Re: Strange runtime error: AbstractMethodError

From:

Lew <lew@nospam.lewscanon.com>

Newsgroups:

comp.lang.java.programmer

Date:

Wed, 07 Feb 2007 17:52:31 -0500

Message-ID:

<DOOdnb4wAK-txVfYnZ2dnUVZ_qmpnZ2d@comcast.com>

Oliver Wong wrote:

After a bit for trimming, I've formed an SSCCE that doesn't require any
external libraries:

<SSCCE>
interface Root {
public Root someMethod();
}

interface Intermediary extends Root {
public Leaf someMethod();
}

class Leaf implements Intermediary {
@Override
public Leaf someMethod() {
  return null;
}
}

public class BugTest {
public static void main(String[] args) {
  Leaf leafReference = new Leaf();
  leafReference.someMethod();
  Root rootReference = leafReference;
  rootReference.someMethod(); /* throws error */
}
}
</SSCCE>

<output>
Exception in thread "main" java.lang.AbstractMethodError:
Leaf.someMethod()LRoot;
at BugTest.main(BugTest.java:21)
</output>

But I'm still unsure if this is a bug in the JVM, the compiler, or if this
is the correct behaviour of the Java language, using some rule I'm not
familiar with.

I think we're dealing with
<http://java.sun.com/docs/books/jls/third_edition/html/classes.html#8.4.8.4>

When I try your example I immediately but not consistently get an error from
Netbeans on the @Override,

   "method does not override a method from its superclass"

but javac doesn't show me this, not even with -Xlint:overrides.

Now, try adding these two classes to the source:

interface Othif extends Root
{
     public String someMethod(); // String is not a subtype of Root
}

class OthImpl implements Othif
{
     @Override
     public String someMethod()
     {
         return null;
     }
}

These give the expected compilation errors:

src/testit/BugTest.java:30: someMethod() in testit.Othif clashes with
someMethod() in testit.Root; attempting to use incompatible return type
found : java.lang.String
required: testit.Root
     public String someMethod();

src/testit/BugTest.java:33: testit.OthImpl is not abstract and does not
override abstract method someMethod() in testit.Root
class OthImpl implements Othif

src/testit/BugTest.java:36: someMethod() in testit.OthImpl cannot implement
someMethod() in testit.Root; attempting to use incompatible return type
found : java.lang.String
required: testit.Root
     public String someMethod()

- Lew