CODEWITHSUNDEEP
javagenerics
MarkPenn
MarkPenn

Reputation: 53

Implementing interface with generic type

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?
    }
}

Upvotes: 4

Views: 6194

Answers (4)

unholysampler
unholysampler

Reputation: 17321

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.

Upvotes: 3

Michael D
Michael D

Reputation: 1942

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;
    }
}

Upvotes: 12

duffymo
duffymo

Reputation: 308743

Try 

public class UserMapper implements RowMapper<User> {

Upvotes: 3

Marvin Cohrs
Marvin Cohrs

Reputation: 21

Because T != User.

Upvotes: 0

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