The official sample code and tutorials can be found here: http://developer.android.com/resources/index.html

For reading code I can recommend these repositories:

More sample applications for the Android platform: http://code.google.com/p/apps-for-android/

Open Source examples from the books http://commonsware.com/AndTutorials/ and http://commonsware.com/AdvAndroid/ are available at http://github.com/commonsguy/cw-andtutorials and http://github.com/commonsguy/cw-advandroid/ . They helped me a lot by simply reading the code and searching for classes I needed.

You have to access your android device via terminal to get the APK from it. Moreover you'll need to root the device if the app has been installed from the app store. Once you got the APK you can decompile it in java code.

Root your device: http://www.digitaltrends.com/mobile/how-to-root-android/

Get the APK from the phone:

• Connect the phone to your computer and open a terminal.
• Run adb shell so you get into your phone.
• Run find /data/app -name '*.apk' to list the apks installed and copy the name of the one in your interest.
• Run adb pull /data/app/appname.apk

Now that you have the APK on your computer you have to decompile the APK.