Source code

001/*
002 * Licensed to the Apache Software Foundation (ASF) under one
003 * or more contributor license agreements.  See the NOTICE file
004 * distributed with this work for additional information
005 * regarding copyright ownership.  The ASF licenses this file
006 * to you under the Apache License, Version 2.0 (the
007 * "License"); you may not use this file except in compliance
008 * with the License.  You may obtain a copy of the License at
009 *
010 *   http://www.apache.org/licenses/LICENSE-2.0
011 *
012 * Unless required by applicable law or agreed to in writing,
013 * software distributed under the License is distributed on an
014 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
015 * KIND, either express or implied.  See the License for the
016 * specific language governing permissions and limitations
017 * under the License.
018 */
019package org.apache.reef.util;
020
021import net.jcip.annotations.Immutable;
022import net.jcip.annotations.ThreadSafe;
023
024import java.io.Serializable;
025
026/**
027 * Represents an optional value. Loosely based on
028 * <a href="http://download.java.net/jdk8/docs/api/java/util/Optional.html">The Java 8 version</a>, but filtered for
029 * Java 7 compatibility.
030 */
031@Immutable
032@ThreadSafe
033public final class Optional<T> implements Serializable {
034
035  private static final long serialVersionUID = 42L;
036
037  private final T value;
038  private static final Optional<?> EMPTY = new Optional<>();
039  private static final String EMPTY_VALUE_STR = "Optional.empty";
040
041  private Optional(final T value) {
042    this.value = value;
043  }
044
045  private Optional() {
046    this.value = null;
047  }
048
049  /**
050   * @return An Optional with the given value.
051   * @throws IllegalArgumentException if the value is null
052   */
053  public static <T> Optional<T> of(final T value) throws IllegalArgumentException {
054    if (null == value) {
055      throw new IllegalArgumentException("Passed a null value. Use ofNullable() instead");
056    }
057    return new Optional<>(value);
058  }
059
060  /**
061   * @return an Optional with no value.
062   */
063  public static <T> Optional<T> empty() {
064    return (Optional<T>) EMPTY;
065  }
066
067  /**
068   * @return An optional representing the given value, or an empty Optional.
069   */
070  public static <T> Optional<T> ofNullable(final T value) {
071    if (null == value) {
072      return Optional.empty();
073    } else {
074      return Optional.of(value);
075    }
076  }
077
078  /**
079   * @return the value represented or null, if isPresent() is false.
080   */
081  public T get() {
082    return this.value;
083  }
084
085  /**
086   * @param other
087   * @return the value of this Optional or other, if no value exists.
088   */
089  public T orElse(final T other) {
090    if (isPresent()) {
091      return this.get();
092    } else {
093      return other;
094    }
095  }
096
097  /**
098   * @return true if there is a value, false otherwise.
099   */
100  public boolean isPresent() {
101    return null != this.value;
102  }
103
104  @Override
105  public boolean equals(final Object obj) {
106
107    if (this == obj) {
108      return true;
109    }
110
111    if (obj == null || getClass() != obj.getClass()) {
112      return false;
113    }
114
115    final Optional that = (Optional) obj;
116    return this.value == that.value || this.value != null && this.value.equals(that.value);
117  }
118
119  @Override
120  public int hashCode() {
121    return this.value == null ? 0 : this.value.hashCode();
122  }
123
124  @Override
125  public String toString() {
126    return this.value == null ? EMPTY_VALUE_STR : "Optional:{" + this.value + "}";
127  }
128}