org.apache.spark.api.java.Optional.java Source code

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Here is the source code for org.apache.spark.api.java.Optional.java

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/*
 * 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 java.util.Objects;

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 was not used because at the time, the
 * project did 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 Objects.equals(value, 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);
    }

}