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15 Questions to Ask During a Ruby Interview

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Originally published in June 2008

When hiring Ruby on Rails programmers, knowing the right questions to ask during an interview was a real challenge for me at first. In 30 minutes or less, it's difficult to get a solid read on a candidate's skill set without looking at code they've previously written. And in the corporate/enterprise world, I often don't have access to their previous work.

To ensure we hired competent ruby developers at my last job, I created a list of 15 ruby questions -- a ruby measuring stick if you will -- to select the cream of the crop that walked through our doors.

What to expect

Candidates will typically give you a range of responses based on their experience and personality. So it's up to you to decide the correctness of their answer.

There are many solutions to most of these questions -- some aren't listed here. Candidates get my respect for knowing esoteric solutions, but I'm looking for developers with similar practices to my own.

Make no mistake, this list is not complete, by any means. But it does provide a useful tool to put a candidate's reaction to the same series of questions in perspective. Especially during an interview, when your mouth goes dry and and your hands get all sweaty.

Begin!

Senior programmers won't have a problem with these, while junior programmers will usually give only half-answers.

What is a class?

A text-book answer: classes are a blue-print for constructing computer models for real or virtual objects... boring.

In reality: classes hold data, have methods that interact with that data, and are used to instantiate objects.

Like this.

class WhatAreClasses
  def initialize
    @data = "I'm instance data of this object. Hello."
  end

  def method
    puts @data.gsub("instance", "altered")
  end
end

object = WhatAreClasses.new
object.method
 #=> I'm altered data of this object. Hello.

What is an object?

An instance of a class.

To some, it's also the root class in ruby (Object).

Classes themselves descend from the Object root class. (Kudos to Ezra)

What is a module? Can you tell me the difference between classes and modules?

Modules serve as a mechanism for namespaces.

module ANamespace
  class AClass
    def initialize
      puts "Another object, coming right up!"
    end
  end
end

ANamespace::AClass.new
 #=> Another object, coming right up!

Also, modules provide as a mechanism for multiple inheritance via mix-ins and cannot be instantiated like classes can.

module AMixIn
  def who_am_i?
    puts "An existentialist, that's who."
  end
end

# String is already the parent class
class DeepString < String
  # extend adds instance methods from AMixIn as class methods
  extend AMixIn
end

DeepString.who_am_i?
 #=> An existentialist, that's who.

AMixIn.new
 #=> NoMethodError: undefined method ‘new’ for AMixIn:Module

Can you tell me the three levels of method access control for classes and modules? What do they imply about the method?

All methods, no matter the access control, can be accessed within the class. But what about outside callers?

Public methods enforce no access control -- they can be called in any scope.

Protected methods are only accessible to other objects of the same class.

Private methods are only accessible within the context of the current object.

class AccessLevel
  def something_interesting
    another = AccessLevel.new
    another.public_method
    another.protected_method
    another.private_method
  end

  def public_method
    puts "Public method. Nice to meet you."
  end

  protected

  def protected_method
    puts "Protected method. Sweet!"
  end

  private 

  def private_method
    puts "Incoming exception!"
  end
end

AccessLevel.new.something_interesting
 #=> Public method.  Nice to meet you.
 #=> Protected method.  Sweet!
 #=> NoMethodError: private method ‘private_method’ called for
 #=>  #<AccessLevel:0x898c8>

There are three ways to invoke a method in ruby. Can you give me at least two?

Here, I'm looking for the dot operator (or period operator), the Object#send method, or method(:foo).call

object = Object.new
puts object.object_id
 #=> 282660

puts object.send(:object_id)
 #=> 282660

puts object.method(:object_id).call # (Kudos to Ezra)
 #=> 282660

Separating the professional from the hobbyist

Senior programmers should be able to give competent answers for all questions. Junior programmers should answer some correct, but usually won't know them all.

Explain this ruby idiom: a ||= b

A common idiom that strong ruby developers use all the time.

# a = b when a == false
# otherwise a remains unchanged
a || a = b # (Kudos to Markus Prinz)
a = 1
b = 2
a ||= b #=> a = 1
a = nil
b = 2
a ||= b #=> a = 2
a = false
b = 2
a ||= b #=> a = 2

What does self mean?

self always refers to the current object. But this question is more difficult than it seems because Classes are also objects in ruby. (Kudos to Stephen)

class WhatIsSelf
  def test
    puts "At the instance level, self is #{self}"
  end

  def self.test
    puts "At the class level, self is #{self}"
  end
end

WhatIsSelf.test 
 #=> At the class level, self is WhatIsSelf

WhatIsSelf.new.test 
 #=> At the instance level, self is #<WhatIsSelf:0x28190>

This short snippet indicates two things:

  • at the class level, self is the class, in this case WhatIsSelf.
  • at the instance level, self is the instance in context, in this case the instance of WhatIsSelf at memory location 0x28190.

What is a Proc?

Everyone usually confuses procs with blocks, but the strongest rubyist can grok the true meaning of the question.

Essentially, Procs are anonymous methods (or nameless functions) containing code. They can be placed inside a variable and passed around like any other object or scalar value. They are created by Proc.new, lambda, and blocks (invoked by the yield keyword).

Note: Procs and lambdas do have subtle, but important, differences in ruby v1.8.6. However, I wouldn't expect a candidate talk about these nitty-gritty details during an interview. (Kudos to Noah Thorp)

# wants a proc, a lambda, AND a block
def three_ways(proc, lambda, &block)
  proc.call
  lambda.call
  yield # like block.call
  puts "#{proc.inspect} #{lambda.inspect} #{block.inspect}"
end

anonymous = Proc.new { puts "I'm a Proc for sure." }
nameless  = lambda { puts "But what about me?" }

three_ways(anonymous, nameless) do
  puts "I'm a block, but could it be???"
end
 #=> I'm a Proc for sure.
 #=> But what about me?
 #=> I'm a block, but could it be???
 #=> #<Proc:0x00089d64> #<Proc:0x00089c74> #<Proc:0x00089b34>

What is unit testing (in classical terms)? What is the primary technique when writing a test?

The strongest candidates should be quite comfortable with test or behavior driven development.

Unit testing, simply put, is testing methods -- the smallest unit in object-oriented programming. Strong candidates will argue that it allows a developer to flesh out their API before it's consumed by other systems in the application.

The primary way to achieve this is to assert that the actual result of the method matches an expected result.

require "test/unit"

class Brokened
  def uh_oh
    "I needs fixing"
  end
end

class BrokenedTest < Test::Unit::TestCase
  def test_uh_oh
    actual = Brokened.new
    assert_equal("I'm all better!", actual.uh_oh)
  end
end
 #=> Started
 #=> F
 #=> Finished in 0.663831 seconds.
 #=> 
 #=>   1) Failure:
 #=> test_uh_oh:11
 #=> <"I'm all better!"> expected but was
 #=> <"I needs fixing">.
 #=> 
 #=> 1 tests, 1 assertions, 1 failures, 0 errors

Show me the money!

Variable typing is one of those topics that everyone sort of understands it, but is hard to put it into words. I've iterated and improved the next series of questions to really test a senior level candidate's knowledge of static and dynamic typing. This is my best attempt so far.

What is the primary difference in these two code snippets?

// Java
public boolean isEmpty(String s) {
  return s.length() == 0;
}
# ruby
def empty?(s)
  return s.size == 0
end

The Java method only accepts Strings as arguments and only returns a boolean while...

The ruby method accepts any Object and could return anything, but in this case will return a boolean if executed without exceptions.

What does this say about the advantages of ruby's dynamic (duck) typed system?

That ruby program use less code and are more flexible.

What are some disadvantages (real and potential)?

Developers cannot be 100% certain that all arguments sent this empty? method will have a size method that is publicly accessible. Also, ruby is an interpreted language and it may take longer to run than compiled programs, such as Java, that are programmed similarly.

What could a developer do to address these disadvantages?

She could write unit tests or specs to ensure her application behaves as intended. She could also profile her application with tools like the unix time command, the ruby Benchmark class, and the ruby library called ruby-prof.

A cunning programmer would also argue that these two techniques ought to be used for both static and dynamic languages when developing complex systems.

Wrapping things up

To finish up with, I like to lob in some easy ones again. Plus I like to scratch my own curiosity about a candidates relationship with the ruby community.

What are rubygems? Any favorites not including rails? Any that you've worked on personally?

rubygems is package manager software for ruby libraries (i.e. gems). The package manager has basic CRUD operations, dependency trees, and supports asynchronous communication between multiple gem servers.

What is your favorite api resource for ruby?

I really like gotAPI -- auto-complete searching for both ruby and rails together!

Your Own Questions?

I really want to know how you interview candidates for your ruby positions! Ruby is being used heavily at many companies all over the world for external and internal use alike. Please share your ideas with the rest of us!

If you are interviewing for a Ruby position, why would you assume a knowledge of Java?

I don't know any java, but I have some C++ knowledge and could divine the intent of the question.

This was an awesome preparation tool! Thanks for sharing!

I've also been asked if Ruby has open classes. And what does that mean. Probably since that's huge difference between Ruby and stricter languages.

@cswagerty Ruby does have open classes. This refers to our ability in Ruby to open up classes and add methods at any time after they have been defined. For example, to write your own Array extension, we open the Array class and add our method. Another way to do this with metaprogramming is to use define_method. Rails uses this special ability all the time.

class Array
  def my_silly_method
    # .....
  end
end

This is an awesome gist for an overview on the expectations in a ruby intervew. I had also been asked 'What are anonymous methods, open classes etc. Thanks a lot for sharing this.

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