TDD Katas / Exercises: Multimap (2 / 5+)

The 2nd in series of blog posts in which I describe TDD katas & exercises that I’ve used for training purposes.

Other posts in the series:


A data structure that supports storing multiple values at a key. Think “English language dictionary,” which can contain multiple definitions for a single word.

  • key => [values]
  • You add to the multimap by specifying a key and a single value to be stored at that key:
    dictionary.put('a word', 'a definition')
       // or
    dictionary['a word'] = 'a definition'
  • You retrieve from the multimap by specifying a key:
    var keys = dictionary['a word']
    expect(keys).toEqual(['1st definition', '2nd definition']
  • Answers its size (the number of keys)
  • Answers the total count of values stored across all keys (e.g. “count of all definitions”)
  • Answers whether or not it is empty
  • Throws / returns an error when attempting to store a null key

Duration**: 45 minutes

Core themes:

  • Test ordering
  • Sticking with the R-G-R rhythm
  • Incrementally moving from specific to more generalized implementations

I used the Multimap as an introductory TDD exercise for a while many years ago. In turn working backward, this exercise replaced my use of test-driving a stack as an exercise; I moved off of that due to the negative feedback it garnered because of its over-simplicity (which appeared to help reinforce resistances to TDD).

Test-driving a multimap works well as a first exercise (or second exercise if the first was done using “TDD paint by numbers,” in which case you’d want to discuss a starter test list with the group). It does provide a couple opportunities for the students to create mildly interesting defects. It is “real” enough in that I’ve found a few occasions to use it in production code.

Note that some languages and / or frameworks (e.g. Guava for Java and C++11 / later) already provide a Multimap implementation. In this case, test-driving a multimap can still work as an exercise, though it could also provoke thoughts of “why are we wasting our time building something already readily available?”

Some possibilities for extra credit / extending the exercise.

  • Support adding multiple values at a key in addition to singular values
  • Provide the ability to find a key based on a predicate against the values
  • Support storing the values in a sorted order
  • Support storing only unique values for a single key
  • Support removing keys
  • Support removing a specific value
  • Support replacing a specific value
  • Support interesting iteration

** For pairing TDD novices. Impacts to duration can include:

  • Whether it’s their first TDD exercise or a subsequent one
  • General level of programming proficiency
  • Programming language used
  • Exercise learning mode: pairing, mobbing, or solo?
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