在Scala中编写read-while循环的正确方法是什么？

Question

在Scala中编写标准read-while循环的"正确"是什么？正确地说,我的意思是用类似Scala的方式编写而不是类似Java的方式.

这是我在Java中的代码:

MessageDigest md = MessageDigest.getInstance( "MD5" );
InputStream input = new FileInputStream( "file" );
byte[] buffer = new byte[1024];
int readLen;
while( ( readLen = input.read( buffer ) ) != -1 )
    md.update( buffer, 0, readLen );
return md.digest();

这是我在Scala中的代码:

val md = MessageDigest.getInstance( hashInfo.algorithm )
val input = new FileInputStream( "file" )
val buffer = new Array[ Byte ]( 1024 )
var readLen = 0
while( readLen != -1 )
{
    readLen = input.read( buffer )
    if( readLen != -1 )
        md.update( buffer, 0, readLen )
}
md.digest

Scala代码是正确的并且有效,但感觉非常不可靠.首先,它是Java代码的字面翻译,充分利用了Scala的优势.而且它实际上比Java代码长!我真的觉得我错过了什么,但我无法弄清楚是什么.

我对Scala很新,所以我要问这个问题是为了避免陷入在Scala中编写Java风格代码的陷阱.我对Scala解决此类问题的方式比对Scala API可能提供的任何特定帮助方法更感兴趣,以便对文件进行哈希处理.

(我在此问题中提前为我的临时Scala形容词道歉.)

Answer 1

根据雷克斯的帖子,他提到:

Stream.continually(input.read(buffer)).takeWhile(_ != -1).foreach(md.update(buffer, 0, _))

你应该用它替换var readLen + {...}行,它会产生相同的结果.

正如Rex所说,它适用于scala 2.8.

Answer 2

What Rex Kerr suggests in his comment is the following:

val md = MessageDigest.getInstance("MD5")
val input = new FileInputStream("foo.txt")
val buffer = new Array[ Byte ]( 1024 )
Stream.continually(input.read(buffer))
  .takeWhile(_ != -1)
  .foreach(md.update(buffer, 0, _))
md.digest

The key is the Stream.continually. It gets an expression which is evaluated continually, creating an infinite Stream of the evaluated expression. The takeWhile is the translation from the while-condition. The foreach is the body of the while-loop.