You need to override onSaveInstanceState(Bundle savedInstanceState) and write the application state values you want to change to the Bundle parameter like this:
@Override
public void onSaveInstanceState(Bundle savedInstanceState) {
super.onSaveInstanceState(savedInstanceState);
// Save UI state changes to the savedInstanceState.
// This bundle will be passed to onCreate if the process is
// killed and restarted.
savedInstanceState.putBoolean("MyBoolean", true);
savedInstanceState.putDouble("myDouble", 1.9);
savedInstanceState.putInt("MyInt", 1);
savedInstanceState.putString("MyString", "Welcome back to Android");
// etc.
}
The Bundle is essentially a way of storing a NVP ("Name-Value Pair") map, and it will get passed in to onCreate() and also onRestoreInstanceState() where you would then extract the values from activity like this:
@Override
public void onRestoreInstanceState(Bundle savedInstanceState) {
super.onRestoreInstanceState(savedInstanceState);
// Restore UI state from the savedInstanceState.
// This bundle has also been passed to onCreate.
boolean myBoolean = savedInstanceState.getBoolean("MyBoolean");
double myDouble = savedInstanceState.getDouble("myDouble");
int myInt = savedInstanceState.getInt("MyInt");
String myString = savedInstanceState.getString("MyString");
}
Or from a fragment.
@Override
public void onViewStateRestored(@Nullable Bundle savedInstanceState) {
super.onViewStateRestored(savedInstanceState);
// Restore UI state from the savedInstanceState.
// This bundle has also been passed to onCreate.
boolean myBoolean = savedInstanceState.getBoolean("MyBoolean");
double myDouble = savedInstanceState.getDouble("myDouble");
int myInt = savedInstanceState.getInt("MyInt");
String myString = savedInstanceState.getString("MyString");
}
You would usually use this technique to store instance values for your application (selections, unsaved text, etc.).
Best Solution
Update: Since I wrote this answer two years ago, we productized Mono to run on Android. The work included a few steps: porting Mono to Android, integrating it with Visual Studio, building plugins for MonoDevelop on Mac and Windows and exposing the Java Android APIs to .NET languages. This is now available at http://monodroid.net
Mono on Android is based on the Mono 2.10 runtime, and defaults to 4.0 profile with the C# 4.0 compiler and uses Mono's new SGen garbage collection engine, as well as our new distributed garbage collection system that performs GC across Java and Mono.
The links below reflect Mono on Android as of January of 2009, I have kept them for historical context
Mono now works on Android thanks to the work of Koushik Dutta and Marc Crichton.
You can see a video of it running here: http://www.koushikdutta.com/2009/01/mono-on-android-with-gratuitous-shaky.html
And you can get the instructions to build Mono yourself here: http://www.koushikdutta.com/2009/01/building-mono-for-android.html
You can get a benchmark comparing Mono's JIT vs Dalvik's interpreter here: http://www.koushikdutta.com/2009/01/dalvik-vs-mono.html
And of course, you can get a pre-configured image with Mono here (go to the bottom of the post for details on using that): http://www.koushikdutta.com/2009/01/building-mono-for-android.html