使用Java泛型和接口的编译错误

Question

I'm attempting to make a Repository interface, that our business logic can use, with the idea that if we decide to change the data source that backs the repositories, that the business logic would not be affected. We have many clients that would be using this library, so we have begun making a suite of controllers that can be reused among clients. This is the repository interface:

package //redacted

import java.util.Collection;
import java.util.List;

public interface Repository<T extends Object, R extends RepositoryQuery<T>> {

  T add(T entity);

  Collection<T> add(Collection<T> entities);

  void remove(T entity);

  Collection<T> getAll();

  T get(Integer id) throws InvalidEntryException;

  List<T> get(Collection<Integer> ids);

  List<T> query(R query);

  List<T> query(T query);
}

This is the controller I'm having problems with:

package //redacted

import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;

import /* redacted */.entities.pointbank.PointBankBalance;
import /* redacted */.entities.user.User;
import /* redacted */.queries.PendingPointsBalance;
import /* redacted */.repository.Repository;
import /* redacted */.repository.RepositoryQuery;

public class RetrievePendingPointBalance { 

  private Repository<PointBankBalance, ? extends RepositoryQuery<PointBankBalance>> repository;
  private Constructor<? extends PendingPointsBalance> pendingQuery;

  public RetrievePendingPointBalance(Repository<PointBankBalance, ? extends RepositoryQuery<PointBankBalance>> repository,
          Constructor<? extends PendingPointsBalance> pendingQuery) {
    this.repository = repository;
    this.pendingQuery = pendingQuery;
  }

  public PointBankBalance execute(User user) throws InstantiationException, IllegalAccessException, IllegalArgumentException, InvocationTargetException {
      PendingPointsBalance query = pendingQuery.newInstance();
      query.setUser(user);
      return repository.query(query).get(0);
  }

}

I'm trying to create RepositoryQuery, which is what will be responsible for handling more than just CRUD operations. Here is the interface for that:

package //redacted

import java.util.List;

public interface RepositoryQuery<T extends Object> {

    List<T> execute();
}

The idea there, is that each query can extend can extend the interface with setters for the parameters it will need. Then each implementation can extend that query interface with specifics of what it will need (example: setJdbcTemplate(...) )

On the controller above though, the line:

      return repository.query(query).get(0);

is a compilation error, and I do not know why. Is my approach inherently flawed, or am I just missing something?

Full Error:

[ERROR] /Users/redacted/src/main/java/com/redacted/controllers/RetrievePendingPointBalance.java:[26,28] no suitable method found for query(com.redacted.queries.PendingPointsBalance)
    method com.redated.repository.Repository.query(capture#1 of ? extends com.redated.repository.RepositoryQuery<com.redacted.entities.pointbank.PointBankBalance>) is not applicable
      (argument mismatch; com.redacted.queries.PendingPointsBalance cannot be converted to capture#1 of ? extends com.redacted.repository.RepositoryQuery<com.redacted.entities.pointbank.PointBankBalance>)
    method com.redacted.repository.Repository.query(com.redacted.entities.pointbank.PointBankBalance) is not applicable
      (argument mismatch; com.redacted.queries.PendingPointsBalance cannot be converted to com.redacted.entities.pointbank.PointBankBalance)

And my error from Eclipse:

Edit: PointBankBalance interface

package //redacted

import com./* redacted */.entities.pointbank.PointBankBalance;
import com./* redacted */.entities.user.User;
import com./* redacted */.repository.RepositoryQuery;

public interface PendingPointsBalance extends RepositoryQuery<PointBankBalance> {

    void setUser(User user);

}

Answer 1

Your problem is that the method takes an R and you defined R as ? extends RepositoryQuery<PointBankBalance> ? extends RepositoryQuery<PointBankBalance> in the repository field declaration. It's uncertain which subclass ? refers to, so the compiler won't let you use a specific one. You could potentially set that field to an instance of Repository<PointBankBalance, AnotherPointsBalance> , in which case PendingPointsBalance wouldn't be a valid argument to its query method.

One way to fix it would be replacing Repository<PointBankBalance, ? extends RepositoryQuery<PointBankBalance>> Repository<PointBankBalance, ? extends RepositoryQuery<PointBankBalance>> with Repository<PointBankBalance, PendingPointsBalance> .

Or you could simplify things by removing the R parameter entirely:

public interface Repository<T extends Object> {

    ...

    List<T> query(RepositoryQuery<T> query);

    List<T> query(T query);
}

Then just replace Repository<PointBankBalance, ? extends RepositoryQuery<PointBankBalance>> Repository<PointBankBalance, ? extends RepositoryQuery<PointBankBalance>> with Repository<PointBankBalance> .