add JavaPrimitives #6

src/main/scala/com/spotify/bdrc/scala/JavaPrimitives.scala

@@ -0,0 +1,60 @@
+/*
+ * Copyright 2016 Spotify AB.
+ *
+ * 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 com.spotify.bdrc.scala
+
+/**
+ * Examples for working with Java primitives.
+ */
+object JavaPrimitives {
+
+  // java.lang.Double is a boxed type (object) while double in Java is a primitive type.
+  // scala.Double can be either boxed or primitive depending on the context, e.g. it's boxed when
+  // used as a type parameter in a generic class but primitive when used in an array or on the
+  // stack.
+  // Due to type system limitations, M[java.lang.Double] and M[scala.Double] are incompatible types
+  // but they can be casted safely back and forth since both are implemented as Java boxed types.
+  import java.lang.{Double => JDouble}
+  import java.util.{List => JList}
+
+  import scala.collection.JavaConverters._
+
+  /**
+   * `xs.asScala` returns `mutable.Buffer[JDouble]` where `Buffer` is a sub-type of `Seq` but
+   * `JDouble` is not the same type as `Double` (`scala.Double`). Casting is safe because `JDouble`
+   * and `Double` are equivalent when used as type parameters (boxed objects). It's also cheaper
+   * than `.map(_.toDouble)` which creates a copy of the `Buffer`.
+   */
+  def jDoubleListToSeq(xs: JList[JDouble]): Seq[Double] = xs.asScala.asInstanceOf[Seq[Double]]
+
+  /**
+   * Array[Double] is more efficient since it's implemented as a Java primitive array. Arrays are
+   * also mutable so it'scheaper to pre-allocate and mutate elements. Java iterator and while loop
+   * are faster than `xs.asScala.asInstanceOf[Seq[Double]].toArray`.
+   */
+  def jDoubleListToArray(xs: JList[JDouble]): Array[Double] = {
+    val a = new Array[Double](xs.size())
+    var i = 0
+    val iterator = xs.iterator()
+    while (iterator.hasNext) {
+      a(i) = iterator.next()
+      i += 1
+    }
+    a
+  }
+
+}