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Java implementation of some of the core utilities from Scala
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/**
 * Copyright 2015 Peter Nerg
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file
 * except in compliance with the License. You may obtain a copy of the License at
 *
 * 
http://www.apache.org/licenses/LICENSE-2.0
 *
 * 
Unless required by applicable law or agreed to in writing, software distributed under the
 * License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either
 * express or implied. See the License for the specific language governing permissions and
 * limitations under the License.
 */
package javascalautils;

import java.io.Serializable;
import java.util.NoSuchElementException;
import java.util.function.Predicate;
import java.util.function.Supplier;

import static javascalautils.EitherCompanion.Left;
import static javascalautils.EitherCompanion.Right;

/**
 * Represents an empty {@link Option}. 

 * The {@link None} is a replacement for null values representing a non-existing value.
 * 

 * One can consider {@link None} as a collection of size 0. 

 * Instances of None should not be created directly, rather use the factory methods provided on
 * {@link Option}.
 *
 * 

 *   {@link Option#empty()}
 *   
{@link Option#None()}
 * 
 *
 * Since {@link None} anyways cannot represent a value it is preferable to use it as a singleton
 * saving unnecessary instance creation. 

 * For examples on usage refer to the documentation for {@link Option}.
 *
 * @author Peter Nerg
 * @since 1.0
 * @param  The type of the value represented by this instance
 */
public final class None extends EmptyContainer implements Option, Serializable {

  private static final long serialVersionUID = -5169653193196761412L;

  /**
   * Always throws {@link NoSuchElementException} since None cannot per definition hold any value.
   *
   * @since 1.0
   */
  @Override
  public T get() {
    throw new NoSuchElementException("None cannot hold values");
  }

  /**
   * Always the value provided by the supplier.
   *
   * @since 1.0
   */
  public T getOrElse(Supplier supplier) {
    return supplier.get();
  }

  /**
   * Always returns false. 

   * I.e. the predicate is never used as {@link None} represents nothing/no value.
   *
   * @since 1.0
   */
  public boolean exists(Predicate predicate) {
    return false;
  }

  /**
   * Always returns this. 

   * I.e. the function is never used as {@link None} represents nothing/no value.
   *
   * @since 1.0
   */
  @SuppressWarnings("unchecked")
  public  Option map(ThrowableFunction1 function) {
    return (Option) this;
  }

  /**
   * Always returns this. 

   * I.e. the function is never used as {@link None} represents nothing/no value.
   *
   * @since 1.2
   * @since 1.0
   */
  @Override
  public  Option flatMap(ThrowableFunction1> function) {
    // uses the Map method as it anyways produces the same result
    return map(null);
  }

  /**
   * Always the value provided by the supplier.
   *
   * @return The value provided by the supplier
   * @since 1.0
   */
  public Option orElse(Supplier> supplier) {
    return supplier.get();
  }

  /**
   * Returns a {@link Right} containing the value from the provided supplier.
   *
   * @since 1.4
   * @since 1.0
   */
  @Override
  public  Either toLeft(Supplier right) {
    return Right(right.get());
  }

  /**
   * Returns a {@link Left} containing the value from the provided supplier.
   *
   * @since 1.4
   * @since 1.0
   */
  @Override
  public  Either toRight(Supplier left) {
    return Left(left.get());
  }

  /**
   * Returns true if other is a {@link None} as {@link None} is stateless comparing it
   * to some other None is therefore always the same, false otherwise.
   *
   * @param other The other object to compare to
   * @return true if other is {@link None}, false otherwise
   * @since 1.0
   */
  @Override
  public boolean equals(Object other) {
    return other instanceof None;
  }

  /**
   * Always returns 0 as None is stateless and has no value.
   *
   * @return 0
   * @since 1.0
   */
  @Override
  public int hashCode() {
    return 0;
  }

  /**
   * Returns a String representation of the instance.
   *
   * @since 1.0
   */
  @Override
  public String toString() {
    return "None";
  }
}