My client wan't a app that is bug free. Can anyone tell how the fuck is that possible?

Automated testing. If they're willing to pay for that of course...

Have them test the app and sign off on it. Then if a bug 🐜 is found it is on them.

Writing bug free applications is pretty easy.

as long as you only want to output "Hello World".

@DLMousey automated tests does not guarantee that the code is bug free (even with 100% code coverage), automated testing primarily helps you detect regressions early

Use bug 🐜 repellent .

Get the space shuttle software team to develop it.

https://fastcompany.com/28121/...

Write unit tests and hire a software analyst to do smoke and regression tests. Even then the odd bug here and there will get through.

an app w/o input

Import the magic package or install the magic library it will work magically or you go the Volkswagen way and make it pass all tests by cheating on them :D

Depends what the client considers to be a bug.

Dijcstra Law says it is impossible to be sure that soft is without any bugs

All software is bug free until proven otherwise.

An app can't have bugs if you don't make it in the first place

@ItsNotMyFault bugs can be in envoronment also, i.e. bug in JVM or in Linux itself

In order to do that you need to bring them a very simple made calculator and prove that assuming you haven't made any mistakes attaching the calculator's chips together you can prove your program works using software verification techniques.
Microsoft tried to create a miniature experimental OS like that about 10 years ago and gave up.