97 Things Every Java Programmer Should Know

Chapter 76. Take “Separation of Concerns” Seriously

Dave Farley

If you studied computer science, you may have learned about an idea called separation of concerns.¹ This is best characterized by the sound byte “One class one thing, one method one thing.” The idea is that your classes and methods (and functions) should always be focused on a single outcome.

Think carefully about the responsibilities of your classes and methods. I sometimes teach classes in test-driven design. I use adding fractions as a simple coding kata to explore TDD. The most common first test I see people write often looks something like this:

assertEquals("2/3", Fractions.addFraction("1/3", "1/3"));

For me, this test is screaming “poor design.” First, where is the fraction? It only exists implicitly, presumably inside the addFraction function.

Worse than this, let’s think about what is going on here. How would we describe the behavior of the addFraction function? Perhaps something like “It takes two strings, parses them, and calculates their sum.” As soon as you see, or think, the word “and” in the description of a function, method, or class, you should hear alarm bells ringing inside your head. There are two concerns here: one is string parsing, and the other is fraction adding.

What if we wrote our test like this instead:

 Fraction fraction = new Fraction(1, 3); assertEquals(new Fraction(2,3), fraction.add(new ...