Can one class extend two classes?
Solution 1:
Java does not support multiple inheritance.
There are a few workarounds I can think of:
The first is aggregation: make a class that takes those two activities as fields.
The second is to use interfaces.
The third is to rethink your design: does it make sense for a Preferences class to be both a PreferenceActivity and an AbstractBillingActivity?
Solution 2:
Java doesn't support multiple inheritance. You can implement multiple interfaces, but not extend multiple classes.