitive operations (including reference equality (==), identity hash code, or synchronization) on instances of Optional may have unpredictable results and should be avoided. 
Since:
1.8/**
 * A container object which may or may not contain a non-null value.
 * If a value is present, {@code isPresent()} will return {@code true} and
 * {@code get()} will return the value.
 *
 * <p>Additional methods that depend on the presence or absence of a contained
 * value are provided, such as {@link #orElse(java.lang.Object) orElse()}
 * (return a default value if value not present) and
 * {@link #ifPresent(java.util.function.Consumer) ifPresent()} (execute a block
 * of code if the 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.
 *
 * @since 1.8
 */
public final class Optional<T> {
    
Common instance for empty(). /**
     * Common instance for {@code empty()}.
     */
    private static final Optional<?> EMPTY = new Optional<>();

    
If non-null, the value; if null, indicates no value is present
/**
     * If non-null, the value; if null, indicates no value is present
     */
    private final T value;

    
Constructs an empty instance.
Implementation Note:
Generally only one empty instance, EMPTY, should exist per VM./**
     * 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 Optional instance. No value is present for this Optional. 
Type parameters:
<T> – The type of the non-existent value
API Note:
 Though it may be tempting to do so, avoid testing if an object is empty by comparing with == against instances returned by Optional.empty(). There is no guarantee that it is a singleton. Instead, use isPresent().
Returns:
an empty Optional/**
     * 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.
Params:
value – the non-null value to describe
Throws:
NullPointerException – if value is null/**
     * 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 Optional describing the given non-null value. 
Params:
value – the value to describe, which must be non-null
Type parameters:
<T> – the type of the value
Throws:
NullPointerException – if value is null
Returns:
an Optional with the value present/**
     * 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<>(value);
    }

    
Returns an Optional describing the specified value, if non-null, otherwise returns an empty Optional. 
Params:
value – the possibly-null value to describe
Type parameters:
<T> – the class of the value
Returns:
an Optional with a present value if the specified value is non-null, otherwise an empty Optional/**
     * Returns an {@code Optional} describing the specified value, if non-null,
     * otherwise returns an empty {@code Optional}.
     *
     * @param <T> 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 <T> Optional<T> ofNullable(T value) {
        return value == null ? new Optional<T>() : of(value);
    }

    
If a value is present, returns the value, otherwise throws NoSuchElementException. 
Throws:
NoSuchElementException – if no value is present
API Note:
 The preferred alternative to this method is orElseThrow().
Returns:
the non-null value described by this Optional/**
     * 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 true, otherwise false. 
Returns:
true if a value is present, otherwise false/**
     * 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 true, otherwise false. 
Returns:
 true if a value is not present, otherwise false
Since:
  11/**
     * 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, apply the provided mapping function to it, and if the result is non-null, return an Optional describing the result. Otherwise return an empty Optional. 
Params:
mapper – a mapping function to apply to the value, if present
Type parameters:
<U> – The type of the result of the mapping function
Throws:
NullPointerException – if the mapping function is null
API Note:
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 Optional<FileInputStream>: 

    Optional<FileInputStream> fis =
        names.stream().filter(name -> !isProcessedYet(name))
                      .findFirst()
                      .map(name -> new FileInputStream(name));
 Here, findFirst returns an Optional<String>, and then map returns an Optional<FileInputStream> for the desired file if one exists.
Returns:
an Optional describing the result of applying a mapping function to the value of this Optional, if a value is present, otherwise an empty Optional/**
     * 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}.
     *
     * @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<FileInputStream>}:
     *
     * <pre>{@code
     *     Optional<FileInputStream> fis =
     *         names.stream().filter(name -> !isProcessedYet(name))
     *                       .findFirst()
     *                       .map(name -> new FileInputStream(name));
     * }</pre>
     *
     * Here, {@code findFirst} returns an {@code Optional<String>}, and then
     * {@code map} returns an {@code Optional<FileInputStream>} for the desired
     * file if one exists.
     *
     * @param <U> 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
     */
    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));
        }
    }

    
Return the value if present, otherwise invoke other and return the result of that invocation. 
Params:
other – a Supplier whose result is returned if no value is present
Throws:
NullPointerException – if value is not present and other is null
Returns:
the value if present otherwise the result of other.get()/**
     * 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<? extends T> other) {
        return value != null ? value : other.get();
    }

    
Indicates whether some other object is "equal to" this Optional. The other object is considered equal if: 

it is also an Optional and; 
both instances have no value present or;
the present values are "equal to" each other via equals(). 
Params:
obj – an object to be tested for equality
Returns:
true if the other object is "equal to" this object otherwise false/**
     * 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 0 (zero) if no value is present. 
Returns:
hash code value of the present value or 0 if no value is present/**
     * 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 Optional suitable for debugging. The exact presentation format is unspecified and may vary between implementations and versions. 
Implementation Requirements:
 If a value is present the result must include its string representation in the result. Empty and present Optionals must be unambiguously differentiable.
Returns:
the string representation of this instance/**
     * 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";
    }
}