How to get number of lines of a project in Android Studio?

One way is to search (ctrl+shift+F) for line breaks \n with regular expressions enabled and whole project as the scope. If you want to limit the search to only java and xml files, you can use the following file mask *.java, *.xml

As alternative you can use (Linux, Windows) utility cloc (Count Lines Of Code). Cloc counts, and computes differences of, comment lines, blank lines, and physical lines of source code.

An example basic use (my project):

cloc /home/myHome/StudioProjects/myApp/app/src/
     111 text files.
     111 unique files.
       0 files ignored.

github.com/AlDanial/cloc v 1.70  T=0.36 s (308.1 files/s, 38552.3 lines/s)
-------------------------------------------------------------------------------
Language                     files          blank        comment           code
-------------------------------------------------------------------------------
Java                            50           1323            146           6407
XML                             60            460             50           3203
SQL                              1              0              0           2302
-------------------------------------------------------------------------------
SUM:                           111           1783            196          11912
-------------------------------------------------------------------------------  

Project page https://github.com/AlDanial/cloc (thanks Xam)