Question

I am trying to solve a problem given by the Book “Scala for the Impatient”, which asked to implement java’s BufferedInputStream as a trait. Here is my implementation,

trait Buffering {           
    this:InputStream =>
        private[this] val bis = {
            new JavaBufferedInputStream(this)
        }
        override def read = bis.read
        override def read(byte:Array[Byte], off:Int, len:Int) = bis.read(byte, off, len)
        override def available = bis.available
        override def close() {
            bis.close
        }
        override def skip(n:Long) = bis.skip(n)
}

def main(args:Array[String]) {
    val bfis = new FileInputStream(new File("foo.txt")) with Buffering
    println(bfis.read)
    bfis.close
}

But this give me a java stackoverflow error, so what’s wrong with it? Thanks!

Answer 1

It looks like you are getting a stack overflow where you don’t expect one. The key to troubleshoot these is to look at the repeating cycle of the stack trace. It usually points to what is repeatedly allocating frames. Here it will show something like that:

at C.Buffering$class.read(C.scala:12)
at C.C$$anon$1.read(C.scala:23)
at java.io.BufferedInputStream.read1(BufferedInputStream.java:256)
at java.io.BufferedInputStream.read(BufferedInputStream.java:317)
at C.Buffering$class.read(C.scala:12)
at C.C$$anon$1.read(C.scala:23)
at java.io.BufferedInputStream.read1(BufferedInputStream.java:256)
at java.io.BufferedInputStream.read(BufferedInputStream.java:317)
at C.Buffering$class.read(C.scala:12)

So reading from bottom to top, it looks like your read(byte, ...) is calling bis.read(byte, ...) which is calling BufferedInputStream.read which is then calling your read(byte, ...) again.

It would appear that new BufferedInputStream(this) is calling read on the underlying InputStream but since the underlying this is your object that then delegates calls on bis we have infinite recursion.

I’m guessing that the author wants you to use the abstract override stackable modifications pattern where you can use super to refer to the right read method.