Recently I created a guitar tuner application for Android that is written with pure Java (no C++ or NDK usage). The design was inspired by the Google Chrome team's guitar tuner web app using the WebAudio API. I wanted to code a version written natively for Android that didn't have to rely on a WebView, the WebAudio APIs, or server-side logic. Also, I wanted this application to be available to as many versions of Android as possible (whereas the WebAudio API may only be supported in more recent versions of WebView available only on newer flavors of Android). So, I coded it from scratch. I used a portion of the open source TarsosDSP project (their YIN algorithm) to help with the pitch detection.
The application is available in the Google Play Store for Android: https://play.google.com/store/apps/details?id=com.chrynan.guitartuner. The project is completely open source and the code can be found on the GitHub repository: https://github.com/chRyNaN/Android-Guitar-Tuner. Finally, to get a feel for some of the issues that I faced when creating the application, have a look at this StackOverflow question and answer that I posted: http://stackoverflow.com/questions/34938292/rotation-animation-with-custom-view/35181397#35181397.