The JDK's source code, pre-Java 9, had the following basic organizational schema:

source code / [shared, OS-specific] / [classes / native] / Java API package name / [.file extension]

Let's look at this a bit closer. After the source code, we have two options. If the code is cross-platform, then it is a shared directory; otherwise, it is operating system specific. For example:

src/share/...
src/windows/...

Next, we have the classes directory or a native language directory. For example:

src/share/classes/...
src/share/classes/java/...

Next, we have the name of the Java API package followed by the file extension. The file extensions depend on content such as .java, .c, and more.