Question

Below is a simplified version of code I’ve written to override a class method (using composition), in this case method name i’m overriding is addbuttons(); The class “Screen” adds buttons to a screen whereas class “SubScreen” adds custom buttons to an instance of Screen. Maybe this is not an example of decorator pattern since I’m overriding functionality instead of extending it ? Is there a better way (using a design pattern ?) to achieve same functionality ?

public class Driver {

    public static void main(String args[]){
        AddComponents add1 = new Screen();
        add1.addButtons();

        Screen newScreen = new Screen();
        AddComponents add2 = new SubScreen(newScreen);
        add2.addButtons();
    }

}
    public interface AddComponents {

         public void addButtons();
        }

public class Screen implements AddComponents{

    public void addButtons() {
        //Add Buttons to screen class
    }

}
public class SubScreen implements AddComponents{

    private Screen screen;

    public SubScreen(Screen screen){
        this.screen = screen;
    }

    public void addButtons() {
        //add different custom buttons to Screen class
    }

}

Answer 1

Another possibility would be to call it Proxy. Decorator and Proxy are technically very similar – the difference is – in most cases – not a technical one but based on the intention.
Your example is a little bit minimal and therefore it is hard to guess the intention correctly.

Edit

At the detailed level: Screen and SubScreen do not share any code. If you start adding methods to both implementations and the common interface AddComponents you might find

• that you must duplicate code both in Screen and SubScreen (or delegate to Screen) and
• that you must add methods to AddComponents which make this interface badly named.

If both screen classes are similar both on the abstract logical level and at the implementation level, then a an class AbstractScreen with two derived classed would be better. To bring back pattern speak: Use a Factory Method in AbstractScreen to specialize the behaviour regarding the different buttons.

On your current code there is one strange thing: Why is there a method addButton defined anyway? Simply add the buttons in the appropriate constructor, since the user has to call addButtons in any case and the method does not have arguments.

Another point not explained is this: SubScreen has a reference to Screen which is not used. Why? Will there be more method in all involved classes Screen, SubScreen and AddComponents? Will each method be a in SubScreen delegate to Screen or only half of them?

You see – there are many possibilities we do not know and are not shown in the example code but are very important. I’m sure that your head contains a lot of details saying “This proposed stuff will not work because I want to do this and that one way or the other in the near future.” Sadly we cannot get the content of your head into this site without more writing. 🙂