Multiple oncreate? methods

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Mobile

Android

Basically the app starts on the main screen where i want the inventory array to be. there is a button that takes you to another screen and allows you to enter information and create an item object. When I click the add button id like to send the item back to the main screen via intent and have it add it to the inventory. Or alternatively click the back button and return to the main screen with no item.

Where i'm confused is how the main screen know when its being:
a) Launched for the first time
b) if there is an item to add from being started by the add button
c) to just relaunch and not reinitialize my array