Where can i find the main android "{filename}.java" file where all the core coding goes?

From tutorials ive seen, it sounds like its included in the android holo theme, you just need code to make it display. Im a newbie to java/javascript

Where can i find and android source based file called "MainActivity.java". Ive looked through all the folders under "ANDROID" with no luck. Im trying to implement the native android action bar and i need to find that file.