Android uses a DEX archive in place of JAR. Also, the Java classes are compiled differently. To decompile the classes, you need to go through a few extra steps.
What Will You Need?
- Dex2Jar from https://code.google.com/p/dex2jar/
- A regular Java decompiler, such as JD from http://java.decompiler.free.fr.
Install these programs.
First, extract the .apk file of the Android application.
jar xvf HelloWorld.apk
You will notice classes.dex file in the extracted folder.
Next, run dex2jar to convert the classes.dex file to a JAR file.
This will produce an improbably named classes.dex.dex2jar.jar file.
Launch JD decompiler. Open the classes.dex.dex2jar.jar file and then view the .class files.