attempting to assign weaker access privileges; was public error for isCancelled

Firstly, you need braces around your method body.

Secondly, the way you're currently declaring it uses package access, but the isCancelled method is public, so you'd have to override it with another public method.

Thirdly, the method is final anyway, so you can't override it in the first place. It's not clear what you're trying to achieve, but this isn't the way to go.