> For the complete documentation index, see [llms.txt](https://android-course.cornellappdev.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://android-course.cornellappdev.com/chapters/2.-jetpack-compose/2.1-introduction.md).

# 2.1 Introduction

Unlike Native Android, which is covered in the chapters up until this point, Jetpack Compose uses purely Kotlin without XML layout files. You may be wondering how Jetpack Compose achieves this while knowing where to arrange the components. The secret lies in the building blocks of Jetpack Compose: Composable Functions. Instead of using activities and fragments as we would in Native Android, instead everything we want the users to see is annotated with an @Composable tag. This allows users to define their UI programmatically rather than worrying about the semantics of UI construction.


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