The possible sources of this value are:
* The default value, ".", meaning that user class files are all class files in the current directory (or under it, if in a package).
* The value of the CLASSPATH environment variable, which overrides the default value.
* The value of the -cp or -classpath command line option, which overrides both the default value and the CLASSPATH value.
* The JAR archive specified by the -jar option, which overrides all other values. If this option is used, all user classes come from the specified archive.