content: /*
   * Licensed to the Apache Software Foundation (ASF) under one or more
   * contributor license agreements.  See the NOTICE file distributed with
   * this work for additional information regarding copyright ownership.
   * The ASF licenses this file to You 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 org.apache.spark.api.java;
  
  import java.io.Serializable;
  
  import com.google.common.base.Preconditions;
  
  /**
   * <p>Like {@code java.util.Optional} in Java 8, {@code scala.Option} in Scala, and
   * {@code com.google.common.base.Optional} in Google Guava, this class represents a
   * value of a given type that may or may not exist. It is used in methods that wish
   * to optionally return a value, in preference to returning {@code null}.</p>
   *
   * <p>In fact, the class here is a reimplementation of the essential API of both
   * {@code java.util.Optional} and {@code com.google.common.base.Optional}. From
   * {@code java.util.Optional}, it implements:</p>
   *
   * <ul>
   *   <li>{@link #empty()}</li>
   *   <li>{@link #of(Object)}</li>
   *   <li>{@link #ofNullable(Object)}</li>
   *   <li>{@link #get()}</li>
   *   <li>{@link #orElse(Object)}</li>
   *   <li>{@link #isPresent()}</li>
   * </ul>
   *
   * <p>From {@code com.google.common.base.Optional} it implements:</p>
   *
   * <ul>
   *   <li>{@link #absent()}</li>
   *   <li>{@link #of(Object)}</li>
   *   <li>{@link #fromNullable(Object)}</li>
   *   <li>{@link #get()}</li>
   *   <li>{@link #or(Object)}</li>
   *   <li>{@link #orNull()}</li>
   *   <li>{@link #isPresent()}</li>
   * </ul>
   *
   * <p>{@code java.util.Optional} itself is not used at this time because the
   * project does not require Java 8. Using {@code com.google.common.base.Optional}
   * has in the past caused serious library version conflicts with Guava that can't
   * be resolved by shading. Hence this work-alike clone.</p>
   *
   * @param <T> type of value held inside
   */
  public final class Optional<T> implements Serializable {
  
    private static final Optional<?> EMPTY = new Optional<>();
  
    private final T value;
  
    private Optional() {
      this.value = null;
    }
  
    private Optional(T value) {
      Preconditions.checkNotNull(value);
      this.value = value;
    }
  
    // java.util.Optional API (subset)
  
    /**
     * @return an empty {@code Optional}
     */
    public static <T> Optional<T> empty() {
      @SuppressWarnings("unchecked")
      Optional<T> t = (Optional<T>) EMPTY;
      return t;
    }
  
    /**
     * @param value non-null value to wrap
     * @return {@code Optional} wrapping this value
     * @throws NullPointerException if value is null
     */
    public static <T> Optional<T> of(T value) {
      return new Optional<>(value);
    }
  
    /**
     * @param value value to wrap, which may be null
     * @return {@code Optional} wrapping this value, which may be empty
     */
    public static <T> Optional<T> ofNullable(T value) {
      if (value == null) {
        return empty();
      } else {
        return of(value);
      }
    }
  
    /**
     * @return the value wrapped by this {@code Optional}
     * @throws NullPointerException if this is empty (contains no value)
     */
    public T get() {
      Preconditions.checkNotNull(value);
      return value;
    }
  
    /**
     * @param other value to return if this is empty
     * @return this {@code Optional}'s value if present, or else the given value
     */
    public T orElse(T other) {
      return value != null ? value : other;
    }
  
    /**
     * @return true iff this {@code Optional} contains a value (non-empty)
     */
    public boolean isPresent() {
      return value != null;
    }
  
    // Guava API (subset)
    // of(), get() and isPresent() are identically present in the Guava API
  
    /**
     * @return an empty {@code Optional}
     */
    public static <T> Optional<T> absent() {
      return empty();
    }
  
    /**
     * @param value value to wrap, which may be null
     * @return {@code Optional} wrapping this value, which may be empty
     */
    public static <T> Optional<T> fromNullable(T value) {
      return ofNullable(value);
    }
  
    /**
     * @param other value to return if this is empty
     * @return this {@code Optional}'s value if present, or else the given value
     */
    public T or(T other) {
      return value != null ? value : other;
    }
  
    /**
     * @return this {@code Optional}'s value if present, or else null
     */
    public T orNull() {
      return value;
    }
  
    // Common methods
  
    @Override
    public boolean equals(Object obj) {
      if (!(obj instanceof Optional)) {
        return false;
      }
      Optional<?> other = (Optional<?>) obj;
      return value == null ? other.value == null : value.equals(other.value);
    }
  
    @Override
    public int hashCode() {
      return value == null ? 0 : value.hashCode();
    }
  
    @Override
    public String toString() {
      return value == null ? "Optional.empty" : String.format("Optional[%s]", value);
    }
  
  }
  