Can I have a list of 2 inherited types of classes in java?

Question

I have class Parent and a subclass Child. I want to make an ArrayList where I can store objects of both these classes. Is something like that possible?

I tried following:

ArrayList<Parent> list= new ArrayList<Parent>();
Parent a;
Parent b;
Child c;
list.add(a);
list.add(b);
list.add(c);

To this point the compiler didn't get me any errors, so I suppose it is ok. But when i try to use a method from Child on an object that i got from the ArrayList I am not able to use it! While

list.get(2).countNumbers(); 

This gets me an error, the method countNumbers is in Child.I am very confused, thank you in advance!

Answer 1

The ArrayList<Parent> is a container of Parent , so you can put any object that is a Parent in it. A Child is of cource a Parent , so the ArrayList<Parent> can receive it.

However, since your ArrayList<Parent> only knows that it contains Parent , it has no idea whether an object in it is a Child or not. So the compiler won't allow you to treat the elements in it as Child ren, though it may be a Child at runtime. So if you are sure that the Parent object is also a Child , you can downcast it by Child child = (Child) object; . Then you can treat it as a Child at compile time. But note that downcast is not type safe, and it may throw runtime exceptions.

Answer 2

You want to declare your List:

List<Parent> list = new ArrayList<>();

This will tell Java that you accept any Class extending or being Parent.

Since the List will contain Parent types, to use a Child object, you have to cast it:

Parent o = list.get(x);
if (o instanceof Child) {
    Child child = (Child) o;
    // Do something with child
}