cannot find method in generic class

Question

I am trying to create a function that takes in a generic Class, does some work with a function of that Class, then return an instance of said Class.

I'm getting a compiler error: error: cannot select from a type variable - [ERROR] T.getManagers(), T.class);. Here is the function:

public static <T extends Bar> T getFoo(Class<T> fooClass , String person)
{
    T foo = null;
    try
    {
        Configuration config = Configuration.getConfiguration(person);
        foo = (T) Bar.getInstance(config,T.getManagers(), T.class);
    }
    catch (Exception e)
    {
        logger.error("Error", e);
    }
    return foo;
}

EDIT: Is there a way I can get around calling Bar.getManagers()? What I'm trying to do with generics is that I have say 5 classes very similar to Bar but not exactly the same. They all have getManagers(). By using a generic like T.getManagers() I'm trying to get around calling the explicit names in the code and using a generic one instead.

You cannot call a static on a generic. There is no way to ensure that every implementation of Bar implements a static getManagers() method. — jgitter, Apr 17 '14 at 13:57
The `T.getManagers()` should probably be `Bar.getManager()`. And `T.class` should probably be `fooClass`. But you might have to post your `Bar` class and the `getInstance` method for more focussed help. — Marco13, Apr 17 '14 at 13:59
Please post the code of `Bar`, and in particular `getInstance()`. I definitely agree with @Marco13, we can't really help you with what you currently provided. — Joffrey, Apr 17 '14 at 14:19

Joffrey · Answer 1 · 2014-04-17T14:17:02.120

2

You're trying to use constructs that are not allowed in Java:

You can't use T.getManagers(), because it is a static method, and the class T won't be available at runtime (type erasure). If the getManagers() method exists in class Bar (and it should*), then use Bar.getManagers() instead.
You can't use T.class for the same reason, the class T won't be available at runtime. Use fooClass instead.

The code you provided should look like:

public static <T extends Bar> T getFoo(Class<T> fooClass , String person) {
    T foo = null;
    try {
        Configuration config = Configuration.getConfiguration(person);
        foo = (T) Bar.getInstance(config, Bar.getManagers(), fooClass);
    } catch (Exception e) {
        logger.error("Error", e);
    }
    return foo;
}

(*) This method should be in Bar because it seems to be a common behaviour that you use here, and that you seem to expect from all Ts that could extend Bar. There is no such thing as inheritance for static methods.

edited Apr 17 '14 at 14:17

answered Apr 17 '14 at 14:01

Joffrey

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I tried to use fooClass in place of T and got this error: `error: cannot find symbol [ERROR] fooClass.getManagers(), fooClass.class);`. – gjw80 Apr 17 '14 at 14:06
Try `fooClass`, not `fooClass.class` – Joffrey Apr 17 '14 at 14:07
`fooClass.getManagers()` is forbidden. You should use `Bar` instead, as I stated in my answer. – Joffrey Apr 17 '14 at 14:09
I can't use Bar, the point of the method is there are 4 other classes I need to call and I'm trying to do it with a generic. So if I always call `Bar` then when I need to get a manager for `XYZ` I'm still calling `Bar`'s getManagers(), not XYZ's. – gjw80 Apr 17 '14 at 14:13
You can't use static methods this way. Use instance methods instead, then you'll be able to use inheritance. I think you're misusing generics here, I don't understand what you're trying to do. It's hard to give design advice without more info on your use case. Please provide your actual code. – Joffrey Apr 17 '14 at 14:15

score 1 · Answer 2 · edited May 23 '17 at 12:11

1

T.class is prohibited construction, you cannot use it because of type erasure. Please take a look on a workaround

edited May 23 '17 at 12:11

Community

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answered Apr 17 '14 at 13:59

nikis

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score 0 · Answer 3 · answered Apr 17 '14 at 15:11

You can use reflection on the fooClass object to obtain and execute the getManagers() method. It will fail at runtime if the class doesn't have the method though.

public static <T extends Bar> T getFoo(Class<T> fooClass , String person)
{
    T foo = null;
    try
    {
        Configuration config = Configuration.getConfiguration(person);
        Method getManagers = fooClass.getDeclaredMethod("getManagers");
        foo = (T) Bar.getInstance(config, getManagers.invoke(null), T.class);
    }
    catch (Exception e)
    {
        logger.error("Error", e);
    }
    return foo;
}

This is not a great practice, but should work.

cannot find method in generic class

3 Answers3