Question

我正在Java中向泛型类添加泛型。我在修复代码时遇到麻烦，以避免在方法root@VM-0-8-ubuntu:/usr/bin# python -V Python 3.7.2 root@VM-0-8-ubuntu:/usr/bin# pip3 -V Traceback (most recent call last): File "/usr/bin/pip3", line 9, in <module> from pip import main ModuleNotFoundError: No module named 'pip' root@VM-0-8-ubuntu:/usr/bin# sudo apt-get install python3-pip Reading package lists... Done Building dependency tree Reading state information... Done python3-pip is already the newest version (8.1.1-2ubuntu0.4). 0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded. root@VM-0-8-ubuntu:/usr/bin# pip3 -V Traceback (most recent call last): File "/usr/bin/pip3", line 9, in <module> from pip import main ModuleNotFoundError: No module named 'pip' root@VM-0-8-ubuntu:/usr/bin# sudo apt updaate E: Invalid operation updaate root@VM-0-8-ubuntu:/usr/bin# sudo apt update Hit:1 http://mirrors.tencentyun.com/ubuntu xenial InRelease Hit:2 http://mirrors.tencentyun.com/ubuntu xenial-security InRelease Ign:3 http://mirrors.aliyun.com/ubuntu trusty InRelease Hit:4 http://mirrors.tencentyun.com/ubuntu xenial-updates InRelease Hit:5 http://mirrors.aliyun.com/ubuntu trusty-security InRelease Hit:6 http://mirrors.aliyun.com/ubuntu trusty-updates InRelease Hit:7 http://mirrors.aliyun.com/ubuntu trusty-proposed InRelease Hit:8 http://mirrors.aliyun.com/ubuntu trusty-backports InRelease Hit:9 http://mirrors.aliyun.com/ubuntu trusty Release Reading package lists... Done Building dependency tree Reading state information... Done All packages are up to date. root@VM-0-8-ubuntu:/usr/bin# sudo apt upgrade Reading package lists... Done Building dependency tree Reading state information... Done Calculating upgrade... Done 0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded. root@VM-0-8-ubuntu:/usr/bin# sudo apt-get install python3-pip Reading package lists... Done Building dependency tree Reading state information... Done python3-pip is already the newest version (8.1.1-2ubuntu0.4). 0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded. root@VM-0-8-ubuntu:/usr/bin# pip3 -V Traceback (most recent call last): File "/usr/bin/pip3", line 9, in <module> from pip import main ModuleNotFoundError: No module named 'pip' root@VM-0-8-ubuntu:/usr/bin#

上出现这些“未经检查的转换”警告

警告：java：未经检查的演员
必需：java.util.ArrayList
发现：java.util.ArrayList
关于：
public ArrayList<T> getNextPage()

警告：java：未经检查的演员
必需：java.util.ArrayList
发现：java.util.ArrayList
关于：
return (ArrayList<T>) getPaginatedContracts(startRow, pageSize);

目前return (ArrayList<T>) getPaginatedJobs(startRow, pageSize);可以是T或Contract。

还有没有其他方法可以返回Job，而它正确地使用了泛型而没有任何警告？

或者因为我的构造函数使用ArrayList<T>并进行了一些类型检查：
Class<T>
我可以在if (clazz.isAssignableFrom(Contract.class)) {...} 上使用@SuppressWarnings("unchecked")吗？

getNextPage()

Answer 1

您可以将该类抽象化，然后子类必须实现实际的数据库调用：

public abstract class PaginationWrapper<T> {
    private int currentPage = 1;
    private int pageSize = 100;

    public ArrayList<T> getNextPage() {
        currentPage++;
        int startRow = currentPage * pageSize;

        // here we delegate to the implementation
        return getNextPage(startRow, pageSize);
    } 

    protected abstract ArrayList<T> getNextPage(int startRow, int pageSize);
}

然后创建2个子类

public class JobPagination extends PaginationWrapper<Job> {
    protected ArrayList<Job> getNextPage(int startRow, int pageSize) {
         return /* list from database */
    }
}

public class ContractPagination extends PaginationWrapper<Contract> {
    protected ArrayList<Contract> getNextPage(int startRow, int pageSize) {
         return /* list from database */
    }
}

现在，一般逻辑被打包到PaginationWrapper中，但是实际的数据库逻辑现在位于特定的子类中。

Java Unchecked Cast警告：必需ArrayList <t>，找到ArrayList <contract>

1 个答案: