I'm new to Java, in fact I know next to nothing. I've been interested in the type of data an android app is holding on me. So, I can see within an XML file this app has created strings which appear to to be encrypted using AES128-CBC.

So I've decompiled their .apk file and looking through their source I can see a method they call to decode stuff (DecodeMe). Can I pass my encrypted strings through this using their method and it will decrypt it? 

Sorry, I'm not too clued up on this.
['https://Stackoverflow.com/questions/17809234', 'https://Stackoverflow.com', 'https://Stackoverflow.com/users/2359639/']
I have no experience with Android development at all, I can only comment from the Java side of things.

If you have been able to decompile the program, you should be able to use the code as you like - so, assuming that DecodeMe really does what it's name implies, then yes it should decode the text.

Add the decompiled code to a new project in your favourite Java IDE, create your own main(...) class/method and try calling the DecodeMe - it *shouldn't* take too much effort to set up and test.

EDIT: Yes, you can use a decompiled method - you simply need to make sure all methods/classes it uses are also available.
That is a java method you just mined from the APK.

> 
> private void doNothing() {}
> 
> 
> 

Whoa, this is another. You can use it.
Sorry, just some fun. :)
Yes, you can actually use it, but check if it uses some class that is also defined in the application. You have to mine all custom classes, too.