If a variable must persist between executions, you have to do some disk persistence. The easiest way, as suggested by Raghunandan, is using Shared Preferences.
To do this, first request a shared preferences:
SharedPreferences prefs = getSharedPreferences(PREFS_NAME, Context.MODE_PRIVATE);
SharedPreferences.Editor editor = prefs.edit();
editor.putString("key_name", "key_value");
editor.commit(); //important, otherwise it wouldn't save.
You can do this, for instance, in the onStop or onPause of the activity (onPause is always called, but there's no guarantee onStop will be called prior to Honeycomb).
In your onCreate (or when you need the value), you read from SharedPreferences and check if there's a value set, otherwise use a default value:
SharedPreferences prefs = getSharedPreferences(PREFS_NAME, 0);
String myVariable = prefs.getString("key_name", "default_value");
Now you can use myVariable
with the persisted value or the initial (default) value if nothing was saved before.
For more, see https://developer.android.com/guide/topics/data/data-storage.html#pref
To clarify, PREFS_NAME
is the name of your shared preferences file. Usually, prepend the package name for your application, it's good practice.