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Index › Programming Questions & Help › Syntax Questions › Constructors and Inheritance
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Constructors and Inheritance (Read 772 times)
phrontist
Constructors and Inheritance
Mar 3rd, 2010, 7:29am  
I've got a superclass that defines a dozen variables. Each subclass adds at most, one or two additional variables. How can I avoid copying and pasting what is essentially the same constructor in to each subclass? Is there some syntactic shortcut that will let me tack a few extra fields on the input variable list?
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PhiLho 
Re: Constructors and Inheritance
Reply #1 - Mar 3rd, 2010, 7:50am  
I think the Class default values thread can bring you some elements of answer.
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phrontist
Re: Constructors and Inheritance
Reply #2 - Mar 3rd, 2010, 8:02am  
Hmm... I'd seen that thread, and unless I'm misunderstanding it doesn't really address my problem.

Maybe a concrete example will help. Let's say I have a superclass like this:

Code:

class Sprite { 
  float x; 
  float y;
  String name;

  Sprite(float _x, float _y, String _name) {
    x = _x;
    y = _y;
    name = _name;
  }
}


Now I want to have a subclass, which will have all of those variables plus a few more...

Code:

class Agent extends Sprite { 
  color c;

  Agent(float _x, float _y, String _name, color _c) {
    x = _x;
    y = _y;
    name = _name;
    c = _c;
  }
}


It seems silly to write out all of those variable assignments again for the second class. Do I really have to do this? If I change the number or names of the variables in the superclass, I'd have to make changes to the constructor in every subclass.
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koogy
Re: Constructors and Inheritance
Reply #3 - Mar 3rd, 2010, 8:15am  
you can call the constructor for the class you're extending using super()

super(_x, _y, _name);
c = _c;

(i think, haven't tested it)

http://java.sun.com/docs/books/tutorial/java/IandI/super.html
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phrontist
Re: Constructors and Inheritance
Reply #4 - Mar 3rd, 2010, 8:22am  
That's helpful (and it does work, I've used it before), but I still have to write out all the temporary variables in the constructor definition for the subclass.
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PhiLho 
Re: Constructors and Inheritance
Reply #5 - Mar 3rd, 2010, 8:50am  
Yes, the super() wasn't mentioned in this thread, but I was more thinking of setters, with the chaining calls trick. Ie. you don't necessarily have to set all the variables at once in the constructor itself.

Java is a quite verbose language, it doesn't have syntax sugar to say something like Agent(<Sprite.parameters>, color _c) for example. Anyway, which Sprite constructor to take, if there are several?

The idea is to add setters in a fluid API way:
Agent a = (new Agent(7, 20, "Arthur")).setFrontColor(#55FF77).setBackColor(#110033).setSpeed(55).isMoving(tr
ue);
With an additional advantage of being slightly more explicit than a Agent a = new Agent(7, 20, "Arthur", #55FF77, #110033, 55, true); call, no? Closer of named parameters...
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