如何以通用方式访问Java构造函数？

Question

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

Answer 1

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)

Answer 2

The way it is written now, I can't see that the T type parameter in build() is of any use whatsoever. Can't you just drop it and use Model in its place? If so, that would solve your construction problem.