Adding ChildClass object to ArrayList

Question

If we have a User base class with properties username and password.

And a child class SpecialUser, with an extra property special.

I know I can declare ArrayList<User> users = new ArrayList<User>(); and then add SpecialUser object to users, but I can only access the properties inherited from base class (username and password) that way.

Is there a way to add a SpecialUser to the ArrayList and be able to access special property? Or should they be stored seperately?

Kind regards

Rafael Paulino · Accepted Answer · 2018-03-02T19:02:41.977

4

Sure, you absolutely can!

Just after you get the object, cast it to SpecialUser:

SpecialUser spUser = (SpecialUser) users.get(0);

If you are not sure if the User is SpecialUser, test it before:

if (users.get(0) instanceof SpecialUser) {
    SpecialUser spUser = (SpecialUser) users.get(0);
    // ....
}

edited Mar 02 '18 at 19:02

answered Mar 02 '18 at 18:57

Rafael Paulino

570
2
9

score 0 · Answer 2 · answered Mar 02 '18 at 19:04

If you really have to, you can do it with instanceof:

User user = users.get(0);
if (user instanceof SpecialUser) {
    SpecialUser sUser = (SpecialUser) user; // class casting
    sUser.getSpecial(); // whatever subclass call you need
}

Do you really need this? Good polymorphism is about using the User -s methods and do something special in the subclasses.

Usually we prefer code without instanceof. But sometimes this is the right choice. It depends on your actual problem.

Adding ChildClass object to ArrayList

2 Answers2