You probably have two files open already, in fact: activity_main.xml and MainActivity.java. Try switching between these, if you so wish. Over on the left is a hierarchy.

An Android APK file contains all the open-source Java files used to set up and run the Google Android app. ... Press "Enter" and double-click the file to open the Android SDK interface.

Open the Java file in which you want to carry out your image processing. This may be an Android Activity class file or any other Java class you have in your application.

This post introduces the basics of Java syntax so that you can better start to understand what all those lines do. Developing good practices now will save you a lot of trouble as an Android ...