That would depend on what the boolean variable is used for. We would need to know 1) Are there ever any more that one object of that class, that the variable is in, in the world at one time? 2) If possible more than one, could both (or each) of them carry different values for that boolean, or is the variable more general to the class, than to the specific objects of that class? 3) Is(are) there a reference(s) to the object(s) of that class in the world class?

1) no there is only ever one of the object 2) no, because only one is ever required/can be made 3) what do you mean by references?

reference: a variable that holds the actor; but nevermind that. Since you only ever have one, you can make the boolean variable a 'static' one, making it a class variable instead of an instance variable. Then in the world class you can get the value by way of the class name. If your actor class was called 'Player' and the boolean variable was 'mutating', then in the world: if (Player.mutating) addObject(... Just by changing 'boolean mutating = false;' to 'static boolean mutating = false;'

I want you to keep in mind, however, that making the variable 'static' makes it a class variable; meaning that there is one, and only one, variable for that class (no matter how many objects you create in that class). This also means that the variable does NOT belong to any one object of that class, but all have access to the one variable. Also, make sure the world changes the value back to false when it acts on a true value.

There were ways of getting the value without making it static, but a bit more involved. Like

One reason to avoid static is that the value won't be reset when you use the "reset" button in Greenfoot. FWIW one of the tutorials (see the documentation) answers your question quite thoroughly.