Generic method that accept only part of classes

Question

Suppose I have following class structure:

class Base {}
class A extends Base {}
class B extends Base {}
class C extends Base {}

I want to write method, that accepts instances of A and B but not instances of C.
Can I achieve it in Java?

I know this is not good inheritance situation (A and B should have common parent different from C), but I am only curious is in Java way to handle situation like this.

EDIT:
!!!
I know that better inheritance will resolve problem. I am only curious, if Java have some standard mechanism to solve problem like that.
!!!

score 43 · Answer 1 · edited Dec 21 '12 at 18:18

43

Use an interface and have only A and B implement it.

edited Dec 21 '12 at 18:18

Paul Bellora

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answered Dec 21 '12 at 12:47

NimChimpsky

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score 13 · Answer 2 · answered Dec 21 '12 at 12:48

Can you use an interface? e.g.

class Base {}
interface MyGenericInterface {}
class A extends Base implements MyGenericInterface {}
class B extends Base implements MyGenericInterface {}
class C extends Base {}

That way, the method can accept implementations of MyGenericInterface, and since A and B implement the interface but C does not, it will accept instances of A and B but not C, as required.

Jimmy Thompson · Answer 3 · 2012-12-21T12:55:55.787

If something should accept A, B but not C then you should reconsider why C is inheriting from Base in the first place. You could add an extra layer of inheritance:

   ---Base---
   |        |
--Sub--     C
|     |
A     B

Then you could allow your method to only accept objects of type Sub. Sub doesn't even have to do anything, and they're all, technically, also Base.

A quicker solution would be a dirty if( arg.instanceof(C) ) though...

score 8 · Answer 4 · answered Dec 21 '12 at 12:49

8

You can handle it at runtime using exception:

void genericMethod(Base arg_in)
{
    if(arg_in instanceof C)
    {
        throw IllegalArgumentException("C class not accepted."); 
    }

    ...
    ...
}

answered Dec 21 '12 at 12:49

Azodious

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score 8 · Answer 5 · edited Dec 23 '12 at 03:08

8

A type-hierarchy should not be changed only because you want to have such a method. But the case that you want to have such a method may indicate you should think over your type-hierarchy.

But if you are sure your types are well designed, what about this approach:

    public class T {

        public void doSomething(final A a) {
            doSomthing(a);
        }

        public void doSomething(final B b) {
            doSomthing(b);
        }

        private void doSomthing(final Base b) {

            // Here is the implementation

        }

    }

edited Dec 23 '12 at 03:08

Daniel Pryden

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answered Dec 21 '12 at 13:11

MrSmith42

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I don't know what you were intending with the `final` keyword there, but it won't prevent a widening conversion from occurring at the call site. Since every instance of `C` is an instance of `B`, it will always be possible to pass instances of `C` into your `doSomething()` method. – Daniel Pryden Dec 22 '12 at 01:32
Why do you think "C is an instance of B" ? A,B,C all only extend Base – MrSmith42 Dec 22 '12 at 14:01
'final' has nothing to do wthe the functionality. Just a codeing convention I am used to. – MrSmith42 Dec 22 '12 at 14:03
Hmmm... on revisiting this, it appears I misread the question. My apologies. – Daniel Pryden Dec 23 '12 at 03:06

score 3 · Accepted Answer · edited May 23 '17 at 10:29

3

I guess the answer to your questions is "No, java does not have a standard way of dealing with such a problem". And if you have this problem then you can look at other answers here, like fixing the inheritance or using the clever technique given by MrSmith42.

edited May 23 '17 at 10:29

Community

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answered Dec 21 '12 at 13:16

Waqas Ilyas

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@MichałHerman correct ? 17 - 1 ... blimey, this is like being at work. – NimChimpsky Dec 21 '12 at 15:17
@NimChimpsky don't worry, there's badges for this sort of thing. :) – Mike G Dec 21 '12 at 16:15
3

Yes, I for me this answer is correct, because I asked IF Java have some build mechanism to deal this kind of problems. Not HOW to deal with this type of problems by myself, because this I already know. – Michał Herman Dec 21 '12 at 19:43

score 2 · Answer 7 · answered Dec 21 '12 at 12:47

2

You can use instanceof to check if an object is of a certain type.

Better idea would be better inheritance structure, of course.

answered Dec 21 '12 at 12:47

gefei

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To be clear: In Java, `instanceof` works for subclasses. That is, `C c = new C(); c instanceof B` will return true. If you want to exclude `C`, you need to check for it explicitly. (Adding a comment because at first glance this answer wasn't clear to me.) – Daniel Pryden Dec 22 '12 at 01:34

score 2 · Answer 8 · answered Dec 21 '12 at 19:48

2

Generics is for type safety. Period. Not to "allow" or "disallow" arbitrary things. There is no type-safety reason to allow A and B and not C.

answered Dec 21 '12 at 19:48

newacct

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score 0 · Answer 9 · answered Mar 18 '21 at 06:39

0

Try below snippet

public <T extends Base> void setAuditorDetails(T model) 
{
         if(!(T instanceof C)){

              // Your Business logic

         }
}

answered Mar 18 '21 at 06:39

Vikram

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Generic method that accept only part of classes

9 Answers9