泛型的集合“无法实例化ArrayList类型”？扩展OpDTO> "

问题描述

根据我读过的内容，我认为这是无法完成的，但我想确定一下。
我有一个类 $ b

  public   T 必须继承您的类扩展OpDTO>列表与LT; T> getLastOp（List< T> listDTOs）{
  

According to what I've read, I think this can't be done, but I'd like to be sure.I have a class OpDTO and several other *DTO extends OpDTO.Then, I want to have a method to extract just certain elements from lists of these child DTOs, and return the extracted elements in another list:

public List<? extends OpDTO> getLastOp (List<? extends OpDTO> listDTOs) {
        List<? extends OpDTO> last = new ArrayList<? extends OpDTO>(); //compile error: Cannot instantiate the type ArrayList<? extends OpDTO>

        //processing

        return last;
    }

I want ult to be a list of elements of the same kind as elements in listDTOs, and use only OpDTO's methods, but it produces a compile error:

I also tried doing something like:

public <T> List<T> getLastOp (List<T> listDTOs) {
        List<T> last = new ArrayList<T>();
        //processing

        return last;
    }

But then I can't enforce elements in listDTOs to be a subclass of OpDTO, and can't instantiate T.Any idea?

EDIT

It also occurred to me passing the type as parameter, then I can instantiate it. Would that be ok or is it some kind of bad practice?

private <T extends OpDTO> List<T> getLastOp (List<T> listDTOs, Class<? extends OpDTO> clazz) {
    List<T> ult = new ArrayList<T>();

    //processing
    OpDTO op = clazz.newInstance();
    //processing
    ult.add((T) op);
    op = listDTOs.get(i);

    return ult;
}

解决方案

List<? extends OpDTO> is a covariant view of List<T>; that means that any List<T> type can be converted to it, as long as the T matches.
You cannot create an instance of such a type; you can only use it as a variable or parameter type to hold existing instances of concrete generic types.

You need to create a generic method with a constraint that T must inherit your class:

public <T extends OpDTO> List<T> getLastOp (List<T> listDTOs) {

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