Sorry, I worded this pretty badly initially.
I’ve made a text editor already, I know how to load and save files, within the sketch/application.
What I want to be able to do is set my sketch as the default program for a file to open in. Like on windows for example, the default program for a text file is notepad. I would like to know if it’s possible to change that to my processing sketch so it can be used much easier as a text editing application.