Java通配符错误

Question

我有一个 BaseDao 接口(interface)，可以使用 hibernate 将数据持久化到 mysql

public interface BaseDao<T extends Serializable> {
    public void saveAll(Collection<T> objects) throws PersistenceException;
}

我有一个实体类 hirechy，它们的基类是 BaseActivity(所有扩展可序列化)其中之一:

public class Activity1 extends BaseACtivity{...}

在某些类中我声明了 ActivityDao:

 protected abstract ActivityDao<Activity1> createActivityDao1();

和一个方法:

 private void  persistData(ActivityDao<? extends BaseActivity> activityDao, Collection<? extends BaseActivity> data){
    EntityTransaction transaction = activityDao.getEntityManager().getTransaction();

    try {
        transaction.begin();

        activityDao.saveAll(data);
        transaction.commit();
    }
    catch (HibernateException ex) {

在 activityDao.saveAll(data) 我得到一个编译时异常:

 Error:(62, 24) java: method saveAll in interface com.matomy.persistence.dao.BaseDao<T>
 cannot be applied to given types;
 required: java.util.Collection<capture#1 of ? extends
 com.matomy.persistence.entity.activities.BaseActivity>
 found: java.util.Collection<capture#2 of ? extends   com.matomy.persistence.entity.activities.BaseActivity>
 reason: actual argument java.util.Collection<capture#2 of ? extends     com.matomy.persistence.entity.activities.BaseActivity>
 cannot be converted to java.util.Collection<capture#1 of ? extends     com.matomy.persistence.entity.activities.BaseActivity> 
 by method invocation conversion

基础 Activity :

public abstract class BaseActivity implements Serializable, Comparable<BaseActivity> {

请帮忙谢谢!

罗伊

更新:

private <T extends BaseActivity> void  persistData(ActivityDao<T> activityDao, Collection<T> col){
    EntityTransaction transaction = activityDao.getEntityManager().getTransaction();

    try {
        transaction.begin();

        activityDao.saveAll(col);
        transaction.commit();
    }
    catch (HibernateException ex) {

Answer 1

使用具有相同边界的两个有界通配符并不意味着它们是同一类型。在这种情况下，ActivityDao<? extends BaseActivity> 的实际类型参数可能与 Collection<? extends BaseActivity> 附带的不一样.所以，saveAll()方法行不通。

为了进一步详细说明，编译器用唯一的占位符替换了每个出现的通配符，这不过是对该通配符的捕获。这意味着，Collection<?>变成 Collection<CAP#1 of ?> , 同样 Collection<? extends T>变成 Collection<CAP#1 of ? extends T> .如果 Collection<?>再次遇到，将替换为Collection<CAP#2 of ?> .

因此，您的方法声明有点类似于:

private void  persistData(ActivityDao<CAP#1-of-? extends BaseActivity> activityDao, Collection<CAP#2-of-? extends BaseActivity> data){
    EntityTransaction transaction = activityDao.getEntityManager().getTransaction();

    try {
        transaction.begin();

        activityDao.saveAll(data);
        transaction.commit();
    }
    catch (HibernateException ex) {
    }
}

哪里CAP#1和 CAP#2是编译器的不同类型参数。当然是saveAll的签名对于那个方法变成这样:

public void saveAll(Collection<CAP#1-of-? extends BaseActivity> objects) throws PersistenceException;

自从 Collection<T1>与 Collection<T2> 不同 用于两个不同类型的参数，因此方法调用失败。

您可以改用通用方法解决此问题:

private <T extends BaseActivity> void persistData(ActivityDao<T> activityDao, Collection<T> data){