ArrayList of Objects That Inherit from Parent Classes in Java

Question

I have a class, controlBase, that is inherited by other child classes. How can I make a list of any of these child classes?

I have tried:

private ArrayList<? extends controlBase> controllers = new ArrayList();

However, this doesn't allow any classes to be added to it. With the above example, this throws an error:

public <T extends controlBase> void registerController(Class<T> c) {
    controllers.add(c);
}

An error is also thrown when I execute:

controllers.add(new controlBase());

or

controllers.add(new joystick()); // Joystick is a child-class of controlBase

This does not allow any classes to be added, even in the first example where it explicitly states that the class extends controlBase.

How can I make this arrayList be only of objects that extend the controlBase class?

Silvio Mayolo · Accepted Answer · 2022-07-01 02:11:25Z

2

You're misunderstanding the point of the call-site variance annotations. ? extends ControlBase is meant to imply that data can be safely read from the container as ControlBase but that nothing can be written to it. See my answer on a similar question for more details about when you would want to use this syntax, or PECS for a summary of the general rules.

In your case, you want, quite simply, ArrayList<ControlBase>. Every Joystick is a ControlBase, by the rules of inheritance, so you can add any subclass to that ArrayList whenever you like.

answered Jul 1, 2022 at 2:11

Silvio Mayolo

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1 Comment

Silvio Mayolo Over a year ago

Class<T> is a class, not an instance. ControlBase itself (as in, the idea of a control base) is an instance of Class<ControlBase>. In your first example, you're looking for void registerController(T c). You probably won't see Class<T> until you get into some more advanced Java, as it's only used for reflection.

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