Question

What is the “proper” of writing the standard read-while loop in Scala? By proper I mean written in a Scala-like way as opposed to a Java-like way.

Here is the code I have in Java:

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();

Here is the code I have in Scala:

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

The Scala code is correct and works, but feels very un-Scala-ish. For one it is a literal translation of the Java code, taking advantage of none of the advantages of Scala. Further it is actually longer than the Java code! I really feel like I’m missing something, but I can’t figure out what.

I’m fairly new to Scala, and so I’m asking the question to avoid falling into the pitfall of writing Java-style code in Scala. I’m more interested in the Scala way to solve this kind of problem than in any specific helper method that might be provided by the Scala API to hash a file.

(I apologize in advance for my ad hoc Scala adjectives throughout this question.)

Answer 1

Based on Rex’s post that he mentioned:

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

You should replace the var readLen + while {…} lines with it, it produces the same result.

As Rex mentioned, it works with scala 2.8.