Design of class hierarchy for a object formatter API forces derivatives to know too much
This one is screaming for an enum. What you want is what you get:

public enum FormatType {
    XML, JSON, //maybe further support later
}


And your constructor then becomes:

public Formatter (Object object, FormatType formatType) {
    switch (formatType) {
        case XML:
            output = getXMLFormat(object);
            break;
        case JSON:
            output = getJSONFormat(object);
            break;
    }
}


In addition to that a small nitpick on naming:
Your get...Format() methods break the camelCase you kept until then. I suggest you ignore that these formats are acronyms and instead use following method names:

getXmlFormat (Object object);
getJsonFormat (Object object);


But I'd say that's personal preference. All else you didn't give us much to review :(