Erratic AbstractMethodError when calling equals() on interfaces

Question

I am getting java.lang.AbstractMethodError or NPEs when calling methods that were defined in java.lang.Object on objects declared as interfaces. The following code executes normally, but sometimes (erratically) I am getting these errors. The code is part of a complex application; I have not been able to reproduce the errors in a small example.
The following code snipped shows where the error occurrs. Important to know is that the variable 'settings' is declared as private final IInterfaceSettings;, where IInterfaceSettings is declared as Java interface.
    public int hashCode(){
        int result = 83;
        result+=(this.settings==null)?0:this.settings.hashCode();  // error occurs here
        return result;
    }An explicit cast to Object seems to solve the problem; it never occurred again until now.
    public int hashCode(){
        int result = 83;
        result+=(this.settings==null)?0:((Object)this.settings).hashCode();
        return result;
    }Daniel

Answer 1

Define hashCode() in the interface itself. You will probably end up with something like the following:
interface IInterfaceSettings {
abstract public int hashCode();
// other methods follow
class InterfaceSettingsImpl implements IInterfaceSettings {
final public int hashCode() {
return super.hashCode(); // should resolve to Object
Also, if you are going to implement hashCode, you might as well implement equals() so collections behavior is as exepcted.
- Saish
"My karma ran over your dogma." - Anon