强制类型转换问题

Question

[code="java"]CategoryDao dao = new CategoryDaoImpl();[/code]很好理解

但是下面代码我几有点迷惑了
[code="java"]package cn.factory;

import cn.dao.CategoryDao;
import cn.dao.impl.CategoryDaoImpl;
import cn.domain.Category;

public class DaoFactory {

private DaoFactory(){}
private static DaoFactory instance = new DaoFactory();

public static DaoFactory getinstance(){
    return instance;          
}


public <T> T createDao(String classname,Class<T> clazz){

    try {
        T t = (T) Class.forName(classname).newInstance();
        return t;
    } catch (Exception e) {
        throw new RuntimeException(e);
    }
}

}[/code]

不用泛型
[code="java"]
public CategoryDao createCategoryDao(String classname){

    try {
        CategoryDao dao = (CategoryDao) Class.forName(classname).newInstance();
        return dao;
    } catch (Exception e) {
        throw new RuntimeException(e);
    } 

}

[/code]

得到实例
[code="java"]
CategoryDao dao = DaoFactory.getinstance().createDao("cn.dao.impl.CategoryDaoImpl",CategoryDao.class);
//CategoryDao dao = DaoFactory.getinstance().createDao("cn.dao.impl.CategoryDaoImpl");
[/code]

很明显Class.forName(classname).newInstance()得到是接口实现类为什么还需要强转

想了很久[code="java"]CategoryDao dao = new CategoryDaoImpl();[/code]两个感觉矛盾了 :cry: .
求解... :oops:

Answer 1

动作还是慢了，为了确保能拿到这分，其实你的DaoFactory可以再完善一点
[code="java"]
public class DaoFactory {

private DaoFactory(){}   
private static DaoFactory instance = new DaoFactory();   

public static DaoFactory getinstance(){   
    return instance;             
}   


public <T> T createDao(Class<T> clazz){   

    try {   
        return  (T) Class.forName(clazz.getName).newInstance();  
    } catch (Exception e) {   
        throw new RuntimeException(e);   
    }   
}   

}
[/code]