When I execute following code in scala REPL console:
java.util.Collections.max(new java.util.ArrayList开发者_如何学JAVA[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
).
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);
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