Following a suggestion by extempore recently about how to get scala to tell me whether there was boxing going on by looking at the bytecode, I created this class:
class X { def foo(ls : Array[Long]) = ls map (_.toDouble)
Had a look at the bytecode for foo:
public double[] foo(long[]);
Code:
Stack=4, Locals=2, Args_size=2
0: getstatic #11; //Field scala/Predef$.MODULE$:Lscala/Predef$;
3: aload_1
4: invokevirtual #16; //Method scala/Predef$.longArrayOps:([J)Lscala/collection/mutable/ArrayOps;
7: new #18; //class X$$anonfun$foo$1
10: dup
11: aload_0
12: invokespecial #22; //Method X$$anonfun$foo$1."<init>":(LX;)V
15: getstatic #27; //Field scala/Array$.MODULE$:Lscala/Array$;
18: getstatic #32; //Field scala/reflect/Manifest$.MODULE$:Lscala/reflect/Manifest$;
21: invokevirtual #36; //Method scala/reflect/Manifest$.Double:()Lscala/reflect/AnyValManifest;
24: invokevirtual #40; //Method scala/Array$.canBuildFrom:(Lscala/reflect/ClassManifest;)Lscala/collection/generic/CanBuildFrom;
27: invokeinterface #46, 3; //InterfaceMethod scala/collection/TraversableLike.map:(Lscala/Function1;Lscala/collection/generic/CanBuildFrom;)Ljava/lan g/Object;
32: checkcast #48; //class "[D"
35: areturn
LineNumberTable:
line 7: 0
No signs of box/unbox there. But I'm still suspicious, so I compiled it with -print ():
[[syntax trees at end of cleanup]]// Scala source: X.scala
package <empty> {
class X extends java.lang.Object with ScalaObject {
def foo(ls: Array[Long]): Array[Double] = scala.this.Predef.longArrayOps(ls).map({
(new anonymous class X$$anonfun$foo$1(X.this): Function1)
}, scala.this.Array.canBuildFrom(reflect.this.Manifest.Double())).$asInstanceOf[Array[Double]]();
def this(): X = {
X.super.this();
()
}
};
@SerialVersionUID(0) final <synthetic> class X$$anonfun$foo$1 extends scala.runtime.AbstractFunction1$mcDJ$sp with Serializable {
final def apply(x$1: Long): Double = X$$anonfun$foo$1.this.apply$mcDJ$sp(x$1);
<specialized> def apply$mcDJ$sp(v1: Long): Double = v1.toDouble();
final <bridge> def apply(v1: java.lang.Object): java.lang.Object = scala.Double.box(X$$anonfun$foo$1.this.apply(scala.Long.unbox(v1)));
def this($outer: X): anonymous class X$$anonfun$foo$1 = {
X$$anonfun$foo$1.super.this();
()
}
}
}
The main observations about this code are that the created anonymous function has been specialized for Long => Double and that the map functionality is provided by longArrayOps(ls).map (ArrayOps is not specialized).
The question is: "is boxing/unboxing occurring in this example?"