Inheritance in Java Is More Useful Than You Think

A friend of mine came to me with a code problem that I am going to attempt to explain in this article. My friend is using a pre-existing graphics framework. In that framework, you have to extend from a certain interface, which has a specific function that takes only one parameter (which is another class). Up to this moment, there is no problem, but my friend says, "I need to send one more parameter along with that parameter." This means that the function's signature has to change. Then, this becomes another function, which that framework is not aware of.

I will try to simulate the same condition below for you and how to solve this kind of situation without changing a lot of code.

Here is the interface code that you want if you are going to add one more parameter next to Colours. However, you are not allowed to change this interface. Even if you could change this interface, there are tons of other classes that are already using this interface.

// This is part of the framework 
// You are not allowed to change 
public interface IColourMaker {
    public void printColour(Colours colours);
}

Below is the Colours class:

//You should not change this class
public class Colours {

    private String[] colourArray;

    public Colours() {
        colourArray = new String[2];
        colourArray[0] = "Blue";
        colourArray[1] = "Red";
    }

    public String[] getColours() {
        return colourArray;
    }
}

Below are two sample classes that come with the framework:

//You should not change this class
public class PaleMaker implements IColourMaker {

    @Override
    public void printColour(Colours colours) {
        System.out.println("PaleMaker Colour Maker");
        for (String string : colours.getColours()) {
            System.out.println("Pale: " + string);
        }
        System.out.println("PaleMaker Colour Maker");
    }
}

// You should not change this class
public class ShinyMaker implements IColourMaker {

    @Override
    public void printColour(Colours colours) {
        System.out.println("ShinyMaker Colour Maker");
        for (String string : colours.getColours()) {
            System.out.println("Shiny: " + string);
        }
        System.out.println("ShinyMaker Colour Maker");
    }
}

Here is a new class that you might want to add, but it needs one more parameter.

public class AdvancedColourMaker implements IColourMaker {

/*
 * Problem here AdvancedColourMaker needs one more parameter to do its job And
 * AdvancedColourMaker does need one more parameter The printColour signature
 * below is not helping us since we can not call polymorphically What do you
 * offer as a solution? Which does not have ripple effect on other classes
 * Assume there are tons of different Maker classes - and those are not ours, we
 * can not modify *
 */

    public void printColour(Colours colours, int count) {
        System.out.println("Advanced Colour Maker Count is: " + count);
        for (String string : colours.getColours()) {
            System.out.println("Advanced: " + string);
        }
        System.out.println("Advanced Colour Maker"); 
    }

}

Below is the main method. This does not work currently since the AdvancedColourMaker is not implemented yet.

import java.util.ArrayList;

public class Main {

public static void main(String[] args) {
System.out.println("Before Refactoring");

Colours colours = new Colours();

ArrayList<IColourMaker> maker = new ArrayList<>();
maker.add(new PaleMaker());
maker.add(new ShinyMaker());
maker.add(new AdvancedColourMaker());
// This code wants to use its own method however it needs two parameters
// However we do not know tomorrow customer may want to send three or for
// parameters...
// How can we fix it, without changing base class or other derived classes
// except ColourMakerAdvanced
// And this fix should accommodate upcoming new parameters as well.

for (IColourMaker colourMaker : maker) {
colourMaker.printColour(colours);
}
}
}

Wrong Direction

Now, I am trying to push one more parameter to the interface, insisting on changing the function signature. However, this is not helping us and making life complicated by bringing us cascading new changes.

You may have already seen the solution, but this was the mental journey I went through, and it was wrong!

The solution was hidden in just the extending Colours class!

public class MyColour extends Colours {
    public int count;
}

Now, I can add as many parameters as I want without changing the function signature.

Here is my AdvancedColourMaker :

public class AdvancedColourMaker implements IColourMaker {

public void printColour(Colours colours) {
        MyColour myColour = (MyColour) colours;
        System.out.println("Advanced Colour Maker Count is: " + myColour.count);
        for (String string : colours.getColours()) {
            System.out.println("Advanced: " + string);
        }
        System.out.println("Advanced Colour Maker");
    }
}

Here is the running main method:

import java.util.ArrayList;

public class Main {

public static void main(String[] args) {

MyColour colours = new MyColour();
colours.count = 10;

ArrayList<IColourMaker> maker = new ArrayList<>();
maker.add(new PaleMaker());
maker.add(new ShinyMaker());
maker.add(new AdvancedColourMaker());

for (IColourMaker colourMaker : maker) {
colourMaker.printColour(colours);
}
}
}

Below is the sample run:

PaleMaker Colour Maker
Pale: Blue
Pale: Red
PaleMaker Colour Maker
ShinyMaker Colour Maker
Shiny: Blue
Shiny: Red
ShinyMaker Colour Maker
Advanced Colour Maker Count is: 10
Advanced: Blue
Advanced: Red
Advanced Colour Maker

Conclusion

Sometimes, we try to search for solutions in the wrong place and try to force a change in others' code. Instead of extending it and adding your own style without disturbing other codes, I noticed that I can extend the Colour class and add as many parameters as I want — this is fabulous. I did not have to change the pre-existing classes. Because of this, I realized just how powerful is the inheritance.

Hope you enjoyed this demonstration!