Android App development - Force Garbage collector

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http://stackoverflow.com – I'm writing an Android app and my app functions as expected except when it is left running in the background for long periods of time. When I resume the app after several hours, it crashes. I have waited the time period and watched the response in LogCat, which pointed to a null pointer exception in my code, however after surrounding that area of code in a try catch (and executing correctional code in the catch) it still crashes after several hours of inactivity. My question is, is there a way I can manually force the garbage collector (or whatever the Android O/S is doing after several hou (HowTos)