Android :: Can Update A List View In One Activity While Im In Another Activity

I currently have a tab layout with 2 tabs, one tab with a list view and one with the option make strings so I can add them in the list view. Both tabs have their own activity because this made the code much more structured, and I dont have to repeat my self later.

Lets say im in the tab that offer me to create an string, and i press the update list button, how do I update the list view without startActivity()? If i use startActivity(), it starts List.java, and instead of displaying the list in the list view tab, it takes full screen, which defies the purpose of the tab view. In other words, the startActivity() steals the focus from the tab view of the list, and sends it fulscreen.

I want to update the activity in my list view tab, without starting a new activity that goes to fullscreen, and doesnt update the one in the tab.