Inheritance is evil!

Read this on Bernie's blog:

"All of the pain caused by inheritance can be traced back to the fact that inheritance forces 'is-a' rather than 'has-a' relationships. If class R2Unit extends Droid, then a R2Unit is-a Droid. If class Jedi contains an instance variable of type Lightsabre, then a Jedi has-a Lightsabre.

The difference between is-a and has-a relationships is well known and a fundamental part of OOAD, but what is less well known is that almost every is-a relationship would be better off re-articulated as a has-a relationship."

I suggest you read the full story, as it's very interesting! Bottom line is that you should be careful using OO inheritance, and use the Strategy pattern instead.

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2 responses

  1. Avatar for Sam Goutsmit
    Sam Goutsmit October 12th, 2007

    Which just goes to prove that inheritance is a tool like any others, ie it can be used well or poorly. Strategy Pattern is indeed a good alternative in his examples.

    However, to draw a general "inheritance is evil" conclusion from that, is just a bit over-the-top. But I understand that a blog entry reads better when it has a catchy title :-)

  2. Avatar for maartenba
    maartenba October 12th, 2007

    Catchy blog titles are important tools too ;-)

    But you are right in your opinion. You should not stop using inheritance, as it is a powerful tool. Just remember to refactor some things into a Strategy pattern if you come across an issue with Jedi's...