[Docs] Android code style guide

docs/style_guide.md

@@ -1,4 +1,4 @@
-# Style Guide
+# C++ Style Guide
 
 The style guide can be summed up as 'clang-format with the Google style set'.
 In addition, the [Google Style Guide](https://google.github.io/styleguide/cppguide.html)
@@ -40,7 +40,7 @@ The buildbot runs `xb lint --all` on the master branch, and will run
 `xb lint --origin` on pull requests. Run `xb format` before you commit each
 local change so that you are consistently clean, otherwise you may have to
 rebase. If you forget, run `xb format --origin` and rebase your changes (so you
-don't end up with 5 changes and then a 6th 'whoops' one - that's nasty).
+don't end up with 5 changes and then a 6th 'whoops' one — that's nasty).
 
 The buildbot is running LLVM 3.8.0. If you are noticing style differences
 between your local lint/format and the buildbot, ensure you are running that
@@ -82,3 +82,44 @@ tabs or linefeeds or whatever again.
 
 TODO(benvanik): write a cool script to do this/editor plugins.
 In the future, the linter will run as a git commit hook and on travis.
+
+# Android Style Guide
+
+Android Java and Groovy code and XML files currently don't have automatic format
+verification during builds, however, stick to the [AOSP Java Code Style Rules](https://source.android.com/setup/contribute/code-style),
+which contain guidelines not only for code formatting, but for the usage of
+language features as well.
+
+The formatting rules used in Xenia match the default Android Studio settings.
+They diverge from the C++ code style rules of Xenia in many areas, such as
+indentation width and the maximum line length, however, the goal for Android
+formatting in Xenia is to ensure quick development environment setup.
+
+In Java code, limit the length of each line to 100 characters. If an assignment
+doesn't fit in the limit, move the right-hand side of it to a separate line with
+8-space indentation. Similarly, if the argument list of a method declaration or
+a call is too long, start the entire argument list on a new line, also indented
+with 8 spaces — this is one of the differences from the C++ code style in Xenia,
+where arguments may be aligned with the opening bracket. In general, follow the
+[rectangle rule](https://github.com/google/google-java-format/wiki/The-Rectangle-Rule)
+so expressions in the code constitute a hierarchy of their bounding rectangles,
+ensuring that with 4-space indentation for block contents and 8-space
+indentation for subexpressions.
+
+In XML files, if the width of the line with an element exceeds 100 characters,
+or in most cases when there are multiple attributes, each attribute should be
+placed on a separate line with 4-space indentation, with the exception of the
+first `xmlns`, which should stay on the same line as the element name.
+
+In Groovy, use 4-space indentation for blocks and 8-space indentation for
+splitting arguments into multiple lines. String literals should be written in
+single quotes unless string interpolation is used.
+
+You can use the Code -> Reformat Code and Code -> Reformat File options in
+Android Studio to apply coarse formatting rules for different kinds of files
+supported by Android Studio, such as Java, XML and Groovy. While clang-format is
+very strict and generates code with the single allowed way of formatting,
+Android Studio, however, preserves many style choices in the original code, so
+it's recommended to approximate the final style manually instead of relying
+entirely on automatic formatting. Also use Code -> Rearrange Code to maintain a
+consistent structure of Java class declarations.