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Warning! Interfaces Cannot Grow

Suppose that you want to add some functionality to StockWatcher. For instance, suppose that you want to add a method that reports the current stock price, regardless of whether the value changed:

public interface StockWatcher {
    void valueChanged(TickerSymbol tickerSymbol,
                      BigDecimal newValue); 
    void currentValue(TickerSymbol tickerSymbol,
                      BigDecimal newValue); 
}

However, if you make this change, all classes that implement the old StockWatcher interface will break because they don't implement the interface anymore! Programmers relying on this interface will protest loudly.

Try to anticipate all uses for your interface up front and specify it completely from the beginning. Given that this is often impossible, you may need either to create more interfaces later or to break your customer's code. For example, you could create a StockWatcher subinterface called StockTracker that declared the new method:

public interface StockTracker extends StockWatcher {
    void currentValue(TickerSymbol tickerSymbol,
                      BigDecimal newValue);
}

Now users of your code can choose to upgrade to the new interface or to stick with the old interface.

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