Java泛型，绑定不匹配：类型不是bounded参数的有效替代品

Question

I have a class that should return a list of objects that extend a BaseDto class.

So I wrote this

public class Worker<T extends BaseDto> {

   private T t;

     ArrayList<T> getList() {
      ....
     }  
}

and this is the object that should be returned in a list

public class MyDTO extends  BaseDto implements Serializable {
...
}

but when I write :

Worker<MyDTO> q = new Worker<MyDTO>();

it doesn't compile with the following error:

Bound mismatch: The type MyDTO is not a valid substitute for the bounded parameter of the type Worker

What's wrong?

Answer 1

Worker<MyDTO> q = new Worker<MyDTO>(); // this is fine

The code you provided is supposed to work, I tried it here and it compiles normally.

However, this line will not compile in Java:

Worker<BaseDto> dto = q; // this is not

Because generic types in Java are invariant , which means that two types List<X> and List<Y> are not compatible even if X or Y are subtypes from each other. In languages with support to covariance , this line would compile as MyDTO is a subtype of BaseDto.