AbstractMethodError occurred when package private method override

Question

I noticed that AbstractMethodError occurred when execute the following codes. ``` package javapkg; public abstract class JavaClass{ abstract String foo(int b); public String bar(int b){ return foo(b); } } ``` ``` package scalapkg import javapkg._ class ScalaClass extends JavaClass{ def foo(a:Int) = a.toString } object Main extends App{ println(new ScalaClass().bar(10)) } ``` ``` [error] (run-main) java.lang.AbstractMethodError: javapkg.JavaClass.foo(I)Ljava/lang/String; java.lang.AbstractMethodError: javapkg.JavaClass.foo(I)Ljava/lang/String; at javapkg.JavaClass.bar(JavaClass.java:7) at scalapkg.Main$delayedInit$body.apply(ScalaClass.scala:10) at scala.Function0$class.apply$mcV$sp(Function0.scala:34) at scala.runtime.AbstractFunction0.apply$mcV$sp(AbstractFunction0.scala:12) at scala.App$$anonfun$main$1.apply(App.scala:60) at scala.App$$anonfun$main$1.apply(App.scala:60) at scala.collection.LinearSeqOptimized$class.foreach(LinearSeqOptimized.scala:59) at scala.collection.immutable.Li…

Accepted Answer

It's a bit complicated. It's a bug--the Scala compiler should emit an error--but it's a somewhat subtle one.

The problem is that since you have left `public` off of the abstract method, its visibility is package-private. If you write this in Java:

```
package javapkgimpl;

public class JavaClassImpl extends javapkg.JavaClass {
  String foo(int b) { return (new java.lang.Integer(b)).toString(); }
  public static void main(String[] args) {
    System.out.println(new JavaClassImpl().bar(10));
  }
}
```

then the Java compiler will complain:

```
JavaClassImpl.java:3: javapkgimpl.JavaClassImpl is not abstract and does not
  override abstract method foo(int) in javapkg.JavaClass
public class JavaClassImpl extends javapkg.JavaClass {
       ^
1 error
```

It clearly is trying to override, but it doesn't actually work since it's not in the right package and therefore can't actually access (or override) the original `foo`. If we change the package to `javapkg`, then everything works. If we try this Scala instead:

```
package javapkg

class ScalaClass extends JavaClass{
  def foo(a:Int) = a.toString
}

object ScalaClass extends App{
  println(new ScalaClass().bar(10))
}
```

then Scala works fine also.

It is the fault of Scala's compiler that it doesn't notice this mistake--it should emit the same error as the Java compiler (or a more helpful one that actually tells you that the access is screwy!) rather than emitting bytecode that cannot work.

But it's easy to fix: just change the package on the Scala code (or the access modifier in the Java code).

AbstractMethodError occurred when package private method override

Solution

Problem