以下是Scala的TraversableOnce特性的reduceLeft方法的来源。读取var acc: B = 0.asInstanceOf[B]的行发生了什么？
在我看来，如果我在字符串列表上调用它，例如List("a", "b", "c")，这将导致出现0.asInstanceOf[String]。但是，如果我直接尝试0.asInstanceOf[String]，则会在运行时抛出ClassCastException。
该行发生了什么，为什么与在字符串列表中调用时直接调用0.asInstanceOf[String]有何不同？
def reduceLeft[B >: A](op: (B, A) => B): B = {
if (isEmpty)
throw new UnsupportedOperationException("empty.reduceLeft")
var first = true
var acc: B = 0.asInstanceOf[B]
for (x <- self) {
if (first) {
acc = x
first = false
}
else acc = op(acc, x)
}
acc
}
额外的问题:为什么
acc甚至被初始化为该值？看来for循环的第一次迭代中的代码只会用TraversableOnce对象中的第一个元素覆盖该值。 最佳答案
好吧，这在运行时不会因ClassCastException而失败的原因是:
Object(或AnyRef)，因此类型转换实际上是0.asInstanceOf[Object] 您可以通过检查字节码来测试第一种可能性，但这似乎毫无意义。如果不删除它，那么将有不必要的开销，即在每次调用该方法时都将
Int装箱(尽管不是创建对象-参见下文)!找出编译器产生的结果: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
所以现在:
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$1
const #24 = Asciz java/lang/Object;
const #25 = class #24; // java/lang/Object
const #26 = Asciz scala/ScalaObject;
const #27 = class #26; // scala/ScalaObject
{
public abstract java.lang.Object foreach(scala.Function1);
Signature: length = 0x2
00 05
public abstract boolean isEmpty();
public abstract java.lang.Object reduceLeft(scala.Function2);
Signature: length = 0x2
00 0B
}
做到这一点，你会!
找出编译器产生的结果:2
另一种可能性是将其放在源文件和compile it, printing out the intermediate code phase中:
./scalac -Xprint:icode X.scala
然后你得到...
def reduceLeft($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$1): Function1)
});
acc.elem
};
看起来好像里面有不必要的拳击!一点是，这不会涉及对象的创建，而只是查找，因为-127到127的框值被缓存。
您可以通过将上述打印命令中的编译器阶段更改为“擦除”来检查已擦除的行。嘿，presto:
var acc: java.lang.Object = scala.Int.box(0).$asInstanceOf[java.lang.Object]();