How to use a generic result as a generic parameter when the type is a wildcard?

generics, java, wildcard

Solution

When you use wildcard, compiler can't see that the return type of `x.makeT()` and the parameter type of `x.useT()` are the same. In order to guarantee that they are the same, you should use generic method here:

public class Foo { 
    public <T> void bar(Inter<T> x) { 
        x.useT(x.makeT()); 
    } 
}

Problem

[UPDATE] The real situation was a bit more complicated than my initial question made it seem. I've changed the code a bit to reflect that.[/UPDATE] I'm a bit stumped by the following behavior. Given code like: ``` interface Inter<T> { T makeT(); void useT(T t); } public class Foo { public void bar(Qux q) { Inter<?> x = getInterForQux(q); x.useT(x.makeT()); } Inter<?> getInterForQux(Qux q) { if( someTest(q) ) { return (Inter<Integer>) mkAnInterInt(); } else { return (Inter<Double>) mkAnInterDouble(); } } } ``` Javac gives me the error: useT(capture#478 of ?) in Inter<capture#478 of ?> cannot be applied to (java.lang.Object) Whereas Eclipse gives me: The method useT(capture#1-of ?) in the type Inter<capture#1-of ?> is not applicable for the arguments (capture#2-of ?) Obviously, no matter what `T` is the result type of `makeT()` is the same as the parameter type of `useT()`. Why can't I do this? Is there a workaround?

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