从另一个类调用对象的方法

I have three classes, the first class makes a box on screen and there is a method to screen capture the contents of the box. In the second class several boxes are instantiated and in the third class there is a jframe with a button. When the button is pressed I want to call the captureComponent method but just for one specific box which has been created. I am new to Java and I have been really struggling to see how I can achieve this. I have been staring at this code for several days. I think that it will be something simple. All the individual classes work fine and ideally the class arrangement should stay as it is.I have three classes, the first class makes a b