Android Question how can i write to a file and read from a file in android studio?!

hi everyone i used to work with c and c++ but now i need some java odes for my ndk.

i really need to know about files in java

i created an app but it doesnt work in this part !!!
i tried to create a file

write string A

read string A

then show it in my program

but now i cant find my file and A string too

can you help a simple and useful way to do that?