How can I change the window when I click on the icon?

Question

Java

I want to make a simple messenger of messages, which consists of two
conditional windows. In the left window, contacts, and in the second, the functionality for sending messages should appear. So here's how to make it so that when you click on a contact in the second window, this functionality appears, for example, like in Skype. I do it all in JavaFX Scene Builder
PS I just started learning programming and I apologize for the "stupid" questions.

Answer 1

Apparently you need to use the click handler for this button and then execute the algorithm to display what you need in another window, I just can’t understand, “I’m just starting to learn”, and you already want to do something incredible, you just need to understand that you are doing nothing you will not learn and every problem you have will be here on Toster in a way that you will not be able to solve it