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Trail: Learning the Java Language
Lesson: Interfaces and Packages

## What Is an Interface?

> This section shows you how to create and to use interfaces and talks about why you would use an interface instead of a class.

> An interface defines a protocol of behavior that can be implemented by any class anywhere in the class hierarchy. An interface defines a set of methods but does not implement them. A class that implements the interface agrees to implement all the methods defined in the interface, thereby agreeing to certain behavior.

> > Definition: An interface is a named collection of method definitions (without implementations). An interface can also declare constants.

> Because an interface is simply a list of unimplemented, and therefore abstract, methods, you might wonder how an interface differs from an abstract class. The differences are significant.

> - An interface cannot implement any methods, whereas an abstract class can.
> - A class can implement many interfaces but can have only one superclass.
> - An interface is not part of the class hierarchy. Unrelated classes can implement the same interface.

> Let's set up the example we'll be using in this section. Suppose that you have written a class that can watch stock prices coming over a data feed. This class allows other classes to register to be notified when the value of a particular stock changes. First, your class, which we'll call `StockMonitor`, would implement a method that lets other objects register for notification:

> >     public class StockMonitor {
> >          public void watchStock(StockWatcher watcher,
> >          String tickerSymbol, double delta) {
> >          ...
> >          }
> >     }

> The first argument to this method is a `StockWatcher` object. `StockWatcher` is the name of an interface whose code you will see in the next section. That interface declares one method: `valueChanged`. An object that wants to be notified of stock changes must be an instance of a class that implements this interface and thus implements the `valueChanged` method. The other two arguments provide the symbol of the stock to watch and the amount of change that the watcher considers interesting enough to be notified of. When the `StockMonitor` class detects an interesting change, it calls the `valueChanged` method of the watcher.

> The `watchStock` method ensures, through the data type of its first argument, that all registered objects implement the `valueChanged` method. It makes sense to use an interface data type here because it matters only that registrants implement a particular method. If `StockMonitor` had used a class name as the data type, that would artificially force a class relationship on its users. Because a class can have only one superclass, it would also limit what type of objects can use this service. By using an interface, the registered objects class could be anything--`Applet` or `Thread`--for instance, thus allowing any class anywhere in the class hierarchy to use this service.

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