Callbacks in Java

Goal

Work with any type of data.
We can use callbacks for this purpose. In Java callbacks can be implemented using interfaces.

Callbacks

Client passes object to work routine.
The routine callback Objects function to perform appropriate actions for that type as needed.

• Client
• routine
• Object
• Interface
  (behind Object)

The interface provides common functions for all Object types.

public interface Portable {
     public void selectResource();
}

Lets create few Object types:

public class Mobile implements Portable {
    public String type;

    public Mobile(String type){
        this.type = type;
    }

    public void selectResource(){
        System.out.println("Device: "+ type);
        System.out.println(">Selecting mobile image");
    }
}

And one more:

public class Tablet implements Portable {
    public String type;

    public Tablet(String type){
        this.type = type;
    }

    public void selectResource(){
        System.out.println("Device: "+ type);
        System.out.println(">Selecting Hi-Res image for Tablet");

    }
}

The routine method implementation will be oblivious to type of device:

public class App {
    public void displayResource(Portable type){
        type.selectResource();
    }
}

Finally, program that uses routine:

public class InstalledClient {
    public static void main(String[] args){

        App instance1 = new App();
        Portable type1 = new Mobile("Wildfire");

        instance1.displayResource(type1);



        //test for another type

        System.out.println("Another device...");
        type1 = new Tablet("Nexus 7");

        instance1.displayResource(type1);

    } 
}

The client calls displayResource() routine, which does not know type of device.
displayResource() call back Objects method to perform right function.

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