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* 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.
 
*
 
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*/
package java.util;

import java.util.function.Consumer;
import java.util.function.Function;
import java.util.function.Predicate;
import java.util.function.Supplier;

/**
 
* 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 {@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.
     
*
     
* @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
 
.
     
*
     
* @param <T> 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 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 <T> 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 <T> Optional<T> 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 <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 ? 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<? super T> 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<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, 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));
        
}
    
}

    
/**
     
* 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 <U> 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<U> Optional<U> flatMap(Function<? super T, Optional<U>> 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<? extends T> other) {
        
return value != null ? value : other.get();
    
}

    
/**
     
* Return the contained value, if present, otherwise throw an exception
     
* to be created by the provided supplier.
     
*
     
* @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 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
     
*/

    
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 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 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 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.
     
*
     
* @implSpec If a value is present the result must include its string
     
* representation in the result. Empty and present Optionals 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";
    
}
}