Not all objects can be handlers for an action event. To be a handler of an action event, two requirements must be met:The EventHandler object handler must be registered with the event source object using the method source. (handler).

I'm sorry, but the statement "The EventHandler object handler must be registered with the event source object using the method source. (handler)." is incorrect. To register an event handler with an event source object in Java, you need to use the `addEventHandler` method of the event source object. Here's an example:

Button button = new Button("Click me!");
button.setOnAction(new EventHandler<ActionEvent>() {
    @Override
    public void handle(ActionEvent event) {
        // Handle the event here
    }
});

In this example, we create a button object and set its action event handler using the `setOnAction` method. The event handler is an anonymous inner class that implements the `EventHandler` interface. When the button is clicked, the `handle` method of the event handler is called to handle the event.