[returning error message on abort]how to interrupt normal process in a method

I have a method that return an array, something like this :

public String[] getAllClassName(Indi indi) {
        /* check for existing classes */
        /* not very efficient as we do not have a class names list */
        ArrayList<String>classnamelist=new ArrayList<String>();
        String classname;
        Boolean errorflag=false;
        Property classnameproperties[];
        Indi indi_;
        Gedcom gedcom=indi.getGedcom();
        Collator myCollator =gedcom.getCollator();
        Collection indis=gedcom.getEntities(Gedcom.INDI);
        for (Iterator it=indis.iterator(); it.hasNext();) {
                /* we get all _CLAS properties from all INDI's */
                indi_=(Indi)it.next();
                /* we get all _CLAS property values from all INDI's */
                classnameproperties=indi_.getProperties(ClassNameTag);
                /* if no _CLAS tags we do nothing */
                if (classnameproperties.length != 0) {
                        /* there are _CLAS tags */
                        /* we want to collect the _CLAS tag value */
                        for (int i=0; i<classnameproperties.length; i++) {
                                classname=classnameproperties[i].getValue();
                                if (classname.equals("")) {
                                        /* here we want to abort */
                                        println("ERROR: empty _CLAS tag found for :"+indi);
                                        break; /* is this good */
                                        }
                                else {
                                        if (!classnamelist.contains(classname)) {
                                                /* classname not included yet in classnamelist */
                                                /* we add it */
                                                classnamelist.add(classname);
                                        }
                                }
                        }
                        if (classnameproperties.length == 2) {
                                /* here we want to abort */
                                println("ERROR: more than one _CLAS tag found for :"+indi);
                                break;
                        }
                }
        }
        /* we sort classnamenamelist alphabetically */
        Collections.sort(classnamelist,myCollator);
        return classnamelist.toArray(new String[classnamelist.size()]);
}

So when everything goes well I normally get an array of string values but in
some cases I want to abort the method and return an error message ; as a
method can just return an object (?) how to proceed neatly for the caller
of the method to get either :
- the array he wants,
- or possibly a notification of an error when aborting.

The error message can either :
- (1) be processed inside the method then in this case we just need to
terminate properly the called method but returning what ? : a dummy array
(null ?) for notification,
- -(2) or outside of the method(s) by getting an error number generated by
the method that avorts ; in this case the main advantage is that you have a
general method that can process error messages but we still have to collect
this error number from the method.

Of course I can have a static "int ErrorNumber =0" set up front and each
method can set it with its own error number but is this the best way to
proceed ?

With Java I am a little puzzled as the way to think is different.

Thanks.