Answer 1

https://code.sololearn.com/cuwpen4HvEay/?ref=app

Made a program demonstrating what you asked. This way you can quickly run it to see how it works.

Answer 2

boolean x = true;
x = !x;

Answer 3

example code:
boolean b = true;
b = !b;

value of b is now false

Answer 4

normaly you would just use x = !x
wich will set true to "not true"/ false