Question

I’ve heard people saying that good design involves using inheritance instead of littering your code with if block. In what situation should we use inheritance, and when condition block is just fine?

Answer 1

Generally, it isn’t good object-oriented programming practice to have code like the following:

var chioce = PAINT_SCREEN_BLUE

if (choice == PAINT_SCREEN_BLUE):
    paintScreen(blue)
else if (choice == PAINT_SCREEN_RED):
    paintScreen(red)
else if (choice == PAINT_SCREEN_GREEN):
    paintScreen(green)

The above can utilize polymorphism to call some method that will perform the action that is abstracted away in related classes with a common ancestor:

interface ScreenPainter:
    function perform()

class BlueScreenPainter implements ScreenPainter:
    function perform():
        paintScreen(blue)

...

// The conditional block can be replaced with the following call:
var choice = PAINT_SCREEN_BLUE

ScreenPainter p = Painters.getPainter(choice)
p.perform()

However, this practice definitely should not apply for all conditionals, however, when it comes to long switch blocks or if-elseif-else blocks, in many cases, use of inheritance will make your code more extensible and more robust.