Unable to assign the result returned by the Method that receives a generic List and returns a List<?>

Question

I'm trying to filter a List of objects that implements an interface. And I'm trying to create a generic method for all the classes.

Something like:

interface SomeInterface {
    String getFlag();
}


class SomeObject implements SomeInterface {
    public String getFlag() {
        return "X";
    }
}


List<SomeObject> someObjectList = new ArrayList<>();

// Compilation error here
List<SomeObject> filterList = filterGenericList(someObjectList, "X");


private List<?> filterGenericList(List<? extends SomeInterface> objects, String flag) {
        return objects.stream()
                .filter(it -> it.getFlag().equals(flag))
                .collect(Collectors.toList());
}

How do I run away from the compilation Error?

 Incompatible types. 
 Found: 'java.util.List<capture<?>>', 
 Required: 'java.util.List<SomeObject>'

Answer 1

When you return List<?> that means the method returns a list of some unknown type. Java doesn't know that it's the same type of list as the input list. To signal that, create a generic type T and have both the input and output lists be List<T>.

private <T extends SomeInterface> List<T> filterGenericList(List<T> objects, String flag) {
    return objects.stream()
        .filter(it -> it.getFlag().equals(flag))
        .collect(Collectors.toList());
}

Answer 2

Unbounded wildcard <?> also called the unknown type is a way to tell the compiler that a generic type can't be predicted, and therefore the compiler will disallow to assign a list of unknown type to a list of SomeObject because it's impossible to ensure that this assignment is safe.

If you have a list of objects that belong to different classes implementing SomeInterface in order to distinguish between them, you can explicitly pass an instance of target class Class<T> into a method as an argument.

Seems like you're trying to reinvent the same mechanism by utilizing method getFlag(). It doesn't look reliable because nothing can stop you from making a typo or accidentally implementing getFlag() in two different classes in such a way that it'll return the same value.

private <T extends SomeInterface> List<T> filterGenericList(List<? extends SomeInterface> objects, 
                                                            Class<T> targetClass) {
    return objects.stream()
        .filter(it -> targetClass.isAssignableFrom(it.getClass()))
        .map(targetClass::cast)
        .collect(Collectors.toList());
}

Answer 3

first of all List<?> doesn't have any type information. You expect a List of SomeObject but your method just returns a List of any type.

To achieve your goal you should define your filter method as

private static  <T extends SomeInterface> List<T> filterGenericList(List<T> objects, String flag)

In this way you guarantee that the returned list is of the same type like the input list