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?
    }
}

How is the compiler supposed to know that T represents User? Perhaps you mean to implement the interface as implements RowMapper<User> instead? — Kirk Woll
– Kirk Woll, Commented Oct 7, 2010 at 18:10
I thought the compiler would know from the instantiation of the class, i.e. : new UserMapper<User>(); — MarkPenn
– MarkPenn, Commented Oct 7, 2010 at 18:16
But when you say the class is UserMapper<T>, you're saying that any type can be specified for a new instance of it. For example, new UserMapper<String>(). Yet the class will only ever return a User! — ColinD
– ColinD, Commented Oct 7, 2010 at 18:20

Michael D · Accepted Answer · 2010-10-07 18:11:39Z

12

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;
    }
}

answered Oct 7, 2010 at 18:11

Michael D

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Comments

duffymo · Accepted Answer · 2010-10-07 18:11:35Z

3

Try

public class UserMapper implements RowMapper<User> {

answered Oct 7, 2010 at 18:11

duffymo

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4 Comments

MarkPenn Over a year ago

Why isn't it UserMapper<User>?

duffymo Over a year ago

Because the generic only applies to RowMapper.

Carl Manaster Over a year ago

Because RowMapper wants to know the type, but UserMapper is only for that type. So UserMapper is telling RowMapper, "this is my type". But nobody else is allowed to tell UserMapper what its type is - it already knows it.

matt b Over a year ago

Here is how I often think of generics and translate something like UserMapper implements RowMapper<User> in my head: "UserMapper is a RowMapper of User"

unholysampler · Accepted Answer · 2010-10-07 18:13:24Z

3

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.

answered Oct 7, 2010 at 18:13

unholysampler

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Comments

Marvin Cohrs · Accepted Answer · 2010-10-07 18:10:47Z

0

Because T != User.

answered Oct 7, 2010 at 18:10

Marvin Cohrs

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