creativen:
In more case, is it safe to plug in usb cable, dc adaptor, and 4 AA battery at the same time?
If the DC adaptor is pulgged into the barrel jack and the AA battery pack is correctly connected to VIN and GND pins, it won't cause any harm... However, you wouldn't want to do that without at least putting a diode in between the positive lead from the battery pack and the VIN pin. This will make sure the current is only being drawn from AA batteries when the DC adaptor is unplugged or otherwise not providing current.
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What type of diode?