Force a class in Java to call a method

I have a class which holds a private member (which itself is an object). I want to access the member in a legitimate way and modify its properties. The way I thought of doing this is to make the class call a method of my class (where I want to access it) with its private member.

For example, I have class ClassA which has property mItem. I want to get the instance of mItem in ClassB so that I can call mItem.update() at a later point of time. From ClassB, I want to call a method of ClassA (say, aMethod) that actually calls some method of ClassB passing mItem. However, I want to enforce this, so that aMethod implementation mandates calling the function of ClassB with mItem.

Any ideas?