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 http://code.google.com/p/dex2jar/
A regular Java decompiler, such as JD (please, search "java decompiler" on google)
The Steps
First, extract the .apk file of the Android application.
jar xvf HelloWorld.apk
(in alternative you can use the 7zip program)
You will notice classes.dex file in the extracted folder.
Next, run dex2jar to convert the classes.dex file to a JAR file.
(you can extract the dex2jar file in the same folder of classes.dex)
dex2jar.bat classes.dex (for winfows)
dex2jar.sh classes.dex (for linux)
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
