How to create Android UI for Java simple app

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http://stackoverflow.com – I've created a very simple Java code that reads a file (txt), changes a couple of strings on it and then saves the "changed" file creating a path for it on my computer. But i want to port this simple "app" to Android. Currently, i have no GUI for my app, it just runs and does the job.(I run it on Eclipse) What i want to know is how to create a simple "Button" in android to run my code; and I was wondering if it is possible to create it similar to the Java's SWITCH/CASE fuction. I've looking for a while with no luck, i´ve seen some examples but i dont understand if i can actually call my m (HowTos)