How can it be determined what actions and reactions an object in the system will be subjected to?

To determine what actions and reactions an object in a system will undergo, you can analyze its behavior and usage context. In object-oriented programming, this is usually done through:

• Interfaces and abstract classes that define a set of methods (actions) available to the object.
• The state of the object, which can influence its reaction (State pattern).
• Event-driven programming, where the object responds to events or messages.

For example, if you have an object "Button," its actions are pressing, and its reactions are executing the attached handler. In code, it might look like this:

interface Clickable {
    void onClick();
}

class Button implements Clickable {
    @Override
    public void onClick() {
        System.out.println("Button pressed");
    }
}

Thus, the behavior of an object is determined by its interface and internal logic.