How can I read a file to an InputStream then write it into an OutputStream in Scala?

Question

I'm trying to use basic Java code in Scala to read from a file and write to an OutputStream, but when I use the usual while( != -1 ) in Scala gives me a warning "comparing types of Unit and Int with != will always yield true".

The code is as follows:

    val file = this.cache.get(imageFileEntry).getValue().asInstanceOf[File]
    response.setContentType( "image/%s".format( imageDescription.getFormat() ) )

    val input = new BufferedInputStream( new FileInputStream( file ) )
    val output = response.getOutputStream()

    var read : Int = -1

    while ( ( read = input.read ) != -1 ) {
        output.write( read )
    }

    input.close()
    output.flush()

How am I supposed to write from an input stream to an output stream in Scala?

I'm mostly interested in a Scala-like solution.

Solution 1

You could do this:

Iterator 
.continually (input.read)
.takeWhile (-1 !=)
.foreach (output.write)

Solution 2

If this is slow:

Iterator 
.continually (input.read)
.takeWhile (-1 !=)
.foreach (output.write)

you can expand it:

val bytes = new Array[Byte](1024) //1024 bytes - Buffer size
Iterator
.continually (input.read(bytes))
.takeWhile (-1 !=)
.foreach (read=>output.write(bytes,0,read))
output.close()

Solution 3

Assignment statements always return Unit in Scala, so read = input.read returns Unit, which never equals -1. You can do it like this:

while ({read = input.read; read != -1}) {
  output.write(read)
}

Solution 4

def stream(inputStream: InputStream, outputStream: OutputStream) =
{
  val buffer = new Array[Byte](16384)

  def doStream(total: Int = 0): Int = {
    val n = inputStream.read(buffer)
    if (n == -1)
      total
    else {
      outputStream.write(buffer, 0, n)
      doStream(total + n)
    }
  }

  doStream()
}

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Comments

[Maurício Linhares]

You can't have more than one statement in a while/if/for clause. This code yields a compilation error. But thanks for the assignment thing, i thought it would behave like java.

[AndreasScheinert]

Hi Mauricio! Yes the {} are the key point in his example. Let's call them a block. Everything inside will be executed and the result is eventually returned having the type of the last expression.

[Maurício Linhares]

Ok, this looks like a real Scala solution, but is this continually call load the file into memory or is it going to call this function as the foreach loop runs? Also, can you elaborate a bit on this takeWhile method? Why didn't you have to use the _ parameter or define a parameter yourself?

[Kim Stebel]

sorry, added the braces a little late;)

[Daniel C. Sobral]

@Maurício It's an iterator, so everything is done only on-demand. Until foreach, nothing really happens -- you just get new Iterator objects that do some pre-processing before next or hasNext. On foreach, an output.write is executed for each input.read, and then its value is promptly forgotten and garbage collected.

[gerferra]

It would be nice to have a version using the scala-io incubator project

[Christopher]

This is a better solution. The above solutions are horribly slow. One call per byte read of overhead? Really? How is that acceptable in a bulk data mover?

[lyomi]

It should be noted that for the maximum performance, both input and output stream should have been buffered (using Buffered{Input,Output}Stream). It will still be significantly slower than the non-Scala-like way

[foxtrotuniform6969]

This is assuming that you're ok just reading 1024 bytes though. What if I don't know how much I need to read until I reach something like a delineator?