在Scala reduceLeft实现中的0.asInstanceOf [B]发生了什么

>编译器删除它从未使用的初始化推理(我怀疑这一点)
> B是erased到Object(或AnyRef)，因此转换真的是0.asInstanceOf [Object]

您可以通过检查字节码来测试第一种可能性，但它似乎是一个无意义的代码段。如果没有消除这种情况，那么每次调用该方法时都会有一个不必要的开销来控制Int(尽管不是一个对象创建 – 见下文)！

了解编译器生成的内容：1

Welcome to Scala version 2.9.1.final (Java HotSpot(TM) 64-Bit Server VM,Java 1.6.0_29).
Type in expressions to have them evaluated.
Type :help for more information.

scala> trait X[A] {
 | def foreach[U](f: A => U): U
 | def isEmpty = {
 | var b = false
 | foreach(_ => b = true)
 | !b
 | }
 | def reduceLeft[B >: A](op: (B,A) => B): B = {
 | if (isEmpty) sys.error("Bad")
 | var first = true
 | var acc: B = 0.asInstanceOf[B]
 | for (x <- this) {
 | if (first) { acc = x; first = false } else acc = op(acc,x)
 | }
 | acc
 | }
 | }
defined trait X

那么现在

scala> :javap -v X
Compiled from "<console>"
public interface X extends scala.ScalaObject
  SourceFile: "<console>"
  Scala: length = 0x

  Signature: length = 0x2
   00 0D 
  InnerClass: 
   public abstract #19= #16 of #18; //X=class X of class 
   public final #21; //class X$$anonfun$isEmpty$1
   public final #23; //class X$$anonfun$reduceLeft$1
  minor version: 0
  major version: 49
  Constant pool:
const #1 = Asciz    SourceFile;
const #2 = Asciz    <console>;
const #3 = Asciz    foreach;
const #4 = Asciz    (Lscala/Function1;)Ljava/lang/Object;;
const #5 = Asciz    <U:Ljava/lang/Object;>(Lscala/Function1<TA;TU;>;)TU;;
const #6 = Asciz    Signature;
const #7 = Asciz    isEmpty;
const #8 = Asciz    ()Z;
const #9 = Asciz    reduceLeft;
const #10 = Asciz   (Lscala/Function2;)Ljava/lang/Object;;
const #11 = Asciz   <B:Ljava/lang/Object;>(Lscala/Function2<TB;TA;TB;>;)TB;;
const #12 = Asciz   Scala;
const #13 = Asciz   <A:Ljava/lang/Object;>Ljava/lang/Object;Lscala/ScalaObject;;
const #14 = Asciz   InnerClasses;
const #15 = Asciz   X;
const #16 = class   #15;    //  X
const #17 = Asciz   ;
const #18 = class   #17;    //  
const #19 = Asciz   X;
const #20 = Asciz   X$$anonfun$isEmpty$1;
const #21 = class   #20;    //  X$$anonfun$isEmpty$1
const #22 = Asciz   X$$anonfun$reduceLeft$1;
const #23 = class   #22;    //  X$$anonfun$reduceLeft$1
const #24 = Asciz   java/lang/Object;
const #25 = class   #24;    //  java/lang/Object
const #26 = Asciz   scala/ScalaObject;
const #27 = class   #26;    //  scala/ScalaObject

{
public abstract java.lang.Object foreach(scala.Function1);
  Signature: length = 0x2
   00 05 

public abstract boolean isEmpty();

public abstract java.lang.Object reduceLeft(scala.Function2);
  Signature: length = 0x2
   00 0B 

}

做那个你会的！

了解编译器生成的内容：2

另一种可能性是把它放在一个源文件和compile it,printing out the intermediate code phase：

./scalac -Xprint:icode X.scala

那你得到…

def reduceLeft($this: X,op$1: Function2): java.lang.Object = {
  if ($this.isEmpty())
    scala.sys.`package`.error("Bad")
  else
    ();
  var first$1: scala.runtime.BooleanRef = new scala.runtime.BooleanRef(true);
  var acc$1: scala.runtime.ObjectRef = new scala.runtime.ObjectRef(scala.Int.box(0));
  $this.foreach({
    (new anonymous class X$$anonfun$reduceLeft$1($this,op$1,first$1,acc$1): Function1)
  });
  acc.elem
};

看起来很像是不必要的拳击在那里！一点是，这不会涉及对象创建，只是查找，因为-127到127的boxed值被缓存。

您可以通过将上述打印命令中的编译器阶段更改为“擦除”来检查已擦除的行。嘿presto：

var acc: java.lang.Object = scala.Int.box(0).$asInstanceOf[java.lang.Object]();