Question

In the real world what do people use this for (to solve what types of problems)? Can I see some example code of these working together? All I can find is code about cats and dogs speaking or people drinking milk or coffee…

Do people really implement polymorphism with interfaces? What for?

Answer 1

Sure,

Below is concrete example of the “Observer” pattern, using classes and interfaces to accomplish polymorphic behavior in a logger system:

interface ILogger{

   public void handleEvent (String event);
}

class FileLogger implements ILogger{

   public void handleEvent (String event){
       //write to file
   }
}

class ConsoleLogger implements ILogger{

   public void handleEvent (String event){
       System.out.println( event );
   }
}

class Log {

   public void registerLogger (ILogger logger){

       listeners.add(logger);
   }

   public void log (String event){

       foreach (ILogger logger in listeners){

            logger.handleEvent(event); //pass the log string to both ConsoleLogger and FileLogger!
       }
   }

   private ArrayList<ILogger> listeners;
}

Then, you could use it as follows:

public static void main(String [] args){

     Log myLog();
     FileLogger myFile();
     ConsoleLogger myConsole();

     myLog.registerLogger( myFile );    
     myLog.registerLogger( myConsole );

    myLog.log("Hello World!!");
    myLog.log("Second log event!");
}

Hope this helps your understanding of interfaces and polymorphism.