And here is the section of the CodePath Android guides that deals with TextView (what's displayed) and EditText (what's entered):
...and here are the guides for EventListener (what happens when the user takes X action...clicks on a button, etc):
...adding the following property:
...in this sample app:
...inside your Button in your the XML file in src/res/layout/ (probably named activity_main.xml in this case) links clicking the button with the java method you define (onAddItem in the ToDo app).
This defined java method will be found in the .java file in src/main/java/(whatever you set as the Company Domain)/ (probably named MainActivity.java in this case).
The following code in the java method:
...in this demo app:
...is what connects the java code & the XML layout elements.