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    And don't forget to write a unit test to check that a Car cannot be constructed without a SteeringWheel... Commented Mar 20, 2018 at 14:28
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    I might be missing something, but if it's possible and even common to create a Car without a CarRadio, wouldn't it make sense to create a constructor that doesn't require one to be passed in and not have to worry about an explicit null at all? Commented Mar 20, 2018 at 16:34
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    Self driving cars might not have a steering wheel. Just saying. ☺ Commented Mar 20, 2018 at 16:54
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    @James_Parsons I forgot to add that it was about a class which didn't have null checks because it was only used internally and always received non-null parameters. Other methods of Car would have thrown an NRE with a null CarRadio, but the relevant code for SpeedLimit wouldn't. In the case of the question as posted now you would be correct and I would indeed do that. Commented Mar 20, 2018 at 18:40
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    @mtraceur The way everybody assumed that the class allowing construction with a null argument means null is a valid option made me realize that I should probably have null checks there. The actual issue I would have then is covered by a lot of good, interesting, well-written answers here which I don't want to ruin and which did help me. Also accepting this answer now because I don't like to leave things unfinished and it's the most upvoted (although they all were helpful). Commented Mar 21, 2018 at 11:19