Technical aptitude testing & recruiting

Having spent a reasonable amount of time on both sides of the interview table, plus having co-founded NerdAbility, it may come of little surprise that I am pretty opinionated on the topic.

It's a pretty divisive topic, and a lot of people feel quite strongly about a lot of this stuff, but here's my opinion anyway..

Tech tests

First up is the question of whether or not potential candidates should have to take some kind of tech test. Personally I like them. But with a few caveats:

  1. They should be easy. 
    This might sound counter intuitive, but I prefer a relatively simple tech test.  In reality, I'm not really convinced that you gain that much from the more complex tests, and at worst, probably just get false negatives and end up mistakenly ruling out great candidates.  I have seen tests that take days (unit testing/mocking/designing/coding/reviewing/re-factoring etc.) and if anything they put candidates off, and as mentioned probably don't provide much info.

    Something nice and simple, let's say a modest estimate of an hour all in is probably about right.  As ranted about by many folk, famously Jeff Atwood, a basic FizzBuzz programming test will rule out a lot of people.

    Further more, when I review test submissions I don't really care about if they work - what I am really looking at is the coding style and overall approach. Coding style, class structuring, use of nice libraries/core functionality/data structures, unit tests.  If you have a simple test that should take no more than an hour to code then candidates really don't have any excuse not to make their best efforts with how the code is structured, unit tested etc - so you can set the bar pretty high.
  2. They shouldn't be timed
    Timing the tests just blur the lines - if you are timing the tests then you have to lower the bar. With a simple, non-timed test, you can say you will rule out people who haven't submitted unit-tests for example, but if you set a time limit then you have to excuse people - and will inevitably find yourself saying things like

    "Well, sure its 200 lines of code all in the main() method, and variable names like 'tmpString', and it probably doesn't work, and it's not unit-tested.. but maybe they were rushed for time.. maybe we should bring them in..? "

    It happens, the bar slips lower and lower, and eventually the test is serving no purpose other than ruling out those people who just can't be bothered.
  3. They should be core technology concepts
    There is no point having tests that test specific domain knowledge or expect experience beyond core language competencies. Even if your business is in a very specific niche, you are going to do much better hiring great tech folk if you test core competencies rather than specific libraries/tools/technologies.
  4. They should be done before hand
    On-site testing adds a different dimension to the test - whether it be whiteboard or on a machine, there are other variables that can end up being a distraction. On a whiteboard candidates can end up worrying over exact method signatures and missing semi colons (but whiteboard is much preferable to actual coding - In my opinion, you should never expect a candidate to bring a laptop, and asking a candidate to use another machine/OS/IDE is also fraught with potential distractions).

What's the point?

As mentioned, I am not a big believer in using the tests to really measure if someone is a great programmer. For me, they serve two simple purposes (well three actually, but I will mention the third point later)
  1. They can be bothered. They are actually interested enough in the role and the company to invest their own time and energy. This will rule a few people out who are just machine gunning resumes out to lots of companies blindly, or those who are just after some interview practice.
  2. They have demonstrated an understanding of core technology approaches/patterns - Simple things like Single-responsibility, unit testing (use of good assertions, sensible testing, messaging etc), class organisation shows that they have actually spent a reasonable amount of time programming and keep up technology.  A nice little example of this, that I like to see, is use of Java's Collections/Arrays convenience methods (assuming testing Java!)  Arrays.asList( "1", "2", "3" ) makes declaration of explicit lists easier (nice for testing etc) and shows a knowledge of core Java stuff.

So all we have so far is know they can be bothered and that they have a good understanding/interest in program design/architecture - not much more that can indicate whether or not they are an awesome developer.

The interview

This is where the test really comes into its own.  I think using the test to drive the tech interview is really a great way to go.

You can walk through the code, ask about design decisions, and with a little extra thought you can easily push into variations of the test, how would they handle other constraints and you can continue to push through varying degrees of complexity, and if you are consistent with your tests you get a consistent sliding scale to compare candidates - you know exactly at what point did each candidate get to on the scale of questions. See this article as a great example of this technique more generally (also fun/good practice to try working through the problems the author is asking before reading the answers to see how far you get).

This approach of starting very easy and working up a sliding scale of difficulty is a common practice used by big co's like Google et al.

An example

Here's an approach I quite like

Tech test: the problem

Given a webpage address, find the most common word on the page.

This tests core technology concepts, stuff like good usage of HashMap (or similar) data structure for counting words, extensive enough to need some proper unit testing, but quick enough to complete relatively quickly.

Interview: followup

There are a few follow up questions that can be used to further probe the candidates understanding:
  1. What would you need to change if I wanted the most common word on a whole website (e.g. wikipedia) - this can go into how to crawl webpages and potential pitfalls if that is a relevant area, but otherwise can go into challenges regarding the amount of information needed to be stored, e.g. if you have limited memory, how can you store the info etc
  2. If I wanted the top 5 most common words how would you change it?  This is interesting as there are a variety of solutions, and unless they go straight for the optimal solution you can keep asking if they can think of any better solution. 

    For example, they might just keep track of the top 5 words during the counting, which is pretty efficient, but less flexible when top 5 becomes Top X words;

    Alternatively they may just count all the words, then implement a comparator for the Map entries and sort them all and just take the top X - which is flexible but is always going to be  O(nlogn) performance (sorting is always at best nlogn)

    Another approach is to use a Heap (PriorityQueue in Java), and heapify the counted set (heapify can be completed in O(n) time) then just take the top X elements from the queue (X being a lower order constant not dependent on the size of the dataset, and log n being lower order than the linear time to heapify the data upfront,  so performance is O(n))

    You can also follow up this question with further questioning about performance and Big-O - if thats something that you think is interesting/relevant for the position - which it might not be..

Whatever test you choose, as long as you have some sensible and interesting questions to follow up with, I think it makes for a pretty productive process and in many ways is optimized for the candidate making the best impression they can.


Spring MVC & custom routing conditions

I have recently been building a Spring MVC app (well, actually using the Spring Boot project - which is quite nice in parts, and crazy frustrating in others - but the underlying mechanics are the same).  The application is actually a re-build of another application, so it has involved a lot of playing and exploring Spring source code to try and replicate the apps functionality like for like.

One of the first things I found, was that Spring doesn't really cater for the concept of a single app running on different sub-domains.  I assume the thinking is that you would build separate applications for different sub-domains, but in this case, we just have a single app.

There are two main speed-bumps I have come across so far:
  1. Controller routing based on subdomain
  2. Security considerations based on subdomain

Basically, as you can probably imagine, once you have subdomains on a single web application, the URL path is no longer unique (e.g. is clearly not the same as )

The rest of this post, I will look at the routing element of it. I will do another post later in the week about the Spring security stuff (I haven't solved all of that yet - but got far enough to make it work).

Understanding the subdomain

The first thing you need to sort out is a common and consistent way to determine the subdomain of any given request. There are a variety of ways to do this, for example, you could parse it from the request in apache and set a header, so your app doesn't have to worry about it, or you could just parse it from the request object server name. I will assume you have some bean/service/helper class to do this everywhere (although for now we only need it in one place).

An annotation

First up, we need a new annotation, that we can easily apply to a Controller, just like we would use the @RequestMapping - at the moment, the nice built in Spring RequestMapping handling allows you to define a URL path (plus other bits and pieces) to map a request to a URL to a given controller & method - what we want is to also specify the subdomain element of the requested URL

Defining an annotation is simple:

For the sake of simplicity of this example, I will only allow it to be used at class level (so no method based subdomain routing - but that will be pretty easy to do once you have understood the rest of the post).  You will also note that the value is defined as a String array, this will allow us to define mappings to multiple subdomains if needed.

The mapping condition

So, that was easy. Obviously, at this point the annotation doesn't actually do anything - you can add it to all the controllers you like, but it won't actually make any difference to your request routing.

To get our new annotation involved, we can implement something called a RequestCondition.  This is exactly what it sounds like, Spring lets you implement additional request conditions that must be satisfied for a request mapping.

The condition could be based on any logic you like, but in our case we just simply need to check for an annotation and then examine the value provided.  If the annotation value matches our incoming request then the condition is met, easy!  Returning the condition indicates to Spring that the condition has been met, returning the null value indicates that the condition is not met.

Adding the condition to the mapping handler

Usually, we would use the standard Spring RequestMappingHandlerMapping to handle all the routing of requests based on the URLs, but now we need to also ask Spring to consider our new custom condition from above.

This is a simple case of extending the normal RequestMappingHandlerMapping class and adding our new condition as a custom condition.  Luckily, this is really easy:

Allwe are doing is checking to see if the handler class (our controller) has our new @Subdomain annotation and if it does, we register our new custom condition for consideration.


That's really all there is to it - we can then decorate our controller class with @Subdomain("subdomain") and have them handle the routing of requests. 

As you may have noticed, this is a pretty nice pattern for any kind of custom routing you might want to use - the same template could be used for routing by any request info/header or any user info (e.g. routing requests to different controllers based on their logged in role etc)