Comment passer la liste vide avec le paramètre de type? Comment passer la liste vide avec le paramètre de type?

Question

class User{ 
    private int id; 
    private String name; 

    public User(int id, String name) { 
     this.id = id; 
     this.name = name; 
    } 
} 

class Service<T> { 
    private List<T> data; 
    public void setData(List<T> data) { 
     this.data = data; 
    } 
} 

public class ServiceTest { 
    public static void main(String[] args) { 
     Service<User> result=new Service<User>(); 
     result.setData(Collections.emptyList()); // problem is here 
    } 
} 

compilateur

me donner un message d'erreur:

The method setData(List< User >) in the type Service is not applicable for the arguments (List< Object >)

et si je tente de lancer avec liste l'erreur:

Cannot cast from List< Object > to List< User >

result.setData(new ArrayList<User>()); fonctionne très bien mais je ne veux pas passer.

Answer 1

Collections.emptyList() est générique, mais vous l'utilisez dans sa version brute.

Vous pouvez définir explicitement le type paramètre avec:

result.setData(Collections.<User>emptyList()); 

Answer 2

simplement result.setData(Collections.<User>emptyList());

Answer 3

The issue you're encountering is that even though the method emptyList() returns List, you haven't provided it with the type, so it defaults to returning List. You can supply the type parameter, and have your code behave as expected, like this:

result.setData(Collections.<User>emptyList()); 

Now when you're doing straight assignment, the compiler can figure out the generic type parameters for you. It's called type inference. For example, if you did this:

List<User> emptyList = Collections.emptyList(); 

then the emptyList() call would correctly return a List.