The following source:

  class Props extends Hashtable<String,String> {}

is translated by javac info

  class Props extends Hashtable {
    /*synthetic*/ public Object remove(Object x0) {
        return super.remove(x0);
    }
    /*synthetic*/ public Object put(Object x0, Object x1) {
        return super.put((String)x0, (String)x1);
    }
    /*synthetic*/ public Object get(Object x0) {
        return super.get(x0);
    }
  }

These bridge methods are unnecessary, and interfere with
retrofitting java.util.Properties as Hashtable<String,String>.

See also http://groups.yahoo.com/group/java-spec-report/message/1017