[英]Java generics type parameters
Consider the following code: 考虑以下代码:
public abstract class MyObject {
}
public interface Dao<T extends MyObject> {
}
public abstract class JsonDao<T extends MyObject> implements Dao<T> {
}
public abstract class SqliteDao<T extends MyObject> implements Dao<T> {
}
public interface DaoFactory<*????*> {
public *????* getDao(Class<? extends MyObject> objectClass);
}
Now what I'd like is for getDao to be able to instanciate both JsonDao and SqliteDao, but parametrized with a class extending MyObject, basically something like this: 现在我想要的是getDao能够实例化JsonDao和SqliteDao,但是通过扩展MyObject的类进行参数化,基本上是这样的:
public interface DaoFactory<T<? extends MyObject> extends Dao<? extends MyObject>> {
public <U extends MyObject> T<U> getDao(Class<U> objectClass);
}
Is something similar even possible or does type erasure prevent it? 类似的事情是否可能发生,还是类型擦除阻止了它?
Does this fit your needs? 这符合您的需求吗?
public interface DaoFactory<O extends MyObject, D extends Dao<O>> extends Dao<O> {
public D getDao(Class<O> objectClass);
//or for any subclass
public <RO extends O, RD extends D<RO>> RD getSubDao(Class<RO> objectSubclass);
//or maybe this
public <RO extends O> Dao<RO> getSimpleSubDao(Class<RO> objectSubclass);
}
You could declare DaoFactory
as follows: 您可以声明
DaoFactory
如下:
public interface DaoFactory<D extends Dao<? extends MyObject>> {
<T extends MyObject> D getDao(Class<T> type);
}
Then, JsonDaoFactory
could be implemented as follows: 然后,可以如下实现
JsonDaoFactory
:
public class JsonDaoFactory implements DaoFactory<JsonDao<? extends MyObject>> {
private final Map<Class<? extends MyObject>, JsonDao<? extends MyObject>> daos =
new HashMap<>();
public JsonDaoFactory() {
daos.put(Contact.class, new JsonContactDao());
daos.put(Customer.class, new JsonCustomerDao());
}
@Override
@SuppressWarnings("unchecked")
public <T extends MyObject> JsonDao<T> getDao(Class<T> type) {
return (JsonDao<T>) daos.get(type);
}
}
I've used a Map
to make every descendant of MyObject
match its corresponding JsonDao
(here I've used Contact
and Customer
, and JsonContactDao
and JsonCustomerDao
, respectively). 我使用
Map
使MyObject
每个后代与其对应的JsonDao
匹配(这里我分别使用Contact
和Customer
,以及JsonContactDao
和JsonCustomerDao
)。 I've suppressed an unchecked cast warning because I'm explicitly filling the map in the constructor, so I'm sure there won't be a ClassCastException
. 我取消了未选中的强制转换警告,因为我在构造函数中明确填充了地图,因此我确定不会出现
ClassCastException
。
For completeness, here's the code for SqliteDaoFactory
: 为了完整
SqliteDaoFactory
,这是SqliteDaoFactory
的代码:
public class SqliteDaoFactory implements DaoFactory<SqliteDao<? extends MyObject>> {
private final Map<Class<? extends MyObject>, SqliteDao<? extends MyObject>> daos =
new HashMap<>();
public SqliteDaoFactory() {
daos.put(Contact.class, new SqliteContactDao());
daos.put(Customer.class, new SqliteCustomerDao());
}
@Override
@SuppressWarnings("unchecked")
public <T extends MyObject> SqliteDao<T> getDao(Class<T> type) {
return (SqliteDao<T>) daos.get(type);
}
}
The code for both factories is almost duplicated, but I couldn't find a way to solve this. 两个工厂的代码几乎都是重复的,但是我找不到解决此问题的方法。
Here's how to use these factories: 这是使用这些工厂的方法:
JsonDaoFactory jsonDaoFactory = new JsonDaoFactory();
JsonDao<Contact> contactJsonDao = jsonDaoFactory.getDao(Contact.class);
JsonDao<Customer> customerJsonDao = jsonDaoFactory.getDao(Customer.class);
SqliteDaoFactory sqliteDaoFactory = new SqliteDaoFactory();
SqliteDao<Contact> contactSqliteDao = sqliteDaoFactory.getDao(Contact.class);
SqliteDao<Customer> customerSqliteDao = sqliteDaoFactory.getDao(Customer.class);
声明:本站的技术帖子网页,遵循CC BY-SA 4.0协议,如果您需要转载,请注明本站网址或者原文地址。任何问题请咨询:yoyou2525@163.com.