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Editorial Team
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Asked: 2026-05-25T01:53:15+00:00

I always thought that Predef.println was merely a shortcut for System.out.println , but apparently

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I always thought that Predef.println was merely a shortcut for System.out.println, but apparently I am mistaken, since it doesn’t seem to use System.out at all. Why is that so? And how can I do the “redirecting” of System.out below in Scala?

scala> val baos = new java.io.ByteArrayOutputStream
baos: java.io.ByteArrayOutputStream = 

scala> val ps = new java.io.PrintStream(baos)
ps: java.io.PrintStream = java.io.PrintStream@6c5ac4

scala> System.setOut(ps)

scala> println("hello")
hello

scala> new String(baos.toByteArray)
res2: java.lang.String = ""

scala> System.out.println("hello")

scala> new String(baos.toByteArray)
res7: java.lang.String = 
"hello
"
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1 Answer

  1. Editorial Team

    Predef.println is shortcut for Console.println and you can use Console.setOut or Console.withOut for redirecting.

    Also, Console.setOut only affects the current thread while System.setOut
    affects the whole JVM. Additionally Scala 2.9 repl evaluates each line in its own thread, so Console.setOut is not usable there.

    scala> val baos = new java.io.ByteArrayOutputStream
baos: java.io.ByteArrayOutputStream = 

scala> Console.withOut(baos)(print("hello"))

scala> println(baos)
hello
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