Question

I am having trouble reconciling the Single Responsibility Principle with encapsulation. It seems like splitting responsibilities amongst classes requires exposing a lot of data. As an example, consider some object called DataPoints. DataPoints is filled with x and y coordinates, among other things. I can create a Generator class that fills DataPoints. Now, let’s say I want to plot those data points. Clearly, that’s a separate responsibility that might come from a class called DataPointsPlotter. But to plot the data, I need to know what the internal x and y coordinates are. With a single class handling both, that’s no problem. x and y are internal variables, but both a create() and a print() method have access to those. I can expose x and y (perhaps through getters/setters–ugh) or I can pass the DataPoints structure to the Plotter class, but it still needs to get inside to get x and y. I can have an instance of Plotter declared in the DataPoints class to which I send x and y. But that’s still an exposure.

How can I in this example plot x and y using the plotter without violating encapsulation?

Answer 1

class DataPoint {

    protected x, y

    public function construct(theX, theY) {
        x = theX
        y = theY
    }

    public function getX {
        return x
    }

    public function getY {
        return y
    }

}

This encapsulates the responsibility of representing a datapoint in a known format. Assuming there’s also some sanity checking going on to confirm that x and y have good values, this is one responsibility which is being encapsulated here.

class Plotter {

    public function plot(DataPoint dp) {
        x = dp.getX
        y = dp.getY

        ... plot ...
    }

}

The plotter accepts an instance of DataPoint and plots it. That’s also one responsibility: plot an instance of DataPoint. It does not require Plotter to have any knowledge of the internal structure of DataPoint. It just requires a defined and stable interface how Plotter can get the necessary data from DataPoint. Which are the getters getX and getY. You can change the internals of DataPoint around however you want as long as its interface stays the same. And the responsibility of DataPoint is to hold a datapoint and give other code access to its data in a defined way. If there was no way to get data out of DataPoint, its existence would be quite useless.

So no, SRP and encapsulation are not at odds with each other. You just need to be clear about what exactly the responsibility is.