[SPARK-4819] Remove Guava's "Optional" from public API

Replace Guava `Optional` with (an API clone of) Java 8 `java.util.Optional` (edit: and a clone of Guava `Optional`)

See also https://github.com/apache/spark/pull/10512

Author: Sean Owen <sowen@cloudera.com>

Closes #10513 from srowen/SPARK-4819.

1 files changed, 187 insertions, 0 deletions

diff --git a/core/src/main/java/org/apache/spark/api/java/Optional.java b/core/src/main/java/org/apache/spark/api/java/Optional.java
new file mode 100644
index 0000000000..ca7babc3f0
--- /dev/null
+++ b/core/src/main/java/org/apache/spark/api/java/Optional.java
@@ -0,0 +1,187 @@
+/*
+ * 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);
+  }
+
+}