This example shows how a program can get input from four different places,

  1) a file
  2) an environment variable
  3) a command line argument
  4) the program's standard input stream.

Any one of these can be used for "inter-process communication" (IPC).


The program UseAdder.java "calls" the Adder.java program as
if it were a kind of "external function". The UseAdder process
passes parameters to the Adder process by the environment variable
method of inter-process communication (UseAdder.java could be
rewritten to use either command line argument IPC or file base IPC).