Move Java style guide to source code repository.

styleguide/java/java.md

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+# Chromium Java style guide
+
+_For other languages, please see the [Chromium style
+guides](https://chromium.googlesource.com/chromium/src/+/master/styleguide/styleguide.md)._
+
+Chromium follows the [Android Open Source style
+guide](http://source.android.com/source/code-style.html) unless an exception
+is listed below.
+
+## Style
+
+* Copyright header should use
+  [Chromium](https://chromium.googlesource.com/chromium/src/+/master/styleguide/styleguide.md)
+  style.
+* TODO should follow chromium convention i.e. `TODO(username)`.
+* Use of ```assert``` statements are encouraged.
+* Fields should not be explicitly initialized to default values (see
+  [here](https://groups.google.com/a/chromium.org/d/topic/chromium-dev/ylbLOvLs0bs/discussion)).
+* For automated style checking install
+  [checkstyle](https://sites.google.com/a/chromium.org/dev/developers/checkstyle).
+
+## Location
+
+"Top level directories" are defined as directories with a GN file, such as
+[//base](https://chromium.googlesource.com/chromium/src/+/master/base/)
+and
+[//content](https://chromium.googlesource.com/chromium/src/+/master/content/),
+Chromium Java should live in a directory named
+`<top level directory>/android/java`, with a package name
+`org.chromium.<top level directory>`.  Each top level directory's Java should
+build into a distinct JAR that honors the abstraction specified in a native
+[checkdeps](https://chromium.googlesource.com/chromium/buildtools/+/master/checkdeps/checkdeps.py)
+(e.g. `org.chromium.base` does not import `org.chromium.content`).  The full
+path of any java file should contain the complete package name.
+
+For example, top level directory `//base` might contain a file named
+`base/android/java/org/chromium/base/Class.java`. This would get compiled into a
+`chromium_base.jar` (final JAR name TBD).
+
+`org.chromium.chrome.browser.foo.Class` would live in
+`chrome/android/java/org/chromium/chrome/browser/foo/Class.java`.
+
+New `<top level directory>/android` directories should have an `OWNERS` file
+much like
+[//base/android/OWNERS](https://chromium.googlesource.com/chromium/src/+/master/base/android/OWNERS).
+
+## Asserts
+
+The Chromium build system strips asserts in release builds (via ProGuard) and
+enables them in debug builds (or when `dcheck_always_on=true`) (via a [build
+step](https://codereview.chromium.org/2517203002)). You should use asserts in
+the [same
+scenarios](https://chromium.googlesource.com/chromium/src/+/master/styleguide/c++/c++.md#CHECK_DCHECK_and-NOTREACHED)
+where C++ DCHECK()s make sense. For multi-statement asserts, use
+`org.chromium.base.BuildConfig.DCHECK_IS_ON` to guard your code.
+
+Example assert:
+
+```java
+assert someCallWithSideEffects() : "assert description";
+```
+
+Example use of `DCHECK_IS_ON`:
+
+```java
+if (org.chromium.base.BuildConfig.DCHECK_IS_ON) {
+   if (!someCallWithSideEffects()) {
+     throw new AssertionError("assert description");
+   }
+}
+```