Best way to determine proper downcast

Question

I have an abstract superclass called C and multiple subclasses of it: SubC1, SubC2, ... Now I have a collection of the superclass C and I want to iterate over it, but do something different depending on which subclass it is. Here is the pseudo-code:

abstract class C{}
class SubC1 extends C{}
class SubC2 extends C{}

...

for(C c : cs){
    if(c is a SubC1){
    //do one thing
    }
    if(c is a SubC2){ 
    //do another
    }
}

However Im not really sure how to set up the if conditionals. My first thought was to use instanceof but then I realized that it doesnt work for downcasts. Then I considered adding a String field to C and make each subclass define it and base my conditionals around that, but that seems clunky. I also considered trying to use process of elimination by forcing a downcast and catching the ClassCastExceptions, but that is also clunky and seems kinda dangerous. So how can I get what I want without having stupid code?

Edit: I forgot to add that it would not make sense for me to add a doSomething() function in C and just override it.

Edit 2: For some reason I thought doing c instanceof SubC1 caused a compiler error. Now that I realize it didnt I have my answer.

This is what polymorphism is for. Define an abstract method in the base class, and then put the specific behaviours in overrides in each subclass. — Oliver Charlesworth, Jun 20 '13 at 00:51
Take a look on this: http://www.artima.com/interfacedesign/PreferPoly.html — Cob013, Jun 20 '13 at 00:52
@OliCharlesworth I edited my question in response to your comment. — David says Reinstate Monica, Jun 20 '13 at 00:54

score 3 · Answer 1 · answered Jun 20 '13 at 00:52

3

Have an abstract method doSomething in C and override it in all of your subclasses, then call C#doSomething

answered Jun 20 '13 at 00:52

Zim-Zam O'Pootertoot

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I forgot to add, I cant really do that. It wouldn't make much sense. – David says Reinstate Monica Jun 20 '13 at 00:53
1

In that case you're stuck with `if(c instanceof SubC1) { SubC1 subc1 = (SubC1)c; }` – Zim-Zam O'Pootertoot Jun 20 '13 at 00:54
Oh... for some reason I thought that caused a compiler error. Nevermind then. – David says Reinstate Monica Jun 20 '13 at 00:56
Nope, it might cause a runtime error if `c` isn't a `SubC1`, but that's what the `if(c instanceof SubC1)` guard is for – Zim-Zam O'Pootertoot Jun 20 '13 at 00:56

Best way to determine proper downcast

1 Answers1