编译生成以下错误：“错误：不兼容的类型：推理变量 T 具有不兼容的等式约束 Todo,TodoCouchDB”

Question

I'm trying to learn Java by writing a little todo list console app that can save todos to either the filesystem or to CouchDB. Unfortunately, I'm hitting a compiler issue reading todos from CouchDB (using the LightCouch library) and have absolutely no idea how to move forward.

I have the following classes:

• Todo , TodoFilesystem , TodoCouchDB
• Listx , ListFilesystem , ListCouchDB

Here's the content of ListCouchDB :

public class ListCouchDB extends Listx {
    public void load() {
        CouchDbClient dbClient = new CouchDbClient();
        list = dbClient.view("_all_docs").includeDocs(true).query(TodoCouchDB.class);
    }
}

The list variable is an instance variable defined on the superclass:

public class Listx {
    public List<Todo> list = new ArrayList<Todo>();

    ...
}

Compiling this code throws the following compiler error:

ListCouchDB.java:12: error: incompatible types: inference variable T has incompatible equality constraints Todo,TodoCouchDB
        list = dbClient.view("_all_docs").includeDocs(true).query(TodoCouchDB.class);
                                                                 ^
  where T is a type-variable:
    T extends Object declared in method <T>query(Class<T>)
1 error

When I change query(TodoCouchDB.class) to query(Todo.class) in the ListCouchDB class it actually compiles and correctly populates my list; however, the list contains an array of Todo items and therefore if I try to call any methods on an item in the list it incorrectly calls the methods on the Todo superclass and not the TodoCouchDB subclass.

Apologies if there is insufficient info. here, but appreciate any input anyone is able to offer.

Answer 1

You may try to use generic types.

Define your Listx using generics

public class Listx<T extends Todo> {
 public List<T> list = new ArrayList<T>();
 ...
}

Define ListCouchDB as an extension of Listx<TodoCouchDB>

public class ListCouchDB extends Listx<TodoCouchDB> {
    public void load() {
        CouchDbClient dbClient = new CouchDbClient();
        list = dbClient.view("_all_docs").includeDocs(true).query(TodoCouchDB.class);
    }
}

Now, your list property is a list of TodoCouchDB instances