Why doesn't these assignments work when casting with Generics?

[Ronnie Phelps]

List<Object> objectList = new ArrayList();
List<String> stringList = new ArrayList();


objectList = stringList; // This doesn't work

objectList = (List<Object>)stringList; //this doesn't work either
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Can anyone explain why these two assignments won't work and what needs to be done in order to perform this assignment?

[Henry Wong]

Ronnie Phelps wrote:List<Object> objectList = new ArrayList();
List<String> stringList = new ArrayList();

You actually should *not* be doing this -- assigning a non generics collection to a generics one. This is allowed in Java in order to maintain backward compatibility with Java 1.4 and eariler.

Ronnie Phelps wrote:objectList = stringList; // This doesn't work

It doesn't work because it is not a valid assignment. A list of strings can only hold string objects. If you are allowed to assign it to a reference of a list of objects, then you will be able to add any object type list, that another reference believes it to only hold strings.

Ronnie Phelps wrote:
objectList = (List<Object>)stringList; //this doesn't work either

Same reason as previous. Regardless, if you really want to break generics (by casting), then you can do it by casting it to a List that doesn't use generics first.

Henry

[Ronnie Phelps]

Thanks for your timely response Henry. I also noticed that the first assignment will work if I change the signature of the objectList to be

List<? extends Object> objectList = new ArrayList();

objectList = stringList; // does work now

I understand that it doesn't make since to use generics since we are extending Object. The types in this posting are String and Object in order to simplify my posting. I'm actually using other types in my code. Thanks again.