Description Lukas Eder 2016-05-06 07:06:16 EDT

I run into this issue quite frequently, and then I have to resort to a lot of manual work, which I imagine might be possible for Eclipse to refactor automatically. For instance, I have the following method, which doesn't compile:

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    public static void swap(List<?> list) {
        list.set(0, list.set(1, list.get(0)));
        System.out.println(list);
    }
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It doesn't compile because of the JLS's wildcard capturing specification (which is a bit unfortunate). The "?" type returned from "list.get(0)" is the same "?" as the one from the "list" parameter, but it is a different "?" from the "?" that "list.set(1, ?)" takes as an argument, even if we're always operating on the same parameter "list".

Now, it would be great if I could select some code and refactor the code into a method that introduces a type variable just for that. The result would look like this:

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    public static void swap(List<?> list) {
        refactored(list);
        System.out.println(list);
    }

    private static <T> void refactored(List<T> list) {
        list.set(0, list.set(1, list.get(0)));
    }
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Of course, I could manually introduce this type variable, but let's assume that "swap" is some public API that I don't want to modify. In particular, I don't want the type variable to leak into the public API.