We will provide different implementations for the methods declared in the Modem interface for each of the supported modems. To implement an interface, we use the following notation:

 The class MindStickModem declaration uses the implements keyword to implement the Modem interface. In the class definition, we need to provide an implementation for each method defined in the Modem interface.

To support another modem, we would create another class (say, HuaweiModem). We would define this class as follows:

public class HuaweiModem implements Modem {
                     public boolean open() {

                             // implementation

                     }

                     public boolean close() {

                             // implementation

                     }

                     public int read() {

                             // implementation

                     }

                     public int write(byte[] buffer) {

                             // implementation

                     }


}

 The method implementations in this class would be different from the implementations provided in MindStickModem class. Each implementation would be specific to the modem manufacturer. Once we create such classes specific to each modem manufacturer, we can develop our application software that interfaces easily with each of these modems. To use the MindStickModem class, we would use code similar to the following:

Modem modem = new MindStickModem();
modem.open();
modem.write(buffer);
modem.read();

modem.close();

Note that we instantiate the MindStickModem class and assign the object reference to a variable of type Modem. Remember from earlier posts that an object reference can be assigned to its superclass without explicit typecasting. The Modem interface is a super-interface here. The MindStickModem class implements Modem and therefore the assignment of an object reference from type MindStickModem to the Modem interface is permitted.

Now, to interface our application with Huawei modem, we would use the following code:

The only difference between the earlier code and this code is in the class instantiation. In the earlier case we use MindStickModem and in the latter case we use HuaweiModem. The rest of the code remains the same. This is the greatest advantage of creating interfaces—we don’t need to change much of the code even when we change modems. This makes it easier to write an application that works with a lot of different modems. Here, we can see that our initial concern for how to provide a different implementation for each modem and yet maintaining a single interface to access them is easily resolved using an interface.