Java SE (Java Platform, Standard Edition)

Hello everyone,

I have a question related to the size of a JFrame or JPanel. I add a few JPanels to a JFrame and I wanted to detect when the size of the JFrame window or the JPanel itself had changed (if the user pressed the "Maximize icon" or simply dragged the window size from the one of the corner of the window). I am not sure if I need a listener added to the Frame, and what kind of listener that would be (if any exists, I am not sure).

I have another question regarding the class Dimension. I add JPanel's to my JFrame and I use the class Dimension to get the size of the panel with the getSize() method. I wanted to know when is the sooner that I could call this method to get the right size. For example, I use it in a few methods like the paintComponent, the mouseReleased and mouseDragged method, but I wished to get the correct size as soon as possible and only compute it once (or update it if the JPanel sized change). I am wondering why the size is 0 while I call this method inside the constructor and if there is any method where I can call it before actually doing anything to the JFrame please.

I hope my questions are somewhat clear.

Thanks for all your help, it is very much appreciated.