决定在执行时实例化哪个子类

发布于 2024-11-02 07:27:07 字数 630 浏览 4 评论 0原文

我有一个带有一些子类的 Forum 类，并将子类名称与所有其他字段一起作为字符串字段存储在数据库中。当我检索该对象时，我希望它是同一类的实例。我可以做很多 if-else 语句来查找要调用的子类构造函数，但它不容易扩展。

我找到了这个解决方案，但感觉有点脏：

    public static Forum createForum(int forumId, String kind) {
         try {
           Class cls = Class.forName("forum."+kind);
           Constructor ct = cls.getConstructor(Integer.TYPE, String.class);
           Object retobj = ct.newInstance(forumId, kind);
           return (Forum) retobj;
         }
         catch (Throwable e) {
            System.err.println(e);
         }
         return null;
    }

有更好的解决方案吗？

谢谢。

原文

I have a Forum class with some subclasses, and I store the subclass name as a String field on a database along all other fields. When I retrieve the object, I want it to be an instance of the same class it was. I could do a lot of if-else statements to find which subclass constructor to call, but it wouldn't be easily extensible.

I have found this solution, but it feels somehow dirty:

    public static Forum createForum(int forumId, String kind) {
         try {
           Class cls = Class.forName("forum."+kind);
           Constructor ct = cls.getConstructor(Integer.TYPE, String.class);
           Object retobj = ct.newInstance(forumId, kind);
           return (Forum) retobj;
         }
         catch (Throwable e) {
            System.err.println(e);
         }
         return null;
    }

Is there any better solution?

Thanks.

分享到QQ

分享到微博

如果你对这篇内容有疑问，欢迎到本站社区发帖提问参与讨论，获取更多帮助，或者扫码二维码加入 Web 技术交流群。

发布评论

需要登录才能够评论，你可以免费注册一个本站的账号。

游魂 2024-11-09 07:27:07

我可以做很多 if-else 语句来查找要调用的子类构造函数，但它不容易扩展。

一种替代方法是为 Forum 类创建一个“工厂对象”接口，然后使用工厂实例填充映射以创建每个子类。

public interface ForumFactory {
    public Forum createForum(String arg);
}

public class FooForumFactory implements ForumFactory {
    public Forum createForum(String arg) { return new FooForum(arg); }
}

Map<String, ForumFactory> factories = new HashMap<String, ForumFactory>();
factories.put("foo", new FooForumFactory());
factories.put("bar", new BarForumFactory());

...

// Create an instance ...
Forum forum = factories.get(forumType).createForum(forumName);

如果不同的子类构造函数需要不同的参数，事情会变得更加困难。但这对于您的反思解决方案来说也会有问题。

从长远来看，使用 ORM（如 Hibernate）可能会提供更优雅的解决方案。

I could do a lot of if-else statements to find which subclass constructor to call, but it wouldn't be easily extensible.

One alternative is to create a "factory object" interface for your Forum classes, and then populate a map with factory instances to create each of the subclass.

public interface ForumFactory {
    public Forum createForum(String arg);
}

public class FooForumFactory implements ForumFactory {
    public Forum createForum(String arg) { return new FooForum(arg); }
}

Map<String, ForumFactory> factories = new HashMap<String, ForumFactory>();
factories.put("foo", new FooForumFactory());
factories.put("bar", new BarForumFactory());

...

// Create an instance ...
Forum forum = factories.get(forumType).createForum(forumName);

Things would be more difficult if different subclass constructors required different arguments. But that would also be problematic for your reflective solution.

In the long term, using an ORM (like Hibernate) is likely to give a more elegant solution.

回复收藏 0 原文