Set and Retrieve environment (OS) variables in java

OSEnvVars.java

// OS Environment Variables
// =======================
// http://docs.oracle.com/javase/tutorial/essential/environment/env.html
/*
Windows provides the user name in an environment variable called USERNAME, 
while UNIX implementations might provide the user name in USER, LOGNAME, or both.

To maximize portability, if the env var you need is available as java system property, always use getProperty(). 
For example, if the operating system provides a user name, it will always be available in the system property user.name.
*/

/*
Environment variables are set in the OS, eg in linux
    export HOME=/Users/myusername
or on windows
    SET WINDIR=C:\Windows
etc., and, unlike system properties, may not be set at runtime.

Java chose to expose OS environment variables as Properties with
    System.getEnv() or System.getEnv(variable)
(just as Java exposes current directory and "other stuff" as Properties). 
*/

// Retrieving Env Variables
import java.util.Map;

public class EnvMap {
    public static void main (String[] args) {
        Map<String, String> env = System.getenv();
        for (String envName : env.keySet()) {
            System.out.format("%s=%s%n",
                              envName,
                              env.get(envName));
        }
    }
}

public class Env {
    public static void main (String[] args) {
        for (String env: args) {
            String value = System.getenv(env);
            if (value != null) {
                System.out.format("%s=%s%n",
                                  env, value);
            } else {
                System.out.format("%s is"
                    + " not assigned.%n", env);
            }
        }
    }
}