Generics and Callable causing peculiar error

Question

While working on generics and Callable i am getting following error:

The type of the expression must be an array type but it resolved to capture#1-of ? extends Object[]

 public class Test {

    public Object m1(Callable<? extends Object[]> abc) throws Exception {
        return abc.call()[0];//Getting error here
    }
}

I am really not sure what is producing this error.

It would be great if some one share knowledge on this error as i am not able to understand it.

It is not duplicate of above mentioned question , Please check

@RobinTopper--No No its not duplicate, Please check again I am asking from generics context and not about extending A[] — Sachin Sachdeva, May 02 '17 at 10:26
This error is because method `call()` returns type `? extends Object[]` which can't be cast to array. If you change to `Callable — Jay Smith, May 02 '17 at 10:29
Do you want to know "_what is producing this error_" or _how to solve it_? — Stefan Warminski, May 02 '17 at 10:49
Here is some background on the [interaction of generics and arrays](http://stackoverflow.com/a/18581313/2074605) and some [additional](http://stackoverflow.com/a/530289/2074605) [examples](http://stackoverflow.com/a/1817544/2074605) that may be helpful. — vallismortis, May 02 '17 at 10:51

Maurice Perry · Answer 1 · 2017-05-02T10:32:28.340

0

You can't extend an array. Just use Callable<Object[]>.

UPDATE:

Or, if it's the class of the elements that can be extended, You can do this:

public <T> T m1(Callable<T[]> abc) throws Exception {
    return abc.call()[0];
}

edited May 02 '17 at 10:32

answered May 02 '17 at 10:20

Maurice Perry

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`? extends Object[]` let him call this method with any array (e. g. `Callable) – Stefan Warminski May 02 '17 at 10:24
@StefanWarminski yes, I thought that was the idea, but he's doing it wrong (and so are you) – Maurice Perry May 02 '17 at 10:34
no, he's not. `return ((Object[]) abc.call())[0];` works fine. You're right that you don't write code that `Number[] extends Object[]` but the compiler does. – Stefan Warminski May 02 '17 at 10:41
@StefanWarminski the correct way to do this is in my update. – Maurice Perry May 02 '17 at 10:42

score 0 · Answer 2 · answered May 02 '17 at 10:21

I would suggest a little debugging from yourself

public class Test {
    public Object m1(Callable<? extends Object[]> abc) throws Exception {
        Object[] call = abc.call(); // Maybe your error is here
        System.out.println(call.length); // See what this returns
        return call[0];
    }
}

Also remember that when you say //Getting error here, tell us WHAT error you're getting.

Generics and Callable causing peculiar error

2 Answers2