This is the official documentation for the -s option in javac
Specify the directory where to place generated source files. The directory must already exist; javac will not create it. If a class is part of a package, the compiler puts the source file in a subdirectory reflecting the package name, creating directories as needed. For example, if you specify -s C:\mysrc and the class is called com.mypackage.MyClass, then the source file will be placed in C:\mysrc\com\mypackage\MyClass.java.
However, I want to know the default directory if this flag is not set from an official documentation/sources.