Create list of Callables of Monos of different types [duplicate]

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I have a method, which works with a list of Callable of Mono of different types.

How do I call it with Callable<Mono<Integer>> and Callable<Mono<String>> ?

I tried

void mymethod(List<Callable<Mono<?>>> monoList){

}

void yourMethod1() {
    Callable<Mono<Integer>> callableInt = () -> Mono.just(1);
    Callable<Mono<String>> callableStr = () -> Mono.just("Hello");

    List<Callable<Mono<?>>> callableList = Arrays.asList(callableInt,callableStr);

    mymethod(callableList);
}

but this does not compile with error no instance(s) of type variable(s) exist so that Callable<Mono<Integer>> conforms to Callable<Mono<?>> inference variable T has incompatible bounds: equality constraints: Callable<Mono<?>> lower bounds: Callable<Mono<Integer>>, Callable<Mono<String>>

How can I change mymethod or yourMethod1 to get those objects in a list of Mono in mymethod ?

The reason your attempt doesn’t compile is due to the fact that generic types in Java are not covariant. In other words, Callable<Mono<Integer>> and Callable<Mono<String>> are not considered subtypes of Callable<Mono<?>>, even though Mono<Integer> and Mono<String> are subtypes of Mono<?>.

So, first option to fix it:

void yourMethod1() {
    Callable<Mono<?>> callableInt = () -> Mono.just(1); // change type to Callable<Mono<?>>
    Callable<Mono<?>> callableStr = () -> Mono.just("Hello"); // change type to Callable<Mono<?>>

    List<Callable<Mono<?>>> callableList = Arrays.asList(callableInt, callableStr);

    mymethod(callableList);
}

void mymethod(List<Callable<Mono<?>>> monoList) {
}

Again, since Mono<Integer> and Mono<String> are subtypes of Mono<?>, the second option is to change mymethod argument to List<Callable<? extends Mono<?>>>:

void yourMethod1() {
    Callable<Mono<Integer>> callableInt = () -> Mono.just(1);
    Callable<Mono<String>> callableStr = () -> Mono.just("Hello");

    List<Callable<? extends Mono<?>>> callableList = Arrays.asList(callableInt, callableStr);

    mymethod(callableList);
}

void mymethod(List<Callable<? extends Mono<?>>> monoList) { // change type of argument
    // Your implementation here
}

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