Oakc allows a concrete method to be overridden in a subclass
by an abstract. The Spec specifically prohibits this in section 4.10.5:
Classes/Modifiers/Abstract Methods.
Heres an example.

class C {
    void foo() { System.out.println("C at "+this.toString()); }
}
class D extends C {
    abstract void foo();
}

class bug256 extends D{

    void foo(){ System.out.println("bug256 is "+this.toString() ); }

    public static void main( String x[] )
    {
C c = new bug256();

c.foo();

    }
}