Java List how to set and get children objects of a list of type parent

Question

If I had a linked list of parent objects like:

LinkedList<ParentClass> list = new LinkedList<ParentClass>();

And I wanted to fill them with different types of children objects (that all extend "ParentClass") how would I go about doing this. Also note that I don't know which particular child class will be used. For example I could try to do:

list.add(someInstanceOfAChildClass);

And then be able to access methods and attributes inside that child class. I can clarify if I'm not getting my point across. Thanks in advance!

Answer 1

You would have no problems to fill your List<Parent> with different Child objects but you cannot access Child's methods and attributes directly. If you try to assign a List member to a reference of type Child, you'll get a Type Mismatch error.

To get around this problem you can type cast the object returned by the List explicitly.

Child iThinkItsChild = (Child) listOfParents.get(indexOfChild);

But, to do it safely you should use the instanceof operator first.

Parent parent = listOfParents.get(indexOfChild);
if (parent instanceof Child) {
    Child imSureItsChildNow = (Child) parent;
    imSureItsChildNow.childMethod();
}

Answer 2

You just add your subclasses elements. As they extend ParentClass they can be added to any collections that holds ParentClass type.