How does TDD look like in real world development of microservices or web interfaces? That's what this series is all about.
1. Introduction2. The requirements3. Test case analysis I4. Test case analysis II
Test case analysis III6. Test case analysis IV7. Setting up a project with Spring Boot8. Which tests to write?
9. API design

So many TDD cases, so many posts. And no Spring in sight yet. I promise, we’ll get there. For now, let’s continue to explore the requirements. Where did we last stop?

  • Pressing any digit key, the display adds that digit to the existing display. The exception is ‘0’ which is presented only if a number different from ‘0’ wasn’t already pressed.

What’s the deal with the zero key behaving differently? An example for regular digits looks like this:

“4”,”5″ => “45”

But for zeros it’s different:

“0”,”5″ => “5”

Of course, we still need to remember that a zero still behaves as a regular digit after another was pressed:

“5”,”0″ => “50”

And let’s not forget the ever shifty:


Did you notice something? We’re collecting lots of examples, and none have yet to do any calculation. That display sure is tricky.

What’s the next requirement?

  • Pressing an operation key does not change the display.

What does that mean? An example,

“6”,”+” => “6”
“7”,”8″,”/” => “78”

However, when we add a number after the operation key, we’ll see new that number:

“6”,”+”,”1″ => “1”
“7”,”8″,”/”,”3″ => “3”

That makes sense. We’re starting to input a new number after the operation. Funnily enough, if we key two operations sequentially, the display works the same:

“6”,”+”,”-” => “6”

We don’t yet know what that means in terms of calculation, but that’s ok. We’ll explore that later.
A more complex set still yields the same behavior:

“6”,”+”,”4″,”-” => “4”

Time for a bit of semantic talk. Is “=” an operation key? We don’t expect the same behavior after doing the calculation. However, it’s a non-digit key, so how should we call it?

Product Manager: No, it’s not an operation key. Operations are “+”,”-“,”*” and “/”. Others are not operation keys, although, we don’t have a special name for their group.

Ok, we don’t have long to go. Before calculations.

  • Pressing “C” resets the calculator

That’s easy enough:

“1”, “C” => “0”

Wait. Is that still true after an operation?

PM: Yes.

Ok then.
“1”, “/”, “C” => “0”

Now, how do we know that the calculator is really reset? It may show me zeros, but is the last number still affecting calculation? That would need to be put into the calculation examples.

Which are coming up next.

All this, and the calculator is not helpful yet. Imagine that.

Categories: TDD


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