Extension should be written to anticipate multiple uses for the interface and allow users to upgrade. Sample code for an extension follows:

//create an interface
public interface StockWatcher {
    final String
    sunTicker = "SUNW";
   final String oracleTicker = "ORCL";
    final String ciscoTicker = "CSCO";
    void valueChanged(String tickerSymbol, double newValue);
}