Type mismatch: cannot convert from ArrayList<Data> to MyCollection
I've read similar questions here but I'm still a little confused.
MyCollection extends ArrayList<MyClass>
MyClass implements Data
yet this gives me the "cannot convert from ArrayList to MyCollection"
MyCollection mycollection = som开发者_运维技巧ehandler.getCollection();
where getCollection looks like this
public ArrayList<Data> getCollection()
So my assumptions are obviously wrong. How can I make this work like I would like it to
ArrayList
does not extend MyCollection
. It's the other way round.
There are several ways to fix this problem, depending on the functional requirement:
- You need to provide a
MyCollection
constructor which can take anotherCollection
. - If the actual type is
MyCollection
, then you need to castArrayList
toMyCollection
. - If the actual type is
MyCollection
, then you need to change return type toMyCollection
.
That said, what makes MyCollection
so different that it apparently doesn't adhere the List
's contract and you hence need to cast/convert it? This shouldn't happen, I would rethink the design/approach.
You can not assign to subtype from super type, but you can implement a Copy Constructor, then you could do the same thing like this:
MyCollection myCollection = new MyCollection(somehandler.getCollection());
Edit: the type of the parameter to the constructor could be for example List.
Well, imagine you have
class SecondCollection extends ArrayList<MyClass>
then what would ArrayList<MyClass>
be cast to?
You need to either expect ArrayList
or return MyCollection
, or copy the contents of the ArrayList
to a new instance of MyCollection
, or explicitly cast to MyCollection
. I don't recommend the last option.
you just need to cast the result
MyCollection mycollection = (MyCollection) somehandler.getCollection();
edit : or change the return type of getCollection()
to a MyCollection
Check out page 4 of the generics tutorial.
ArrayList<Data>
cannot be cast to MyCollection
or even to ArrayList<MyClass>
. See this post for a good explanation.
精彩评论