无法从非通用子类转换为通用超类

Question

I'm trying to get interface instance depending on what type T is. Place or something else that extends BaseDictionary.

public static <T extends BaseDictionary> IDictionaryDataSource<T> getEntityDataSourceInstance(Class<T> clazz,
        Context cxt) {
    if (Place.class.isAssignableFrom(clazz)) return (IDictionaryDataSource<T>) new PlaceDataSource(cxt);
            //here some other types, same lines

    return null;
}

public abstract class BaseDictionary{}

public class Place extends BaseDictionary{}

public interface IDictionaryDataSource<T extends BaseDictionary>{}

public abstract class BaseDictionaryDataSource<T extends BaseDictionary> implements   IDictionaryDataSource<T>{}

public class PlaceDataSource extends BaseDictionaryDataSource<Place>{}

And I get

Type mismatch: cannot convert from PlaceDataSource to IDictionaryDataSource<T>

or

Type safety: Unchecked cast from PlaceDataSource to IDictionaryDataSource<T>

if I cast it like above.

Can you explain why do compile error and warning occur?

It will be called here

public static <T extends BaseDictionary> DictionaryElementPickerFragment<T> newInstance(Class<T> clazz, Context cxt){
     //somecode here
     fragment.setDataSource(DictUtils.getEntityDataSourceInstance(clazz, cxt));
}

I've tried to find answer here and in google but no success.I would appreciate any help. Now I think like this

There is no helper method to work around the problem, because the code is fundamentally wrong.

Thanks in advance.

Answer 1

This more concrete example illustrates your problem which is one of type parameter variance .

void foo(List<String> stringList, Integer anInteger) {
  List<Object> objList = (List<Object>) stringList;
  objList.add(anInteger);  // Violation -- adding an object to a list of strings
                           // could cause someone getting a "String" to get an
                           // Integer stead
}

so a List<String> is not a List<Object> although it is a List<? extends Object> List<? extends Object> .

---

In your specific instance you can't cast

PlaceDataSource to IDictionaryDataSource<T>

PlaceDataSource is an IDictionaryDataSource<Place> , but the only thing we know about <T> is that it extends BaseDictionary which is a super-class of BaseDictionary .

So you can cast a PlaceDataSource to

1. an IDictionaryDataSource<Place> or to
2. an IDictionaryDataSource<? super Place> IDictionaryDataSource<? super Place> or to
3. an IDictionaryDataSource<? extends BaseDictionary> IDictionaryDataSource<? extends BaseDictionary>

but not to an IDictionaryDataSource<T> because T is not guaranteed to be Place , and doing so would lead to a mismatch between the actual type parameter Place and the formal type parameter T .

Answer 2

That is not secure implementation and it is dangerous to cast because ( Place.class.isAssignableFrom(clazz) saves you here):

T is a generic type and you're replacing it with definite Place .

What if I have

public class Home extends BaseDictionary{}
public class HomeDataSource extends BaseDictionaryDataSource<Home>{}

And then I invoke getEntityDataSourceInstance with Home class but get PlaceDataSource which cannot be cast to HomeDataSource (IDictionaryDataSource) which I expect. So I'll end up having ClassCastException .

Answer 3

It looks like getEntityDataSourceInstance should be an instance method in the BaseDictionary class, not a static method.

A subclass will know which type of DictionaryDataSource to create.

Answer 4

尝试@SuppressWarnings（“ unchecked”）