NoSuchMethodError when calling java code from scala

Question

When I execute following code in scala REPL console:

java.util.Collections.max(new java.util.ArrayList[String]())

NoSuchMethodError exception is thrown:

java.lang.NoSuchMethodError: java.util.Collections.max(Ljava/util/Collection;)Lj
ava/lang/Comparable;
        at .<init>(<console>:8)
        at .<clinit>(<console>)
        at .<init>(<console>:11)
        at .<clinit>(<console>)
        at $export(<console>)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.
java:39)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAcces
sorImpl.java:25)
        at java.lang.reflect.Method.invoke(Method.java:597)
        at scala.tools.nsc.interpreter.IMain$ReadEvalPrint.call(IMain.scala:592)

        at scala.tools.nsc.interpreter.IMain$Request$$anonfun$10.apply(IMain.sca
la:828)
        at scala.tools.nsc.interpreter.Line$$anonfun$1.apply$mcV$sp(Line.scala:4
3)
        at scala.tools.nsc.io.package$$anon$2.run(package.scala:31)
        at java.lang.Thread.run(Thread.java:662)

Scala 2.9.0.1, Java 1.6.0_25

Why an exception is thrown here? The same code executed from Java behaves as expected (throws NoSuchElementException).

notnoop · Accepted Answer · 2011-05-29 18:29:18Z

5

This is a compiler bug that is affecting both Scala 2.8 and 2.9, where the compiler does not compute the proper erasured method signature. I am not aware of a bug report.

Compiling the method:

object Test { def main(a: Array[String]) {
  val a = new java.util.ArrayList[String]()
  java.util.Collections.max(a)
}}

Results into the following bytecode:

public void main(java.lang.String[]);
  Code:
   Stack=2, Locals=3, Args_size=2
   0:   new #16; //class java/util/ArrayList
   3:   dup
   4:   invokespecial   #18; //Method java/util/ArrayList."<init>":()V
   7:   astore_2
   8:   aload_2
   9:   invokestatic    #24; //Method java/util/Collections.max:(Ljava/util/Collection;)Ljava/lang/Comparable;
   12:  pop
   13:  return

Please note that bytecode at offset 9 invokes a static method with a Comparable return type, while the actual Collections.max has Object as the return type:

$ javap -p java.util.Collections | grep max
    public static java.lang.Object max(java.util.Collection);

answered May 29, 2011 at 18:29

notnoop

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2 Comments

Daniel C. Sobral Over a year ago

Curious. Javadoc says <T extends Object & Comparable<? super T>> T.

notnoop Over a year ago

@Daniel, The JVM looks at the erasured type of the method; so T gets erasured to Object, but Scalac expects it to be erasured to Comparable. You can review my email in the scala-user mailing list for more details.

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