Implementing interface with generic type

Question

I'm trying to implement Spring's RowMapper interface, however, my IDE is prompting me to cast the return object to "T" and I don't understand why. Can anyone explain what I'm missing?

public class UserMapper<T> implements RowMapper<T> {
    public T mapRow(ResultSet rs, int row) throws SQLException {
        User user = new User();
        user.firstName(rs.getInt("fname"));
        user.lastName(rs.getFloat("lname"));
        return user; // Why am I being prompted to cast this to "T", should this be fine?
    }
}

Answer 1

Because T != User.

Answer 2

Try

public class UserMapper implements RowMapper<User> {

Answer 3

If a row maps to a User, then it should be a RowMapper<User>

ie:

public class UserMapper implements RowMapper<User> {
    public User mapRow(ResultSet rs, int row) throws SQLException {
        User user = new User();
        user.firstName(rs.getInt("fname"));
        user.lastName(rs.getFloat("lname"));
        return user;
    }
}

Answer 4

The compiler does not know anything about T. Therefore, it is asking you to cast User to T. If you are only planning on using T as a type of User you can use the following to restrict the generic type and give the compiler more information.

public class UserMapper<T extends User> implements RowMapper<T>
...

If your code actually looks like that, you are always returning User and it is not dependent on T. Therefore you should just make the return type User and not T.