如何以通用方式访问 Java 构造函数？

发布于 2024-10-01 16:20:35 字数 936 浏览 4 评论 0原文

我有一个用于类“Model”的静态构建器方法，它接受 JSON 字符串并返回模型的 ArrayList。我希望它一般地引用模型的构造函数，以便子类可以继承构建器方法。

public class Model
{
    protected int id;

    public Model(String json) throws JSONException 
    {
        JSONObject jsonObject = new JSONObject(json);
        this.id = jsonObject.getInt("id");
    }

    public static <T extends Model> ArrayList<T> build(String json) throws JSONException
    {
        JSONArray jsonArray = new JSONArray(json);

        ArrayList<T> models = new ArrayList<T>(jsonArray.length());

        for(int i = 0; i < jsonArray.length(); i++)
            models.add( new T(jsonArray.get(i)) )

        return models;
    }
}

这是该类的简化实现，相关行是

models.add( new T(jsonArray.get(i)) )

我知道这是不可能的，但我想编写一些调用 T 恰好是什么类型的构造函数的东西。我尝试使用 this() ，这显然不起作用，因为方法“build”是静态的，并且我尝试使用反射来确定 T 的类，但一直不知道如何获取它去工作。非常感谢任何帮助。

谢谢，

罗伊

原文

I have a static builder method for a class "Model" that takes a JSON string and returns an ArrayList of Models. I would like it refer to the Model's constructer generically so that subclasses can inherit the builder method.

public class Model
{
    protected int id;

    public Model(String json) throws JSONException 
    {
        JSONObject jsonObject = new JSONObject(json);
        this.id = jsonObject.getInt("id");
    }

    public static <T extends Model> ArrayList<T> build(String json) throws JSONException
    {
        JSONArray jsonArray = new JSONArray(json);

        ArrayList<T> models = new ArrayList<T>(jsonArray.length());

        for(int i = 0; i < jsonArray.length(); i++)
            models.add( new T(jsonArray.get(i)) )

        return models;
    }
}

This is a simplified implementation of the class, the relevant line being

models.add( new T(jsonArray.get(i)) )

I know this isn't possible, but I would like to write something that calls the constructor of whatever type T happens to be. I have tried to use this(), which obviously doesn't work because the method "build" is static and I've tried to use reflection to determine the class of T but have been at a loss to figure out how to get it to work. Any help is greatly appreciated.

Thanks,

Roy

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孤独陪着我 2024-10-08 16:20:35

使用泛型进行“动态实例化”的解决方法是向类或方法传递提示：

public class Model<T> {
  Class<T> hint;
  public Model(Class<T> hint) {this.hint = hint;}

  public T getObjectAsGenericType(Object input, Class<T> hint) throws Exception {
    return hint.cast(input);
  }

  public T createInstanceOfGenericType(Class<T> hint) throws Exception {
    T result = hint.newInstance();
    result.setValue(/* your JSON object here */);
    return result;
  }
}

我很乐意提供更多帮助/想法，但我不确定您想要实现什么目标实现您的技术解决方案。

（注意：示例有一些过于简化的异常处理）

The workaround for "dynamic instantiation" with generics is to pass a hint to the class or the method:

public class Model<T> {
  Class<T> hint;
  public Model(Class<T> hint) {this.hint = hint;}

  public T getObjectAsGenericType(Object input, Class<T> hint) throws Exception {
    return hint.cast(input);
  }

  public T createInstanceOfGenericType(Class<T> hint) throws Exception {
    T result = hint.newInstance();
    result.setValue(/* your JSON object here */);
    return result;
  }
}

I'm happy to provide more help/ideas but I'm not sure what you want to achieve with your technical solution.

(Note: Example has some over-simplified Exception handling)

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