p2-4 visual + logic (ie: layout + coding)
p2-6 visual: GUI (graphic user interface) using XML
p2-7 Coding: using Java to program
p2-8 Build: make file to genertate .apk (Fig 2-3)

p2-9 
 up Fig: why only 2 files: A:activity_main.xml B:MainActivity.java (# take attentation on files' extension: xml, java)

p2-10 create java (not kotlin)

p2-13 
 layout: element's ID in xml link to java

p2-15 layout and attributes
 layout: elements display 
 attributes: size, color, ...

p2-25 Chap02_Button
 request: check Fig 1st

p2-26
 Chap02_Button
 tw.com.flag.chap02_button #differ

p2-29
 xml --> code (comment out orig one)
 #color (yellow ok, red is BAD!)
<androidx.xxyyzz
..
> END here

</androidx ...> end of androidx 

how to comment?
<!--
here is comment
-->

p2-29
 SELECT 'android.support.constraint.ConstraintLayout' --> practice select using arrow key

p2-31
 canNOT move 'hello world' text --> disable magnetic icon
 add TextView id as txv

p2-33
 add a button

p2-34
 move hello world and button
 change txv text to Bigger (chinese) --> warning (yellow tringle) --> click that triangle --> last there is a Fix (it will open extract resource window)

p2-41
 textApperance is missing, you have to use + to add it in
 add textSize attr in too

p2-45
 public void bigger(V #note Capital V not v)