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Trail: Learning the Java Language
Lesson: Interfaces and Packages

## 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,
> >                                String tickerSymbol, double 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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