I'll Be Backend
Search lessons / @illbebackend
Principles

Single Responsibility: Split Calculation from Formatting

The Single Responsibility Principle says a class should have one reason to change. An invoice report that both adds up the money and writes the text answers to the finance team and to its readers, so a second output copies the tax math. These lessons split the calculation from the formatting, one small refactoring at a time, until the CSV export is one line.

Pick your language

Single Responsibility: split calculation from formatting in Java, step 2: The same invoice as CSVJava · ~5 minSingle Responsibility in JavaSingle Responsibility: split calculation from formatting in C#, step 2: The same invoice as CSVC# · ~5 minSingle Responsibility in C#Single Responsibility: split calculation from formatting in Python, step 2: The same invoice as CSVPython · ~5 minSingle Responsibility in PythonSingle Responsibility: split calculation from formatting in PHP, step 2: The same invoice as CSVPHP · ~5 minSingle Responsibility in PHPSingle Responsibility: split calculation from formatting in TypeScript, step 2: The same invoice as CSVTypeScript · ~5 minSingle Responsibility in TypeScript

When to use it

Use it when

One method computes values and turns them into text, and a second output needs the same values.

Skip it when

A one-off script prints one report once and nobody else needs the numbers.

Spot it by

Arithmetic between string appends, and a second output method that copies the math.