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Main class with companion cannot be run #4861

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scabug opened this issue Aug 1, 2011 · 5 comments
Closed

Main class with companion cannot be run #4861

scabug opened this issue Aug 1, 2011 · 5 comments
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@scabug
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@scabug scabug commented Aug 1, 2011

==========================================================================================
This is a bug, affected versions (at least) 2.9.0.1, 2.9.1.RC1 and 2.10 (a recent nightly)

This code compiles:

object Foo {
def main(args: Array[String]): Unit = {
println("Foo")
}
}
trait Foo

But it won't run:

java.lang.NoSuchMethodException: Foo.main([Ljava.lang.String;)
at java.lang.Class.getMethod(Class.java:1605)
at scala.tools.nsc.util.ScalaClassLoader$class.run(ScalaClassLoader.scala:74)
at scala.tools.nsc.util.ScalaClassLoader$URLClassLoader.run(ScalaClassLoader.scala:101)
at scala.tools.nsc.ObjectRunner$.run(ObjectRunner.scala:33)
at scala.tools.nsc.ObjectRunner$.runAndCatch(ObjectRunner.scala:40)
at scala.tools.nsc.MainGenericRunner.runTarget$1(MainGenericRunner.scala:56)
at scala.tools.nsc.MainGenericRunner.process(MainGenericRunner.scala:80)
at scala.tools.nsc.MainGenericRunner$.main(MainGenericRunner.scala:89)
at scala.tools.nsc.MainGenericRunner.main(MainGenericRunner.scala)

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@scabug scabug commented Aug 1, 2011

Imported From: https://issues.scala-lang.org/browse/SI-4861?orig=1
Reporter: @hseeberger
Affected Versions: 2.9.0, 2.9.1

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@scabug scabug commented Aug 1, 2011

@SethTisue said:
Should work if you change "trait Foo" to "class Foo".

From Java, a Scala trait is an interface, not a class, and interfaces can't have static methods such as "main". So this is unfixable.

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@scabug scabug commented Aug 1, 2011

@paulp said:
Thank you for saving me the trouble of figuring out/remembering that -- I'm reopening because I can recognize the attempt and issue a warning.

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@scabug scabug commented Aug 1, 2011

@hseeberger said:
Seth: Thanks for the explanation. Now that I know it it's obvious ;-)
Paul: A warning would be great.

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@scabug scabug commented Aug 1, 2011

Commit Message Bot (anonymous) said:
(extempore in r25427) Working on jar creation infrastructure. Now if -d foo.jar is used for
output generation (but only then, since otherwise we're not creating the jar):

  1. -Xmain-class foo.Bar will give the jar a Main-Class of foo.Bar
  2. Alternatively, if there is only one runnable program, that will be the Main-Class
  3. Always, the jar's manifest will have an entry for Scala-Compiler-Version.

Not very relatedly, a warning is now issued when a module has a main method but
a runnable program will not be generated. Closes #4861.

This represents an opening step toward automatically recognizing mismatched
bytecode situations: coarse, but useful and safe. Review by mirco.

@scabug scabug closed this Aug 1, 2011
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