A use for the private in a method - Java

See in the file pen I create a method that is private that can't be used in no place but only in the class-pen, so what the use if I run in the main and for the main is invisible, so if I use the class exec inside one that is public(visible for others classes) it will run and you gonna see the output, so is that is like we have sitrons inside the eletronical, and in the electronic circuit, part is isolated from the keypad, but work with it, so is private and only work because is part of something that is public and will work in the main, in the front of yuo remote control, the keypad need it to be there but don't reach it, I can make a mechanism that is isolated and only comunicated with the front by other part of the isolated mechanism that isn't private, why the same shit in the programming, because is something that will never work dispair of that one that is public and we are trying make the main be less busy with code in others files (one class can be write inside in the main, but we want write in other file to be more organized with big codes organized in files to be more understandible, and even corrected, part of the program could be wrong and we can turn it off to analize). Rute Bezerra de Menezes Gondim