Unchecked cast from X to generic type that extends X

Question

I've been tasked with removing as many @SupressWarnings as possible in our codebase, and I'm not sure how to get around this particular issue.

I have this external method that returns a Serializable object, and a generic type T extends Serializable that I would like to cast the object to.

Here is a simplified version of the code:

class A <T extends Serializable> {

    public T someMethod() {
        Serializable result = someExternalMethod(...);
        T convertedObject = (T) result; // produces unchecked cast warning

        return convertedObject;
    }
}

Is it possible to perform this conversion without producing an unchecked cast warning (assuming the external method cannot be changed)?

This is Java 8.

Can you change the method so that it accepts a `Class`? It will then be easy to actually do the checking. As it stands, the warning is right on the money - you have no way of knowing that the method returns the correct object type. — RealSkeptic, Feb 11 '19 at 16:49
@RealSkeptic Yes that is a possibility, and that would fix my issue. I will hold out for other solutions though, as your solution would involve some tricky refactoring. — mario_sunny, Feb 11 '19 at 16:53
Are you sure that `(T) result` is a conform cast in your case? — dehasi, Feb 11 '19 at 16:53

score 3 · Accepted Answer · answered Feb 11 '19 at 16:57

3

To extend the answer of ferpel you can pass the type as a parameter

    class A <T extends Serializable> {

        public T someMethod(Class<T> type) {
            Serializable result = someExternalMethod(...);
            return type.cast(result);
        }
    }

answered Feb 11 '19 at 16:57

zoomout

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Or for greater generality: `public T someMethod(Class type)` if the caller knows that the result is a subtype of `T` and prefers to pass that subtype. It only removes the warning, of course: A `ClassCastException` will happen if `someExternalMethod` doesn’t return the expected type. – Ole V.V. Feb 11 '19 at 17:03

score 0 · Answer 2 · answered Feb 11 '19 at 16:45

0

try replacing the line that produces unchecked cast warning with

T convertedObject = anyClassYouWant.cast(result);

answered Feb 11 '19 at 16:45

ferpel

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Unchecked cast from X to generic type that extends X

2 Answers2