Java generic interface and java generic interface

Java generic interface and java generic interface

Recently, a third-party class library has encountered a wildcard problem. I was a little confused, so I recorded it

public interface QueryApi<T extends QueryApi<?,?>,U>

public abstract class  QueryApiImpl<U> implements QueryApi<QueryApiImpl<?>,U> 

public interface TaskQueryApi<T extends OrderFlow>  extends QueryApi<TaskQueryApi<?>, T> 

Public class TaskQueryApiImpl <T extends OrderFlow> extends QueryApiImpl <T> implements TaskQueryApi <T>

Above, compilation Error
The problem is that public interface TaskQueryApi <T extends OrderFlow> extends QueryApi <TaskQueryApi <?>, T> this sentence.
Change

Public interface TaskQueryApi <T extends OrderFlow> extends QueryApi <QueryApiImpl <?>, T> then it is normal.

Cause Analysis: TaskQueryApi implements QueryApi. The first generic parameter is TaskQueryApi.
QueryApiImpl also implements QueryApi. The first generic parameter is QueryApiImpl.
Therefore, when TaskQueryApiImpl implements TaskQueryApi, it also integrates QueryApiImpl. In this way, there are two upstream generic parameters, but the generic instance types are not the same, resulting in an error.
Of course, TaskQueryApi does not integrate QueryApi. In this way, there is a QueryApi method in the implementation class, but there is no interface definition.