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Using an Interface as a Type

When you define a new interface, you are defining a new reference data type. You can use interface names anywhere you can use any other data type name. Recall that the data type for the first argument to the watchStock method in the StockMonitor class is StockWatcher:

public class StockMonitor {
    public void watchStock(StockWatcher watcher,
                           TickerSymbol tickerSymbol,
                           BigDecimal delta) {
    ...
    }
}

Only an instance of a class that implements the interface can be assigned to a reference variable whose type is an interface name. So only instances of a class that implements the StockWatcher interface can register to be notified of stock value changes. StockWatcher objects are guaranteed to have a valueChanged method.

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