Thursday, 6 October 2011

Java: Inheritance IV

Today, we get to talk about the super keyword, and how static variables work with inheritance!


Super


Remember our Car had an int parameter, for whatever reason. Well if we had a constructor for Vehicle with an integer parameter that we wanted to use, our Car constructor would throw an error.. This is because a default constructor is looked for, but not found (we only have an integer parameter one). What we need to do instead is call the superclass constructor with either a parameter in the Car constructor, or some other value.






The super expression (heh) replaces the automatic call to the superclass constructor so the superclass constructor with the matching parameter is called instead. Just remember that if you are going to use super, it should be the first statement in your subclasses constructor.




Statics


Since static variables and methods are inherited, we should probably know how they work when inherited. Well, pretty much the same as always, 1 variable is shared between all instances of a class, including subclasses. It doesn't work in reverse though. A subclass is an extension of its superclass, but a superclass is not an extension of a subclass. Example:






...would lead to each new instance of Vehicle AND Car to increase the count, since Car uses the Vehicle constructor, but a similar counter inside Car would not increase for a Vehicle being created.




I'm tired now, so that's it for now..Questions welcome! Comment, follow, subscribe, share etc, and see you tomorrow!


And as part of a shameless plug for a friend, if you're interested in classic movies/books/music, visit his site here (fixed), and feel free to throw loads of criticism at us. 

6 comments:

  1. Interesting stuff, I can't say if it worked for me, since I can't try this at the moment, but it looks like you're continuing on with the great teaching :)

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  2. Just realized I wasn't following you. D:

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  3. Very interesting, I'll have to keep checking back to see how to finish this code.

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  4. Each time i come here, i feel dumber. Not because your post are bad, but because i find out the little i know.

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  5. What is your end goal just out of curiosity

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  6. Normally I can understand this stuff but not today. I think I need a nap first.

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