How do you pause a thread until a condition become

How do you pause a thread until a condition becomes true without busy waiting? For example, suppose I'm checking to see whether

playerOne.isWalking()

Is going to be true and I want to run some code when it does become true. Again suppose playerOne is an instance of the class Player which then has the function isWalking(). I do not have access to the what makes isWalking() become true. So everytime I want to check to see if isWalking() is true, I have to call the method from an instance of player explicitly.

I've tried using Wait/Notify in synchronized blocks but that wouldn't ever work because I would have to manually notify the Thread to wake up when isWalking() becomes true and I don't know when it will become true.

I've tried using the observer pattern to make Player a subclass of observable so that I can call the update method when isWalking() becomes true but again I don't know when that will happen.

I've even tried using Propertys but that wouldn't work either.

Is this even possible to check without busy-waiting? Constantly polling and calling that function to see if it is true, then if it is true, execute the code.

This was a bad solution I made involving busy-waiting:

Busy-Waiting solution

The underlaying problem is that you use a getter method to aquire an information and act on it. The principle which is violated here is Tell, don't ask!.

The OO whay to solve this is to introduce an interface (e.g.: PlayerMovingListener).

The player class would hold an instance of an object implementing that interface (or a list of them). As soon as the player starts walking it calls the isMoving() method defined in the interface implemented by the (or all) listener object(s) held by the player.
^{Most likely is triggered by a setter method in your current Payer implementation.}